feat: implementation of gsl::dyn_array (#1228)

* feat: implementation of gsl::dyn_array

Implement gsl::dyn_array<T, Allocator> as specified by the
CppCoreGuidlines here:
https://github.com/isocpp/CppCoreGuidelines/blob/master/docs/dyn_array.md

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

.clang-format

@@ -31,4 +31,4 @@ AlignConsecutiveAssignments: false
 AlignTrailingComments: true
 
 SpaceAfterCStyleCast: true
-CommentPragmas: '^ NO-FORMAT:'
+WhitespaceSensitiveMacros: [GSL_SUPPRESS]

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,29 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: 'Status: Open, Type: Bug'
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+```c++
+#include <gsl>
+
+// your repro here: ...
+```
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Spec (please complete the following information):**
+ - OS: [e.g. Windows]
+ - Compiler: [e.g. MSVC]
+ - C++ Version: [e.g. C++20]
+
+**Additional context**
+Add any other context about the problem here.

.github/copilot-instructions.md

@@ -0,0 +1,84 @@
+# GitHub Copilot Instructions for GSL (Guidelines Support Library)
+
+## Project Overview
+This repository contains the Guidelines Support Library (GSL), a Microsoft implementation of types and functions 
+suggested for use by the C++ Core Guidelines. It's a header-only C++ library with emphasis on safety, 
+correctness, and zero overhead.
+
+## Coding Standards
+
+### General
+- Follow C++ Core Guidelines wherever possible
+- Use meaningful type, function, and template parameter names
+- Keep functions small and focused with clear preconditions/postconditions
+- Include comments for complex code, but prefer self-documenting code
+- Use the Expects() and Ensures() macros for contract verification
+
+### Style Guidelines
+- Use 4 spaces for indentation (not tabs)
+- Maximum line length of 100 characters
+- Follow GSL naming conventions (lowercase with underscores)
+- Keep templates clean and readable with appropriate spacing
+- Use C++14 features since this is the minimum standard supported
+
+### Error Handling
+- Use Expects() for preconditions and Ensures() for postconditions
+- Design for fail-fast semantics (std::terminate) on contract violations
+- Template constraints should use static_assert or SFINAE
+- Don't throw exceptions from basic operations
+
+### Testing
+- Write thorough unit tests for every component using GTest
+- Test for all edge cases and error conditions
+- Ensure cross-platform compatibility in tests
+- Maintain 100% code coverage for changed code
+
+## Project-Specific Conventions
+
+### Architecture
+- All public types must be in the gsl namespace
+- Design for zero overhead abstractions when possible
+- Respect the distinction between Owners and Views
+- Maintain backward compatibility with existing GSL code
+
+### Version Control
+- Link all PRs to related issues
+- Use clear commit messages explaining what and why
+- Follow the contribution guidelines documented in CONTRIBUTING.md
+- PRs should include appropriate tests with 100% coverage for changed code
+
+### Documentation
+- Document all public APIs with clarity on preconditions and postconditions
+- Keep header comments up-to-date
+- Include examples for complex functionality in docs/headers.md
+
+## Technology Stack
+- C++14 (minimum) for core implementation
+- CMake build system (3.14+)
+- Google Test for unit testing
+- Support for multiple compilers (MSVC, GCC, Clang)
+
+## Security Considerations
+- Bounds checking is a core principle - enforce it consistently
+- Design for safety while minimizing overhead
+- Ensure undefined behavior is explicitly detected where possible
+
+## Performance Guidelines
+- Optimize for both safety and performance
+- Constexpr-enable functions wherever possible
+- Avoid hidden allocations
+- Use noexcept appropriately for move operations and other performance-critical functions
+
+## Cross-Platform Support
+- Code must work across:
+  - Windows (MSVC)
+  - Linux (GCC, Clang)
+  - macOS (AppleClang)
+  - Android and iOS where applicable
+
+## Copilot Tasks
+- You can find the CMake artifacts for C++20 in build-cxx20 and C++14 in build-cxx14.
+- Before publishing a PR, verify the following:
+  - There are no compiler warnings or errors when building the test suite.
+  - The test suite passes on all supported platforms and compilers.
+  - The test suite passes for both C++14 and C++20.

.github/workflows/android.yml

@@ -17,13 +17,13 @@ jobs:
       run:
         working-directory: build
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Create build directory
         run: mkdir -p build
         working-directory: .
 
-      - uses: actions/setup-java@v4
+      - uses: actions/setup-java@v5
         with:
           java-version: 8
           distribution: zulu
@@ -38,7 +38,7 @@ jobs:
           echo "Emulator starting in background"
 
       - name: Configure
-        run: cmake -Werror=dev -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK_LATEST_HOME/build/cmake/android.toolchain.cmake -DANDROID_PLATFORM=16 -DANDROID_ABI=x86_64 -DCMAKE_BUILD_TYPE=Debug ..
+        run: cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK_LATEST_HOME/build/cmake/android.toolchain.cmake -DANDROID_PLATFORM=16 -DANDROID_ABI=x86_64 -DCMAKE_BUILD_TYPE=Debug ..
 
       - name: Build
         run: cmake --build . --parallel

.github/workflows/clang-format.yml

@@ -0,0 +1,39 @@
+name: Code Formatting
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+permissions:
+  contents: read
+
+env:
+  CLANG_VERSION: "20"
+
+jobs:
+  clang-format:
+    name: Run clang-format
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      # Install the exact clang-format binary
+      - name: Install clang-format
+        run: |
+          sudo apt install clang-format-${{ env.CLANG_VERSION }}
+
+      # Prints the version of clang-format being used
+      - name: Log clang-format version
+        run: clang-format-${{ env.CLANG_VERSION }} --version
+        
+      # Runs clang-format over the repository codebase
+      - name: Check format
+        run: |
+          {
+            find include/gsl -type f
+            find tests -type f \( -name '*.cpp' -o -name '*.h' \)
+          } | xargs clang-format-${{ env.CLANG_VERSION }} --dry-run --Werror

.github/workflows/cmake/action.yml

@@ -1,56 +1,33 @@
 name: Composite CMake
 inputs:
-  cmake_generator:
-    required: false
-    type: string
-    default: 'Unix Makefiles'
-  cmake_build_type:
+  cmake_preset:
     required: true
     type: string
-    default: ''
-  cmake_cxx_compiler:
-    required: false 
-    type: string 
-  gsl_cxx_standard:
-    required: true
-    type: number
-  extra_cmake_args:
+  extra_cmake_build_args:
     required: false
     type: string
     default: ''
-  build_cmd:
-    required: true 
-    type: string 
-    default: 'make'
-  test_cmd:
+  extra_cmake_configure_args:
     required: false
-    type: string 
-    default: 'make test'
-  shell:
-    required: false 
     type: string
-    default: 'bash'
-
+    default: ''
+  extra_ctest_args:
+    required: false
+    type: string
+    default: ''
 
 runs:
   using: composite
   steps:
-    - name: Create build directory
-      run: mkdir build
-      shell: ${{ inputs.shell }}
-
     - name: Configure CMake
-      working-directory: build
-      run: cmake -G "${{ inputs.cmake_generator }}" -DCMAKE_BUILD_TYPE=${{ inputs.cmake_build_type }} -DCMAKE_CXX_COMPILER=${{ inputs.cmake_cxx_compiler }} -DGSL_CXX_STANDARD=${{ inputs.gsl_cxx_standard }} -DCI_TESTING:BOOL=ON -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -Werror=dev ${{ inputs.extra_cmake_args }} ..
-      shell: ${{ inputs.shell }}
+      run: cmake --preset ${{ inputs.cmake_preset }} ${{ inputs.extra_cmake_configure_args }} -DCI_TESTING:BOOL=ON -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -Werror=dev 
+      shell: ${{ env.RUNNER_OS == 'Windows' && 'pwsh' || 'bash' }}
 
-    - name: Build
-      working-directory: build
-      run: ${{ inputs.build_cmd }}
-      shell: ${{ inputs.shell }}
+    - name: Build (with preset)
+      run: cmake --build --preset ${{ inputs.cmake_preset }} ${{ inputs.extra_cmake_build_args }}
+      shell: ${{ env.RUNNER_OS == 'Windows' && 'pwsh' || 'bash' }}
 
-    - name: Test
-      working-directory: build
-      run: ${{ inputs.test_cmd }}
-      shell: ${{ inputs.shell }} 
+    - name: Test (with preset)
+      run: ctest --preset ${{ inputs.cmake_preset }} ${{ inputs.extra_ctest_args }} --output-on-failure --no-compress-output
+      shell: ${{ env.RUNNER_OS == 'Windows' && 'pwsh' || 'bash' }}
 

.github/workflows/cmake_find_package.yml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         os: [ ubuntu-latest, macos-latest ]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: lukka/get-cmake@latest
         with:
           cmakeVersion: 3.14.0

.github/workflows/compilers.yml

@@ -17,49 +17,56 @@ jobs:
   gcc:
     strategy:
       matrix:
-        gcc_version: [ 10, 11, 12 ]
+        gcc_version: [ 12, 13, 14 ]
         build_type: [ Debug, Release ]
         cxx_version: [ 14, 17, 20, 23 ]
         exclude:
-          - gcc_version: 10
+          # https://github.com/google/googletest/issues/4232
+          # Looks like GoogleTest is not interested in making version 1.14
+          # work with gcc-12.
+          - gcc_version: 12
+            cxx_version: 20
+          - gcc_version: 12
             cxx_version: 23
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Run CMake (configure, build, test)
         uses: ./.github/workflows/cmake
         with:
-          cmake_build_type: ${{ matrix.build_type }}
-          cmake_cxx_compiler: g++-${{ matrix.gcc_version }}
-          gsl_cxx_standard: ${{ matrix.cxx_version }}
+          cmake_preset: gcc-${{ matrix.cxx_version }}-${{ matrix.build_type == 'Debug' && 'debug' || 'release' }}
           
   clang:
     strategy:
       matrix:
-        clang_version: [ 13, 14, 15 ]
+        clang_version: [ 16, 17, 18 ]
         build_type: [ Debug, Release ]
         cxx_version: [ 14, 17, 20, 23 ]
+        exclude:
+          # https://github.com/llvm/llvm-project/issues/93734
+          # Looks like clang fixed this issue in clang-18, but won't backport
+          # the fix.
+          - clang_version: 17
+            cxx_version: 23
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
      
       - name: Run CMake (configure, build, test)
         uses: ./.github/workflows/cmake
         with:
-          cmake_build_type: ${{ matrix.build_type }}
-          cmake_cxx_compiler: clang++-${{ matrix.clang_version }}
-          gsl_cxx_standard: ${{ matrix.cxx_version }}
+          cmake_preset: clang-${{ matrix.cxx_version }}-${{ matrix.build_type == 'Debug' && 'debug' || 'release' }}
 
   xcode:
     strategy:
       matrix:
-        xcode_version: [ '14.3.1', '15.4' ]
+        xcode_version: [ '16.4' ]
         build_type: [ Debug, Release ]
         cxx_version: [ 14, 17, 20, 23 ]
     runs-on: macos-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: select xcode version
         run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode_version }}.app
@@ -67,38 +74,33 @@ jobs:
       - name: Run CMake (configure, build, test)
         uses: ./.github/workflows/cmake
         with:
-          cmake_build_type: ${{ matrix.build_type }}
-          cmake_cxx_compiler: clang++
-          gsl_cxx_standard: ${{ matrix.cxx_version }}
+          cmake_preset: clang-${{ matrix.cxx_version }}-${{ matrix.build_type == 'Debug' && 'debug' || 'release' }}
+          extra_cmake_configure_args: '-DCMAKE_CXX_FLAGS="-isysroot \"$(xcode-select --print-path)/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk\""'
 
-  VisualStudio:
+  msvc:
     strategy:
       matrix:
-        generator: [ 'Visual Studio 16 2019', 'Visual Studio 17 2022' ]
-        image: [ windows-2019, windows-2022 ]
+        image: [ windows-2022, windows-2025 ]
         build_type: [ Debug, Release ]
-        extra_args: [ '', '-T ClangCL' ]
+        toolset: [ 'msvc', 'ClangCL' ]
         cxx_version: [ 14, 17, 20, 23 ]
-        exclude:
-          - generator: 'Visual Studio 17 2022'
-            image: windows-2019
-          - generator: 'Visual Studio 16 2019'
-            image: windows-2022
-          - generator: 'Visual Studio 16 2019'
-            cxx_version: 23
+        include:
+          # Regular MSVC builds use Ninja (from preset)
+          - toolset: 'msvc'
+            generator_override: ''
+          # ClangCL builds require Visual Studio generator
+          - toolset: 'ClangCL'
+            generator_override: '-G "Visual Studio 17 2022" -T ClangCL'
     runs-on: ${{ matrix.image }}
     steps:
-      - uses: actions/checkout@v4
-      - uses: microsoft/setup-msbuild@v2
+      - uses: actions/checkout@v6
+      - uses: microsoft/setup-msbuild@v3
+      - uses: ilammy/msvc-dev-cmd@v1
 
       - name: Run CMake (configure, build, test)
         uses: ./.github/workflows/cmake
         with:
-          cmake_generator: ${{ matrix.generator }}
-          cmake_build_type: ${{ matrix.build_type }}
-          gsl_cxx_standard: ${{ matrix.cxx_version }}
-          extra_cmake_args: ${{ matrix.extra_args }}
-          build_cmd: msbuild GSL.sln
-          test_cmd: ctest . --output-on-failure --no-compress-output
-          shell: pwsh
-
+          cmake_preset: msvc-${{ matrix.cxx_version }}-${{ matrix.build_type == 'Debug' && 'debug' || 'release' }}
+          extra_cmake_configure_args: ${{ matrix.generator_override }}
+          extra_cmake_build_args: ${{ matrix.toolset == 'ClangCL' && format('--config {0}', matrix.build_type) || '' }}
+          extra_ctest_args: ${{ matrix.toolset == 'ClangCL' && format('-C {0}', matrix.build_type) || '' }}

.github/workflows/copilot-setup-steps.yml

@@ -0,0 +1,21 @@
+name: "Copilot Setup Steps"
+
+on: workflow_dispatch
+
+jobs:
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+
+      - name: Install Build Dependencies
+        run: sudo apt-get update && sudo apt-get install -y clang cmake make
+
+      - name: Configure CMake (C++14)
+        run: cmake --preset clang-14-debug
+
+      - name: Configure CMake (C++20)
+        run: cmake --preset clang-20-debug

.github/workflows/ios.yml

@@ -12,7 +12,7 @@ jobs:
       run:
         working-directory: build
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Create build directory
         run: mkdir -p build
@@ -25,11 +25,12 @@ jobs:
             -GXcode \
             -DCMAKE_SYSTEM_NAME=iOS \
             "-DCMAKE_OSX_ARCHITECTURES=arm64;x86_64" \
-            -DCMAKE_OSX_DEPLOYMENT_TARGET=9 \
+            -DCMAKE_OSX_DEPLOYMENT_TARGET=12.0 \
             -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
             "-DMACOSX_BUNDLE_GUI_IDENTIFIER=GSL.\$(EXECUTABLE_NAME)" \
-            -DMACOSX_BUNDLE_BUNDLE_VERSION=3.1.0 \
-            -DMACOSX_BUNDLE_SHORT_VERSION_STRING=3.1.0 \
+            -DMACOSX_BUNDLE_BUNDLE_VERSION=4.2.0 \
+            -DMACOSX_BUNDLE_SHORT_VERSION_STRING=4.2.0 \
+            -DCMAKE_CXX_FLAGS="-Wno-missing-include-dirs" \
             ..
 
       - name: Build

.github/workflows/shell-script-linter.yml

@@ -0,0 +1,32 @@
+name: Shell script linter
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+permissions:
+  contents: read
+
+jobs:
+  clang-format:
+    name: Run shfmt and shellcheck
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      # Install the needed binaries
+      - name: Install shfmt and shellcheck
+        run: |
+          sudo apt install shfmt shellcheck
+
+      - name: Check format
+        run: |
+          find scripts -type f -name '*.sh' -exec shfmt -l {} \;
+
+      - name: Run shellcheck
+        run: |
+          find scripts -type f -name '*.sh' -exec shellcheck {} \;

.gitignore

@@ -1,5 +1,5 @@
 CMakeFiles
-build
+build*/
 tests/CMakeFiles
 tests/Debug
 *.opensdf

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.14...3.16)
 
-project(GSL VERSION 4.1.0 LANGUAGES CXX)
+project(GSL VERSION 4.2.0 LANGUAGES CXX)
 
 add_library(GSL INTERFACE)
 add_library(Microsoft.GSL::GSL ALIAS GSL)

CMakePresets.json

@@ -0,0 +1,459 @@
+{
+    "version": 3,
+    "configurePresets": [
+        {
+            "name": "base",
+            "hidden": true,
+            "binaryDir": "${sourceDir}/build/${presetName}",
+            "installDir": "${sourceDir}/install/${presetName}",
+            "generator": "Ninja",
+            "cacheVariables": {
+                "GSL_CXX_STANDARD": "14",
+                "GSL_TEST": "ON"
+            }
+        },
+        {
+            "name": "msvc-base",
+            "inherits": "base",
+            "hidden": true,
+            "condition": {
+                "type": "equals",
+                "lhs": "${hostSystemName}",
+                "rhs": "Windows"
+            },
+            "cacheVariables": {
+                "CMAKE_CXX_COMPILER": "cl"
+            }
+        },
+        {
+            "name": "gcc-base",
+            "inherits": "base",
+            "hidden": true,
+            "cacheVariables": {
+                "CMAKE_CXX_COMPILER": "g++",
+                "CMAKE_C_COMPILER": "gcc"
+            }
+        },
+        {
+            "name": "clang-base",
+            "inherits": "base",
+            "hidden": true,
+            "cacheVariables": {
+                "CMAKE_CXX_COMPILER": "clang++",
+                "CMAKE_C_COMPILER": "clang"
+            }
+        },
+        {
+            "name": "msvc-14-debug",
+            "displayName": "MSVC C++14 Debug",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "14"
+            }
+        },
+        {
+            "name": "msvc-14-release",
+            "displayName": "MSVC C++14 Release",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "14"
+            }
+        },
+        {
+            "name": "msvc-17-debug",
+            "displayName": "MSVC C++17 Debug",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "17"
+            }
+        },
+        {
+            "name": "msvc-17-release",
+            "displayName": "MSVC C++17 Release",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "17"
+            }
+        },
+        {
+            "name": "msvc-20-debug",
+            "displayName": "MSVC C++20 Debug",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "20"
+            }
+        },
+        {
+            "name": "msvc-20-release",
+            "displayName": "MSVC C++20 Release",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "20"
+            }
+        },
+        {
+            "name": "msvc-23-debug",
+            "displayName": "MSVC C++23 Debug",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "23"
+            }
+        },
+        {
+            "name": "msvc-23-release",
+            "displayName": "MSVC C++23 Release",
+            "inherits": "msvc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "23"
+            }
+        },
+        {
+            "name": "gcc-14-debug",
+            "displayName": "GCC C++14 Debug",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "14"
+            }
+        },
+        {
+            "name": "gcc-14-release",
+            "displayName": "GCC C++14 Release",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "14"
+            }
+        },
+        {
+            "name": "gcc-17-debug",
+            "displayName": "GCC C++17 Debug",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "17"
+            }
+        },
+        {
+            "name": "gcc-17-release",
+            "displayName": "GCC C++17 Release",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "17"
+            }
+        },
+        {
+            "name": "gcc-20-debug",
+            "displayName": "GCC C++20 Debug",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "20"
+            }
+        },
+        {
+            "name": "gcc-20-release",
+            "displayName": "GCC C++20 Release",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "20"
+            }
+        },
+        {
+            "name": "gcc-23-debug",
+            "displayName": "GCC C++23 Debug",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "23"
+            }
+        },
+        {
+            "name": "gcc-23-release",
+            "displayName": "GCC C++23 Release",
+            "inherits": "gcc-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "23"
+            }
+        },
+        {
+            "name": "clang-14-debug",
+            "displayName": "Clang C++14 Debug",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "14"
+            }
+        },
+        {
+            "name": "clang-14-release",
+            "displayName": "Clang C++14 Release",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "14"
+            }
+        },
+        {
+            "name": "clang-17-debug",
+            "displayName": "Clang C++17 Debug",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "17"
+            }
+        },
+        {
+            "name": "clang-17-release",
+            "displayName": "Clang C++17 Release",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "17"
+            }
+        },
+        {
+            "name": "clang-20-debug",
+            "displayName": "Clang C++20 Debug",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "20"
+            }
+        },
+        {
+            "name": "clang-20-release",
+            "displayName": "Clang C++20 Release",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "20"
+            }
+        },
+        {
+            "name": "clang-23-debug",
+            "displayName": "Clang C++23 Debug",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Debug",
+                "GSL_CXX_STANDARD": "23"
+            }
+        },
+        {
+            "name": "clang-23-release",
+            "displayName": "Clang C++23 Release",
+            "inherits": "clang-base",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "GSL_CXX_STANDARD": "23"
+            }
+        }
+    ],
+    "buildPresets": [
+        {
+            "name": "msvc-14-debug",
+            "configurePreset": "msvc-14-debug"
+        },
+        {
+            "name": "msvc-14-release",
+            "configurePreset": "msvc-14-release"
+        },
+        {
+            "name": "msvc-17-debug",
+            "configurePreset": "msvc-17-debug"
+        },
+        {
+            "name": "msvc-17-release",
+            "configurePreset": "msvc-17-release"
+        },
+        {
+            "name": "msvc-20-debug",
+            "configurePreset": "msvc-20-debug"
+        },
+        {
+            "name": "msvc-20-release",
+            "configurePreset": "msvc-20-release"
+        },
+        {
+            "name": "msvc-23-debug",
+            "configurePreset": "msvc-23-debug"
+        },
+        {
+            "name": "msvc-23-release",
+            "configurePreset": "msvc-23-release"
+        },
+        {
+            "name": "gcc-14-debug",
+            "configurePreset": "gcc-14-debug"
+        },
+        {
+            "name": "gcc-14-release",
+            "configurePreset": "gcc-14-release"
+        },
+        {
+            "name": "gcc-17-debug",
+            "configurePreset": "gcc-17-debug"
+        },
+        {
+            "name": "gcc-17-release",
+            "configurePreset": "gcc-17-release"
+        },
+        {
+            "name": "gcc-20-debug",
+            "configurePreset": "gcc-20-debug"
+        },
+        {
+            "name": "gcc-20-release",
+            "configurePreset": "gcc-20-release"
+        },
+        {
+            "name": "gcc-23-debug",
+            "configurePreset": "gcc-23-debug"
+        },
+        {
+            "name": "gcc-23-release",
+            "configurePreset": "gcc-23-release"
+        },
+        {
+            "name": "clang-14-debug",
+            "configurePreset": "clang-14-debug"
+        },
+        {
+            "name": "clang-14-release",
+            "configurePreset": "clang-14-release"
+        },
+        {
+            "name": "clang-17-debug",
+            "configurePreset": "clang-17-debug"
+        },
+        {
+            "name": "clang-17-release",
+            "configurePreset": "clang-17-release"
+        },
+        {
+            "name": "clang-20-debug",
+            "configurePreset": "clang-20-debug"
+        },
+        {
+            "name": "clang-20-release",
+            "configurePreset": "clang-20-release"
+        },
+        {
+            "name": "clang-23-debug",
+            "configurePreset": "clang-23-debug"
+        },
+        {
+            "name": "clang-23-release",
+            "configurePreset": "clang-23-release"
+        }
+    ],
+    "testPresets": [
+        {
+            "name": "msvc-14-debug",
+            "configurePreset": "msvc-14-debug"
+        },
+        {
+            "name": "msvc-14-release",
+            "configurePreset": "msvc-14-release"
+        },
+        {
+            "name": "msvc-17-debug",
+            "configurePreset": "msvc-17-debug"
+        },
+        {
+            "name": "msvc-17-release",
+            "configurePreset": "msvc-17-release"
+        },
+        {
+            "name": "msvc-20-debug",
+            "configurePreset": "msvc-20-debug"
+        },
+        {
+            "name": "msvc-20-release",
+            "configurePreset": "msvc-20-release"
+        },
+        {
+            "name": "msvc-23-debug",
+            "configurePreset": "msvc-23-debug"
+        },
+        {
+            "name": "msvc-23-release",
+            "configurePreset": "msvc-23-release"
+        },
+        {
+            "name": "gcc-14-debug",
+            "configurePreset": "gcc-14-debug"
+        },
+        {
+            "name": "gcc-14-release",
+            "configurePreset": "gcc-14-release"
+        },
+        {
+            "name": "gcc-17-debug",
+            "configurePreset": "gcc-17-debug"
+        },
+        {
+            "name": "gcc-17-release",
+            "configurePreset": "gcc-17-release"
+        },
+        {
+            "name": "gcc-20-debug",
+            "configurePreset": "gcc-20-debug"
+        },
+        {
+            "name": "gcc-20-release",
+            "configurePreset": "gcc-20-release"
+        },
+        {
+            "name": "gcc-23-debug",
+            "configurePreset": "gcc-23-debug"
+        },
+        {
+            "name": "gcc-23-release",
+            "configurePreset": "gcc-23-release"
+        },
+        {
+            "name": "clang-14-debug",
+            "configurePreset": "clang-14-debug"
+        },
+        {
+            "name": "clang-14-release",
+            "configurePreset": "clang-14-release"
+        },
+        {
+            "name": "clang-17-debug",
+            "configurePreset": "clang-17-debug"
+        },
+        {
+            "name": "clang-17-release",
+            "configurePreset": "clang-17-release"
+        },
+        {
+            "name": "clang-20-debug",
+            "configurePreset": "clang-20-debug"
+        },
+        {
+            "name": "clang-20-release",
+            "configurePreset": "clang-20-release"
+        },
+        {
+            "name": "clang-23-debug",
+            "configurePreset": "clang-23-debug"
+        },
+        {
+            "name": "clang-23-release",
+            "configurePreset": "clang-23-release"
+        }
+    ]
+}

CMakeSettings.json

@@ -1,18 +0,0 @@
-{
-  "configurations": [
-    {
-      "name": "x64-Debug",
-      "generator": "Ninja",
-      "configurationType": "Debug",
-      "inheritEnvironments": [
-        "msvc_x64_x64"
-      ],
-      "buildRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\build\\${name}",
-      "installRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\install\\${name}",
-      "cmakeCommandArgs": "-DGSL_CXX_STANDARD=17",
-      "buildCommandArgs": "-v",
-      "ctestCommandArgs": "",
-      "codeAnalysisRuleset": "CppCoreCheckRules.ruleset"
-    }
-  ]
-}

README.md

@@ -1,5 +1,6 @@
 # GSL: Guidelines Support Library
-[![Build Status](https://dev.azure.com/cppstat/GSL/_apis/build/status/microsoft.GSL?branchName=main)](https://dev.azure.com/cppstat/GSL/_build/latest?definitionId=1&branchName=main)
+[![CI](https://github.com/Microsoft/GSL/actions/workflows/compilers.yml/badge.svg)](https://github.com/microsoft/GSL/actions/workflows/compilers.yml?query=branch%3Amain)
+[![vcpkg](https://img.shields.io/vcpkg/v/ms-gsl)](https://vcpkg.io/en/package/ms-gsl)
 
 The Guidelines Support Library (GSL) contains functions and types that are suggested for use by the
 [C++ Core Guidelines](https://github.com/isocpp/CppCoreGuidelines) maintained by the [Standard C++ Foundation](https://isocpp.org).
@@ -39,22 +40,18 @@ span_p                                                                   | &#x26
 [u32zstring](docs/headers.md#user-content-H-zstring)                     | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `char32_t`
 [cu32zstring](docs/headers.md#user-content-H-zstring)                    | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char32_t`
 [**2. Owners**][cg-owners]                                               |            |
-[unique_ptr](docs/headers.md#user-content-H-pointers-unique_ptr)         | ☑   | An alias to `std::unique_ptr`
-[shared_ptr](docs/headers.md#user-content-H-pointers-shared_ptr)         | ☑   | An alias to `std::shared_ptr`
 stack_array                                                              | ☐   | A stack-allocated array
-dyn_array                                                                | ☐   | A heap-allocated array
+dyn_array                                                                | ☑   | A heap-allocated array
 [**3. Assertions**][cg-assertions]                                       |            |
 [Expects](docs/headers.md#user-content-H-assert-expects)                 | ☑   | A precondition assertion; on failure it terminates
 [Ensures](docs/headers.md#user-content-H-assert-ensures)                 | ☑   | A postcondition assertion; on failure it terminates
 [**4. Utilities**][cg-utilities]                                         |            |
 move_owner                                                               | ☐   | A helper function that moves one `owner` to the other
-[byte](docs/headers.md#user-content-H-byte-byte)                         | ☑   | Either an alias to `std::byte` or a byte type
 [final_action](docs/headers.md#user-content-H-util-final_action)         | ☑   | A RAII style class that invokes a functor on its destruction
 [finally](docs/headers.md#user-content-H-util-finally)                   | ☑   | A helper function instantiating [final_action](docs/headers.md#user-content-H-util-final_action)
-[GSL_SUPPRESS](docs/headers.md#user-content-H-assert-gsl_suppress)       | ☑   | A macro that takes an argument and turns it into `[[gsl::suppress(x)]]` or `[[gsl::suppress("x")]]`
+[GSL_SUPPRESS](docs/headers.md#user-content-H-assert-gsl_suppress)       | ☑   | A macro that takes an argument and turns it into `[[gsl::suppress(x)]]` or `[[gsl::suppress("x")]]` depending on the compiler.
 [[implicit]]                                                             | ☐   | A "marker" to put on single-argument constructors to explicitly make them non-explicit
 [index](docs/headers.md#user-content-H-util-index)                       | ☑   | A type to use for all container and array indexing (currently an alias for `std::ptrdiff_t`)
-joining_thread                                                           | ☐   | A RAII style version of `std::thread` that joins
 [narrow](docs/headers.md#user-content-H-narrow-narrow)                   | ☑   | A checked version of `narrow_cast`; it can throw [narrowing_error](docs/headers.md#user-content-H-narrow-narrowing_error)
 [narrow_cast](docs/headers.md#user-content-H-util-narrow_cast)           | ☑   | A narrowing cast for values and a synonym for `static_cast`
 [narrowing_error](docs/headers.md#user-content-H-narrow-narrowing_error) | ☑   | A custom exception type thrown by [narrow](docs/headers.md#user-content-H-narrow-narrow)
@@ -76,6 +73,14 @@ cu16string_span                    | ☐   | Deprecated. An alias to `basic
 u32string_span                     | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `char32_t`
 cu32string_span                    | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `const char32_t`
 
+## The following features have been adopted by WG21. They are deprecated in GSL.
+Feature                                                           | Deprecated Since | Notes
+------------------------------------------------------------------|------------------|------
+[unique_ptr](docs/headers.md#user-content-H-pointers-unique_ptr)  | C++11            | Use std::unique_ptr instead.
+[shared_ptr](docs/headers.md#user-content-H-pointers-shared_ptr)  | C++11            | Use std::shared_ptr instead.
+[byte](docs/headers.md#user-content-H-byte-byte)                  | C++17            | Use std::byte instead.
+joining_thread                                                    | C++20 (Note: Not yet implemented in GSL) | Use std::jthread instead.
+
 This is based on [CppCoreGuidelines semi-specification](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gsl-guidelines-support-library).
 
 [cg-views]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gslview-views
@@ -92,9 +97,9 @@ Below is a table showing the versions currently being tested (also see [.github/
 
 Compiler |Toolset Versions Currently Tested
 :------- |--:
- GCC | 10, 11, 12
+ GCC | 12, 13, 14
  XCode | 14.3.1, 15.4
- Clang | 13, 14, 15
+ Clang | 16, 17, 18
  Visual Studio with MSVC | VS2019, VS2022 
  Visual Studio with LLVM | VS2019, VS2022
 
@@ -106,8 +111,8 @@ If you successfully port GSL to another platform, we would love to hear from you
 
 Target | CI/CD Status
 :------- | -----------:
-iOS | ![CI_iOS](https://github.com/microsoft/GSL/workflows/CI_iOS/badge.svg)
-Android | ![CI_Android](https://github.com/microsoft/GSL/workflows/CI_Android/badge.svg)
+iOS | [![CI_iOS](https://github.com/microsoft/GSL/workflows/CI_iOS/badge.svg?branch=main)](https://github.com/microsoft/GSL/actions/workflows/ios.yml?query=branch%3Amain)
+Android | [![CI_Android](https://github.com/microsoft/GSL/workflows/CI_Android/badge.svg?branch=main)](https://github.com/microsoft/GSL/actions/workflows/android.yml?query=branch%3Amain)
 
 Note: These CI/CD steps are run with each pull request, however failures in them are non-blocking.
 
@@ -197,7 +202,7 @@ include(FetchContent)
 
 FetchContent_Declare(GSL
     GIT_REPOSITORY "https://github.com/microsoft/GSL"
-    GIT_TAG "v4.1.0"
+    GIT_TAG "v4.2.0"
     GIT_SHALLOW ON
 )
 

docs/headers.md

@@ -2,13 +2,14 @@ The Guidelines Support Library (GSL) interface is very lightweight and exposed v
 
 Types and functions are exported in the namespace `gsl`.
 
-See [GSL: Guidelines support library](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#S-gsl)
+See [GSL: Guidelines support library](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#s-gsl)
 
 # <a name="H" />Headers
 
 - [`<algorithms>`](#user-content-H-algorithms)
 - [`<assert>`](#user-content-H-assert)
 - [`<byte>`](#user-content-H-byte)
+- [`<dyn_array>`](#user-content-H-dyn_array)
 - [`<gsl>`](#user-content-H-gsl)
 - [`<narrow>`](#user-content-H-narrow)
 - [`<pointers>`](#user-content-H-pointers)
@@ -38,7 +39,7 @@ that the destination `span` is at least as large as the source `span`.
 
 This header contains some macros used for contract checking and suppressing code analysis warnings.
 
-See [GSL.assert: Assertions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-assertions)
+See [GSL.assert: Assertions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ss-assertions)
 
 - [`GSL_SUPPRESS`](#user-content-H-assert-gsl_suppress)
 - [`Expects`](#user-content-H-assert-expects)
@@ -49,9 +50,9 @@ See [GSL.assert: Assertions](https://isocpp.github.io/CppCoreGuidelines/CppCoreG
 This macro can be used to suppress a code analysis warning.
 
 The core guidelines request tools that check for the rules to respect suppressing a rule by writing
-`[[gsl::suppress(tag)]]` or `[[gsl::suppress(tag, justification: "message")]]`.
+`[[gsl::suppress("tag")]]` or `[[gsl::suppress("tag", justification: "message")]]`.
 
-Clang does not use exactly that syntax, but requires `tag` to be put in double quotes `[[gsl::suppress("tag")]]`.
+Older versions of MSVC (VS 2022 and earlier) only understand `[[gsl::suppress(tag)]]` without the double quotes around `tag`.
 
 For portable code you can use `GSL_SUPPRESS(tag)`.
 
@@ -86,21 +87,21 @@ If `GSL_USE_STD_BYTE` is not defined, then the header file will check if `std::b
 If you do so, you might want to `#define GSL_USE_STD_BYTE 0` to a fixed value to be sure that both projects use exactly
 the same type. Otherwise you might get linker errors.
 
-See [SL.str.5: Use `std::byte` to refer to byte values that do not necessarily represent characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rstr-byte)
+See [SL.str.5: Use `std::byte` to refer to byte values that do not necessarily represent characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#rstr-byte)
 
 ### Non-member functions
 
 ```cpp
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator<<=(byte& b, IntegerType shift) noexcept;
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator<<(byte b, IntegerType shift) noexcept;
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator>>=(byte& b, IntegerType shift) noexcept;
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator>>(byte b, IntegerType shift) noexcept;
 ```
 
@@ -134,7 +135,7 @@ constexpr byte operator~(byte b) noexcept;
 Bitwise negation of a `byte`. Flips all bits. Zeroes become ones, ones become zeroes.
 
 ```cpp
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr IntegerType to_integer(byte b) noexcept;
 ```
 
@@ -155,6 +156,178 @@ constexpr byte to_byte() noexcept;
 
 Convert the given value `I` to a `byte`. The template requires `I` to be in the valid range 0..255 for a `gsl::byte`.
 
+## <a name="H-dyn_array" />`<dyn_array>`
+
+This header contains an owning dynamically allocated array type whose size is fixed between assignments.
+
+- [`gsl::dyn_array`](#user-content-H-dyn_array-dyn_array)
+
+### <a name="H-dyn_array-dyn_array" />`gsl::dyn_array`
+
+```cpp
+template <typename T, typename Allocator = std::allocator<T>>
+class dyn_array;
+```
+
+`gsl::dyn_array` owns a contiguous sequence of `T` objects allocated with `Allocator`.
+The number of elements is established when the object is constructed and remains unchanged until the object is copy-assigned.
+It provides bounds-checked element access and checked random-access iterators.
+
+`gsl::dyn_array` is useful when the number of elements is known only at runtime, but the array should not grow or shrink through container operations.
+
+#### Member Types
+
+```cpp
+using value_type = T;
+using reference = T&;
+using const_reference = const T&;
+using iterator = details::dyn_array_iterator<T>;
+using const_iterator = details::dyn_array_iterator<const T>;
+using reverse_iterator = std::reverse_iterator<iterator>;
+using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+using difference_type = std::ptrdiff_t;
+using size_type = std::size_t;
+
+using allocator_type = Allocator;
+```
+
+#### Member functions
+
+##### Construct/Copy
+
+```cpp
+explicit constexpr dyn_array(const Allocator& alloc = {});
+```
+
+Constructs an empty `dyn_array`.
+No elements are allocated and `data()` returns `nullptr`.
+
+```cpp
+constexpr explicit dyn_array(size_type count, const Allocator& alloc = {});
+
+constexpr dyn_array(size_type count, const T& value, const Allocator& alloc = {});
+```
+
+Constructs a `dyn_array` with `count` elements using `alloc`.
+The first overload default-constructs each element.
+The second overload constructs each element as a copy of `value`.
+
+```cpp
+template <typename InputIt>
+constexpr dyn_array(InputIt first, InputIt last, const Allocator& alloc = {});
+```
+
+Constructs a `dyn_array` by copying the elements in the range `[first, last)`.
+
+```cpp
+template <std::ranges::input_range InputRg>
+constexpr dyn_array(std::from_range_t, InputRg&& rg, const Allocator& alloc = {});
+```
+
+Constructs a `dyn_array` by copying the elements in `rg`.
+This overload is available when container ranges are supported.
+
+```cpp
+constexpr dyn_array(const dyn_array& other, const Allocator& alloc = {});
+
+constexpr dyn_array(std::initializer_list<T> init, const Allocator& alloc = {});
+```
+
+Constructs a `dyn_array` by copying the elements from another `dyn_array` or from an initializer list.
+
+```cpp
+constexpr auto operator=(const dyn_array& other) -> dyn_array&;
+
+constexpr dyn_array(dyn_array&&) = delete;
+dyn_array& operator=(dyn_array&&) = delete;
+```
+
+Copy assignment replaces the contents with copies of the elements in `other`.
+Move construction and move assignment are explicitly deleted.
+
+##### Observers
+
+```cpp
+constexpr auto size() const;
+constexpr auto empty() const;
+constexpr auto max_size() const;
+constexpr auto get_allocator() -> Allocator&;
+```
+
+Returns the number of elements, whether the array is empty, the maximum representable size, or the allocator used by the `dyn_array`.
+
+##### Element access
+
+```cpp
+constexpr auto operator[](size_type pos) -> reference;
+constexpr auto operator[](size_type pos) const -> const_reference;
+```
+
+Returns a reference to the element at the given index.
+[`Expects`](#user-content-H-assert-expects) that `pos` is less than the `dyn_array`'s size.
+
+```cpp
+constexpr auto data();
+constexpr auto data() const -> const T*;
+```
+
+Returns a pointer to the beginning of the contained data.
+If the `dyn_array` is empty, this returns `nullptr`.
+
+##### Iterators
+
+```cpp
+constexpr auto begin();
+constexpr auto begin() const;
+constexpr auto cbegin() const;
+
+constexpr auto end();
+constexpr auto end() const;
+constexpr auto cend() const;
+```
+
+Returns an iterator to the first element or to one past the last element.
+
+```cpp
+constexpr auto rbegin();
+constexpr auto rbegin() const;
+constexpr auto crbegin() const;
+
+constexpr auto rend();
+constexpr auto rend() const;
+constexpr auto crend() const;
+```
+
+Returns a reverse iterator to the first element of the reversed range or to one past the last element of the reversed range.
+
+The iterators are random-access iterators and perform bounds checking.
+Dereferencing `end()`, moving before `begin()` or past `end()`, or comparing iterators from different arrays violates preconditions.
+
+##### Comparisons
+
+```cpp
+constexpr auto operator==(const dyn_array& other) const;
+constexpr auto operator!=(const dyn_array& other) const;
+```
+
+Compares two `dyn_array`s by size and element value.
+
+#### Deduction guides
+
+```cpp
+template <class InputIt,
+          class Alloc = std::allocator<typename std::iterator_traits<InputIt>::value_type>>
+dyn_array(InputIt, InputIt,
+          Alloc = {}) -> dyn_array<typename std::iterator_traits<InputIt>::value_type, Alloc>;
+
+template <std::ranges::input_range InputRg,
+          class Alloc = std::allocator<std::ranges::range_value_t<InputRg>>>
+dyn_array(std::from_range_t, InputRg&&,
+          Alloc = {}) -> dyn_array<std::ranges::range_value_t<InputRg>, Alloc>;
+```
+
+The range deduction guide is available when container ranges are supported.
+
 ## <a name="H-gsl" />`<gsl>`
 
 This header is a convenience header that includes all other [GSL headers](#user-content-H).
@@ -164,7 +337,7 @@ Since `<narrow>` requires exceptions, it will only be included if exceptions are
 
 This header contains utility functions and classes, for narrowing casts, which require exceptions. The narrowing-related utilities that don't require exceptions are found inside [util](#user-content-H-util).
 
-See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-utilities)
+See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ss-utilities)
 
 - [`gsl::narrowing_error`](#user-content-H-narrow-narrowing_error)
 - [`gsl::narrow`](#user-content-H-narrow-narrow)
@@ -180,13 +353,13 @@ If the argument `x` cannot be represented in the target type `T`, then the funct
 
 Note: compare [`gsl::narrow_cast`](#user-content-H-util-narrow_cast) in header [util](#user-content-H-util).
 
-See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named)
+See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#res-casts-named)
 
 ## <a name="H-pointers" />`<pointers>`
 
 This header contains some pointer types.
 
-See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-views)
+See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ss-views)
 
 - [`gsl::unique_ptr`](#user-content-H-pointers-unique_ptr)
 - [`gsl::shared_ptr`](#user-content-H-pointers-shared_ptr)
@@ -198,13 +371,13 @@ See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-v
 
 `gsl::unique_ptr` is an alias to `std::unique_ptr`.
 
-See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-ownership)
+See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ss-ownership)
 
 ### <a name="H-pointers-shared_ptr" />`gsl::shared_ptr`
 
 `gsl::shared_ptr` is an alias to `std::shared_ptr`.
 
-See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-ownership)
+See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ss-ownership)
 
 ### <a name="H-pointers-owner" />`gsl::owner`
 
@@ -213,7 +386,7 @@ See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/C
 
 A `gsl::owner<T>` is a typedef to `T`. It adds no runtime overhead whatsoever, as it is purely syntactic and does not add any runtime checks.  Instead, it serves as an annotation for static analysis tools which check for memory safety, and as a code comprehension guide for human readers.
 
-See Enforcement section of [C.31: All resources acquired by a class must be released by the class’s destructor](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-dtor-release).
+See Enforcement section of [C.31: All resources acquired by a class must be released by the class’s destructor](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#rc-dtor-release).
 
 ### <a name="H-pointers-not_null" />`gsl::not_null`
 
@@ -222,7 +395,15 @@ See Enforcement section of [C.31: All resources acquired by a class must be rele
 The checks for ensuring that the pointer is not null are done in the constructor. There is no overhead when retrieving or dereferencing the checked pointer.
 When a nullptr check fails, `std::terminate` is called.
 
-See [F.23: Use a `not_null<T>` to indicate that “null” is not a valid value](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-nullptr)
+See [F.23: Use a `not_null<T>` to indicate that “null” is not a valid value](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#rf-nullptr)
+
+#### Member Types
+
+```cpp
+using element_type = T;
+```
+
+The type of the pointer or smart pointer that is managed by this object.
 
 #### Member functions
 
@@ -270,7 +451,7 @@ not_null& operator+=(std::ptrdiff_t) = delete;
 not_null& operator-=(std::ptrdiff_t) = delete;
 ```
 
-Explicitly deleted operators. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow these operators.
+Explicitly deleted operators. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ri-array)), so don't allow these operators.
 
 ##### Observers
 
@@ -292,7 +473,13 @@ Dereference the underlying pointer.
 void operator[](std::ptrdiff_t) const = delete;
 ```
 
-Array index operator is explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow treating them as an array.
+Array index operator is explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ri-array)), so don't allow treating them as an array.
+
+```cpp
+void swap(not_null<T>& other) { std::swap(ptr_, other.ptr_); }
+```
+
+Swaps contents with another `gsl::not_null` object.
 
 #### Non-member functions
 
@@ -303,6 +490,13 @@ auto make_not_null(T&& t) noexcept;
 
 Creates a `gsl::not_null` object, deducing the target type from the type of the argument.
 
+```cpp
+template <typename T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value, bool> = true>
+void swap(not_null<T>& a, not_null<T>& b);
+```
+
+Swaps the contents of two `gsl::not_null` objects.
+
 ```cpp
 template <class T, class U>
 auto operator==(const not_null<T>& lhs,
@@ -354,7 +548,7 @@ template <class T>
 not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 ```
 
-Addition and subtraction are explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow these operators.
+Addition and subtraction are explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ri-array)), so don't allow these operators.
 
 ##### STL integration
 
@@ -391,22 +585,36 @@ The `gsl::span` is based on the standardized version of `std::span` which was ad
 deprecate `gsl::span` when `std::span` finished standardization, however that plan changed when the runtime bounds checking
 was removed from `std::span`'s design.
 
-The only difference between `gsl::span` and `std::span` is that `gsl::span` strictly enforces runtime bounds checking.
-Any violations of the bounds check results in termination of the program.
-Like `gsl::span`, `gsl::span`'s iterators also differ from `std::span`'s iterator in that all access operations are bounds checked.
+The key differences between `gsl::span` and `std::span` are:
+- `gsl::span` strictly enforces runtime bounds checking for all access operations
+- Any violations of the bounds check results in termination of the program
+- `gsl::span`'s iterators also perform bounds checking, unlike `std::span`'s iterators
 
 #### Which version of span should I use?
 
-##### Use `gsl::span` if
+The following table compares the different span implementations to help you choose which one is best for your project:
 
-- you want to guarantee bounds safety in your project.
-  - All data accessing operations use bounds checking to ensure you are only accessing valid memory.
-- your project uses C++14 or C++17.
-  - `std::span` is not available as it was not introduced into the STL until C++20.
+| Feature/Version | `std::span` (C++20/23) | Hardened `std::span` (C++26) | `gsl::span` |
+|-----------------|------------------------|------------------------------|-------------|
+| **C++ Standard** | Requires C++20 or later | Requires C++26 or backported implementation | Works with C++14 or later |
+| **Element Access** | No bounds checking | Bounds checking | Bounds checking |
+| **Iterator Safety** | No bounds checking | Implementation-defined, may depend on vendor | Full bounds checking |
+| **Error Behavior** | Undefined behavior on invalid access | Implementation-defined, may be configurable | Always calls [`std::terminate()`](https://en.cppreference.com/w/cpp/error/terminate) via [gsl::details::terminate()](https://github.com/microsoft/GSL/blob/main/include/gsl/assert#L111-L118) |
+| **Performance** | Fastest (no checking) | Varies by implementation and configuration | May have performance impact from bounds checking |
 
-##### Use `std::span` if
+**Recommendations:**
 
-- your project is C++20 and you need the performance offered by `std::span`.
+- **C++14 & C++17 projects**: Use `gsl::span` as `std::span` is not available.
+- **C++20 & C++23 projects**: 
+  - Use `gsl::span` if safety is your priority.
+  - Use `std::span` if performance is critical and you're confident in your index calculations.
+- **C++26 projects**: 
+  - Use `gsl::span` if you need guaranteed iterator safety across all platforms.
+  - Use hardened `std::span` if you want standard library compliance and acceptable safety.
+
+**Implementation notes for hardened `std::span` in C++26:**
+- For MSVC: See [Microsoft STL Hardening](https://github.com/microsoft/STL/wiki/STL-Hardening)
+- For Clang/LLVM: See [libc++ Hardening](https://libcxx.llvm.org/Hardening.html)
 
 #### Types
 
@@ -663,10 +871,6 @@ template <class Container>
 constexpr span<typename Container::value_type> make_span(Container& cont);
 template <class Container>
 constexpr span<const typename Container::value_type> make_span(const Container& cont);
-template <class Ptr>
-constexpr span<typename Ptr::element_type> make_span(Ptr& cont, std::size_t count);
-template <class Ptr>
-constexpr span<typename Ptr::element_type> make_span(Ptr& cont);
 ```
 
 Utility function for creating a `span` with [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent) from
@@ -753,14 +957,14 @@ that a pointer points to a zero terminated C style string. This helps static cod
 `u32zstring` is a zero terminated `char32_t` string.  
 `cu32zstring` is a const zero terminated `char32_t` string.  
 
-See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-views) and [SL.str.3: Use zstring or czstring to refer to a C-style, zero-terminated, sequence of characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rstr-zstring).
+See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ss-views) and [SL.str.3: Use zstring or czstring to refer to a C-style, zero-terminated, sequence of characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#rstr-zstring).
 
 ## <a name="H-util" />`<util>`
 
 This header contains utility functions and classes. This header works without exceptions being available. The parts that require
 exceptions being available are in their own header file [narrow](#user-content-H-narrow).
 
-See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-utilities)
+See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#ss-utilities)
 
 - [`gsl::narrow_cast`](#user-content-H-util-narrow_cast)
 - [`gsl::final_action`](#user-content-H-util-final_action)
@@ -776,7 +980,7 @@ An alias to `std::ptrdiff_t`. It serves as the index type for all container inde
 
 Note: compare the throwing version [`gsl::narrow`](#user-content-H-narrow-narrow) in header [narrow](#user-content-H-narrow).
 
-See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named)
+See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#res-casts-named)
 
 ### <a name="H-util-final_action" />`gsl::final_action`
 
@@ -787,7 +991,7 @@ class final_action { ... };
 
 `final_action` allows you to ensure something gets run at the end of a scope.
 
-See [E.19: Use a final_action object to express cleanup if no suitable resource handle is available](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Re-finally)
+See [E.19: Use a final_action object to express cleanup if no suitable resource handle is available](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#re-finally)
 
 #### Member functions
 
@@ -827,7 +1031,7 @@ The function `gsl::at` offers a safe way to access data with index bounds checki
 
 Note: `gsl::at` supports indexes up to `PTRDIFF_MAX`.
 
-See [ES.42: Keep use of pointers simple and straightforward](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-ptr)
+See [ES.42: Keep use of pointers simple and straightforward](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#res-ptr)
 
 ```cpp
 template <class T, std::size_t N>
@@ -858,3 +1062,10 @@ constexpr auto at(std::span<T, extent> sp, const index i) -> decltype(sp[sp.size
 This overload returns a reference to the `i`s element of the `std::span` `sp`. It [`Expects`](#user-content-H-assert-expects) that the provided index is within the bounds of the array.
 
 For [`gsl::at`](#user-content-H-span_ext-at) for [`gsl::span`](#user-content-H-span-span) see header [`span_ext`](#user-content-H-span_ext).
+
+```cpp
+template <class T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value>>
+void swap(T& a, T& b);
+```
+
+Swaps the contents of two objects. Exists only to specialize `gsl::swap<T>(gsl::not_null<T>&, gsl::not_null<T>&)`.

docs/upgrade_checklist.md

@@ -0,0 +1,19 @@
+> When bumping the version, you need to update the following files:
+
+1. [ ] [CMakeLists.txt](../CMakeLists.txt) Bump `GSL_VERSION`
+1. [ ] [README.md](../README.md) Bump `GIT_TAG`
+1. [ ] [ios.yml](../.github/workflows/ios.yml) Bump `MACOSX_BUNDLE_BUNDLE_VERSION` and
+`MACOSX_BUNDLE_SHORT_VERSION_STRING`
+
+> After updating, you need to create a new GitHub release:
+
+1. [ ] [Microsoft/GSL - Create Release](https://github.com/microsoft/GSL/releases/new)
+
+Be sure to update the release notes accordingly and properly mention open-source
+contributors.
+
+> After a new release exists, update the `ms-gsl` vcpkg port:
+
+1. [ ] [Microsoft/vcpkg - ms-gsl port](https://github.com/microsoft/vcpkg/tree/master/ports/ms-gsl)
+
+Be sure to monitor the PR that updates the port for any feedback from vcpkg maintainers.

include/gsl/algorithm

@@ -17,8 +17,8 @@
 #ifndef GSL_ALGORITHM_H
 #define GSL_ALGORITHM_H
 
-#include "assert" // for Expects
-#include "span"   // for dynamic_extent, span
+#include "./assert" // for Expects
+#include "./span"   // for dynamic_extent, span
 
 #include <algorithm>   // for copy_n
 #include <cstddef>     // for ptrdiff_t
@@ -48,9 +48,7 @@ void copy(span<SrcElementType, SrcExtent> src, span<DestElementType, DestExtent>
                   "Source range is longer than target range");
 
     Expects(dest.size() >= src.size());
-    // clang-format off
-    GSL_SUPPRESS(stl.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(stl.1)
     std::copy_n(src.data(), src.size(), dest.data());
 }
 

include/gsl/assert

@@ -47,13 +47,14 @@
 //
 #if defined(__clang__)
 #define GSL_SUPPRESS(x) [[gsl::suppress(#x)]]
-#else
-#if defined(_MSC_VER) && !defined(__INTEL_COMPILER) && !defined(__NVCC__)
+#elif defined(_MSC_VER) && _MSC_VER >= 1950
+// Visual Studio versions after 2022 (_MSC_VER > 1944) support the justification message.
+#define GSL_SUPPRESS(x) [[gsl::suppress(#x)]]
+#elif defined(_MSC_VER) && !defined(__INTEL_COMPILER) && !defined(__NVCC__)
 #define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
 #else
 #define GSL_SUPPRESS(x)
-#endif // _MSC_VER
-#endif // __clang__
+#endif // defined(__clang__)
 
 #if defined(__clang__) || defined(__GNUC__)
 #define GSL_LIKELY(x) __builtin_expect(!!(x), 1)
@@ -92,9 +93,7 @@ namespace details
 
     typedef void(__cdecl* terminate_handler)();
 
-    // clang-format off
-    GSL_SUPPRESS(f.6) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(f.6)
     [[noreturn]] inline void __cdecl default_terminate_handler()
     {
         __fastfail(RANGE_CHECKS_FAILURE);

include/gsl/byte

@@ -17,6 +17,8 @@
 #ifndef GSL_BYTE_H
 #define GSL_BYTE_H
 
+#include "./util" // for GSL_DEPRECATED
+
 #include <type_traits>
 
 #ifdef _MSC_VER
@@ -80,7 +82,16 @@ namespace gsl
 {
 #if GSL_USE_STD_BYTE
 
-using std::byte;
+namespace impl
+{
+    // impl::byte is used by gsl::as_bytes so our own code does not trigger a deprecation warning as
+    // would be the case when we used gsl::byte. Users of GSL should only use gsl::byte, not
+    // gsl::impl::byte.
+    using byte = std::byte;
+} // namespace impl
+
+using byte GSL_DEPRECATED("Use std::byte instead.") = std::byte;
+
 using std::to_integer;
 
 #else // GSL_USE_STD_BYTE
@@ -91,25 +102,33 @@ enum class byte_may_alias byte : unsigned char
 {
 };
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+namespace impl
+{
+    // impl::byte is used by gsl::as_bytes so our own code does not trigger a deprecation warning as
+    // would be the case when we used gsl::byte. Users of GSL should only use gsl::byte, not
+    // gsl::impl::byte.
+    using byte = gsl::byte;
+} // namespace impl
+
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator<<=(byte& b, IntegerType shift) noexcept
 {
     return b = byte(static_cast<unsigned char>(b) << shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator<<(byte b, IntegerType shift) noexcept
 {
     return byte(static_cast<unsigned char>(b) << shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator>>=(byte& b, IntegerType shift) noexcept
 {
     return b = byte(static_cast<unsigned char>(b) >> shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator>>(byte b, IntegerType shift) noexcept
 {
     return byte(static_cast<unsigned char>(b) >> shift);
@@ -147,7 +166,7 @@ constexpr byte operator^(byte l, byte r) noexcept
 
 constexpr byte operator~(byte b) noexcept { return byte(~static_cast<unsigned char>(b)); }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr IntegerType to_integer(byte b) noexcept
 {
     return static_cast<IntegerType>(b);
@@ -156,22 +175,21 @@ constexpr IntegerType to_integer(byte b) noexcept
 #endif // GSL_USE_STD_BYTE
 
 template <typename T>
-// NOTE: need suppression since c++14 does not allow "return {t}"
-// GSL_SUPPRESS(type.4) // NO-FORMAT: attribute // TODO: suppression does not work
-constexpr byte to_byte(T t) noexcept
+constexpr gsl::impl::byte to_byte(T t) noexcept
 {
-    static_assert(std::is_same<T, unsigned char>::value,
-                  "gsl::to_byte(t) must be provided an unsigned char, otherwise data loss may occur. "
-                  "If you are calling to_byte with an integer constant use: gsl::to_byte<t>() version.");
-    return byte(t);
+    static_assert(
+        std::is_same<T, unsigned char>::value,
+        "gsl::to_byte(t) must be provided an unsigned char, otherwise data loss may occur. "
+        "If you are calling to_byte with an integer constant use: gsl::to_byte<t>() version.");
+    return gsl::impl::byte(t);
 }
 
 template <int I>
-constexpr byte to_byte() noexcept
+constexpr gsl::impl::byte to_byte() noexcept
 {
     static_assert(I >= 0 && I <= 255,
                   "gsl::byte only has 8 bits of storage, values must be in range 0-255");
-    return static_cast<byte>(I);
+    return static_cast<gsl::impl::byte>(I);
 }
 
 } // namespace gsl

include/gsl/dyn_array

@@ -0,0 +1,439 @@
+// -*- C++ -*-
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2026 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef GSL_DYN_ARRAY_H
+#define GSL_DYN_ARRAY_H
+
+#include "./assert"
+#include "./narrow"
+#include "./util"
+
+#include <algorithm>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+
+#if defined(__cpp_lib_ranges) && (__cpp_lib_ranges >= 201911L)
+#include <ranges>
+#endif /* __cpp_lib_ranges >= 201911L */
+
+namespace gsl
+{
+namespace details
+{
+    template <typename T, typename Allocator = std::allocator<T>>
+    class dyn_array_base : public Allocator
+    {
+        using pointer = T*;
+        using size_type = std::size_t;
+
+        template <typename... Args>
+        GSL_CONSTEXPR_SINCE_CPP20 void construct(pointer ptr, Args&&... args)
+        {
+            std::allocator_traits<Allocator>::construct(static_cast<Allocator&>(*this), ptr,
+                                                        std::forward<Args>(args)...);
+        }
+
+        GSL_CONSTEXPR_SINCE_CPP20 void destroy(pointer ptr)
+        {
+            std::allocator_traits<Allocator>::destroy(static_cast<Allocator&>(*this), ptr);
+        }
+
+        GSL_CONSTEXPR_SINCE_CPP20 void destroy_range(pointer first, pointer last)
+        {
+            for (; first != last; ++first) { destroy(first); }
+        }
+
+        GSL_CONSTEXPR_SINCE_CPP20 void rollback_construction(pointer first, pointer last)
+        {
+            destroy_range(first, last);
+            std::allocator_traits<Allocator>::deallocate(static_cast<Allocator&>(*this), _data,
+                                                         _count);
+            _data = nullptr;
+            _count = 0;
+        }
+
+    protected:
+        constexpr auto data() const { return _data; }
+
+        constexpr auto count() const { return _count; }
+
+        GSL_CONSTEXPR_SINCE_CPP20 void resize(size_type count)
+        {
+            // This should only be called when constructing a non-forward iterator.
+            // It neither frees nor copies `_data`.
+            Expects(_data == nullptr && _count == 0);
+            if (count != 0)
+            {
+                _data = std::allocator_traits<Allocator>::allocate(static_cast<Allocator&>(*this),
+                                                                   count);
+                _count = count;
+            }
+        }
+
+        GSL_CONSTEXPR_SINCE_CPP20 void fill(pointer first, size_type count, const T& value)
+        {
+            pointer current = first;
+            try
+            {
+                for (size_type i = 0; i < count; ++i, ++current) { construct(current, value); }
+            } catch (...)
+            {
+                rollback_construction(first, current);
+                throw;
+            }
+        }
+
+        template <typename InputIt>
+        GSL_CONSTEXPR_SINCE_CPP20 void copy(InputIt first, InputIt last, pointer output)
+        {
+            pointer current = output;
+            try
+            {
+                for (; first != last; ++first, ++current) { construct(current, *first); }
+            } catch (...)
+            {
+                rollback_construction(output, current);
+                throw;
+            }
+        }
+
+        GSL_CONSTEXPR_SINCE_CPP20 void default_construct(pointer first, size_type count)
+        {
+            pointer current = first;
+            try
+            {
+                for (size_type i = 0; i < count; ++i, ++current) { construct(current); }
+            } catch (...)
+            {
+                rollback_construction(first, current);
+                throw;
+            }
+        }
+
+    private:
+        pointer _data;
+        size_type _count;
+
+    public:
+        constexpr dyn_array_base(const Allocator& alloc)
+            : Allocator{alloc}, _data{nullptr}, _count{0}
+        {
+            Ensures((_count == 0 && _data == nullptr) || (_count > 0 && _data != nullptr));
+        }
+
+        constexpr dyn_array_base(size_type count, const Allocator& alloc)
+            : Allocator{alloc}
+            , _data{count == 0 ? nullptr
+                               : std::allocator_traits<Allocator>::allocate(
+                                     static_cast<Allocator&>(*this), count)}
+            , _count{count}
+        {
+            Ensures((_count == 0 && _data == nullptr) || (_count > 0 && _data != nullptr));
+        }
+
+        GSL_CONSTEXPR_SINCE_CPP20 ~dyn_array_base()
+        {
+            if (_data)
+            {
+                if (!std::is_trivially_destructible<T>::value)
+                {
+                    destroy_range(_data, _data + _count);
+                }
+                std::allocator_traits<Allocator>::deallocate(static_cast<Allocator&>(*this), _data,
+                                                             _count);
+            }
+        }
+    };
+
+    template <typename T>
+    class dyn_array_iterator
+    {
+        using size_type = std::size_t;
+
+    public:
+        using difference_type = std::ptrdiff_t;
+        using value_type = T;
+        using pointer = T*;
+        using reference = T&;
+        using const_reference = const T&;
+        using iterator_category = std::random_access_iterator_tag;
+
+#if defined(__cpp_lib_ranges) && (__cpp_lib_ranges >= 201911L)
+        constexpr dyn_array_iterator() = default;
+#endif /* __cpp_lib_ranges >= 201911L */
+
+        constexpr dyn_array_iterator(pointer ptr, size_type pos, size_type end_pos)
+            : _ptr{ptr}, _pos{pos}, _end_pos{end_pos}
+        {
+            Ensures((_ptr != nullptr && _end_pos > 0) || (_ptr == nullptr && _end_pos == 0));
+            Ensures(_pos <= _end_pos);
+        }
+
+#if defined(_MSC_VER) && defined(__cpp_lib_ranges) && (__cpp_lib_ranges >= 201911L)
+        constexpr operator pointer() const { return _ptr + gsl::narrow<size_type>(_pos); }
+#endif /* defined(_MSC_VER) && __cpp_lib_ranges >= 201911L */
+
+        constexpr auto operator==(const dyn_array_iterator& other) const
+        {
+            Expects(_ptr == other._ptr);
+            Expects(_end_pos == other._end_pos);
+            return _pos == other._pos;
+        }
+
+        constexpr auto operator!=(const dyn_array_iterator& other) const
+        {
+            return !(*this == other);
+        }
+
+        constexpr auto operator*() const -> reference
+        {
+            Expects(_ptr != nullptr);
+            Expects(_pos < _end_pos);
+            return _ptr[_pos];
+        }
+
+        constexpr auto operator++() -> dyn_array_iterator&
+        {
+            Expects(_pos < _end_pos);
+            ++_pos;
+            return *this;
+        }
+
+        constexpr auto operator++(int)
+        {
+            auto rv = *this;
+            ++(*this);
+            return rv;
+        }
+
+        constexpr auto operator--() -> dyn_array_iterator&
+        {
+            Expects(_pos > 0);
+            --_pos;
+            return *this;
+        }
+
+        constexpr auto operator--(int)
+        {
+            auto rv = *this;
+            --(*this);
+            return rv;
+        }
+
+        constexpr auto operator+=(difference_type diff) -> dyn_array_iterator&
+        {
+            auto new_pos = gsl::narrow<difference_type>(_pos) + diff;
+            Expects(new_pos >= 0);
+            Expects(new_pos <= gsl::narrow<difference_type>(_end_pos));
+            _pos = gsl::narrow<size_type>(new_pos);
+            return *this;
+        }
+
+        constexpr auto operator-=(difference_type diff) -> dyn_array_iterator&
+        {
+            auto new_pos = gsl::narrow<difference_type>(_pos) - diff;
+            Expects(new_pos >= 0);
+            Expects(new_pos <= gsl::narrow<difference_type>(_end_pos));
+            _pos = gsl::narrow<size_type>(new_pos);
+            return *this;
+        }
+
+        constexpr auto operator+(difference_type diff) const
+        {
+            auto new_pos = gsl::narrow<difference_type>(_pos) + diff;
+            return dyn_array_iterator{_ptr, gsl::narrow<size_type>(new_pos), _end_pos};
+        }
+
+        constexpr auto operator-(difference_type diff) const { return *this + (-diff); }
+
+        constexpr auto operator-(const dyn_array_iterator& other) const
+        {
+            Expects(_ptr == other._ptr);
+            Expects(_end_pos == other._end_pos);
+            return gsl::narrow<difference_type>(_pos) - gsl::narrow<difference_type>(other._pos);
+        }
+
+        constexpr auto operator[](size_type pos) -> reference
+        {
+            Expects(_pos + pos < _end_pos);
+            return _ptr[_pos + pos];
+        }
+
+        constexpr auto operator[](size_type pos) const -> const_reference
+        {
+            return const_cast<dyn_array_iterator&>(*this).operator[](pos);
+        }
+
+    private:
+        pointer _ptr{};
+        size_type _pos{};
+        size_type _end_pos{};
+    };
+} // namespace details
+
+template <typename T, typename Allocator = std::allocator<T>>
+class dyn_array : private details::dyn_array_base<T, Allocator>
+{
+    using base = details::dyn_array_base<T, Allocator>;
+    using pointer = T*;
+
+public:
+    using value_type = T;
+    using reference = T&;
+    using const_reference = const T&;
+    using iterator = details::dyn_array_iterator<T>;
+    using const_iterator = details::dyn_array_iterator<const T>;
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+    using difference_type = std::ptrdiff_t;
+    using size_type = std::size_t;
+
+    using allocator_type = Allocator;
+
+    explicit constexpr dyn_array(const Allocator& alloc = {}) : base{alloc} {}
+
+    constexpr dyn_array(size_type count, const T& value, const Allocator& alloc = {})
+        : base{count, alloc}
+    {
+        base::fill(data(), size(), value);
+    }
+
+    template <typename InputIt,
+              std::enable_if_t<details::is_fwd_iterator<InputIt>::value, bool> = true>
+    constexpr dyn_array(InputIt first, InputIt last, const Allocator& alloc = {})
+        : base{gsl::narrow<size_type>(std::distance(first, last)), alloc}
+    {
+        base::copy(first, last, data());
+    }
+
+    template <typename InputIt, std::enable_if_t<!details::is_fwd_iterator<InputIt>::value &&
+                                                     details::is_iterator<InputIt>::value,
+                                                 bool> = true>
+    constexpr dyn_array(InputIt first, InputIt last, const Allocator& alloc = {}) : dyn_array{alloc}
+    {
+        std::vector<T> tmp(first, last);
+        base::resize(tmp.size());
+        base::copy(std::begin(tmp), std::end(tmp), data());
+    }
+
+#if defined(__cpp_lib_containers_ranges) && (__cpp_lib_containers_ranges >= 202202L)
+    template <typename InputRg>
+        requires(std::ranges::input_range<InputRg>)
+    constexpr dyn_array(std::from_range_t, InputRg&& rg, const Allocator& alloc = {})
+        : base{gsl::narrow<size_type>(std::size(rg)), alloc}
+    {
+        base::copy(std::ranges::begin(rg), std::ranges::end(rg), data());
+    }
+#endif /* __cpp_lib_containers_ranges >= 202202L */
+
+    constexpr explicit dyn_array(size_type count, const Allocator& alloc = {}) : base{count, alloc}
+    {
+        base::default_construct(data(), size());
+    }
+
+    constexpr dyn_array(const dyn_array& other, const Allocator& alloc = {})
+        : dyn_array(other.begin(), other.end(), alloc)
+    {}
+
+    constexpr dyn_array(std::initializer_list<T> init, const Allocator& alloc = {})
+        : dyn_array(init.begin(), init.end(), alloc)
+    {}
+
+    constexpr dyn_array(dyn_array&&) = delete;
+    dyn_array& operator=(dyn_array&&) = delete;
+
+    constexpr auto operator==(const dyn_array& other) const
+    {
+        return size() == other.size() && std::equal(begin(), end(), other.begin(), other.end());
+    }
+
+    constexpr auto operator!=(const dyn_array& other) const { return !(*this == other); }
+
+    constexpr auto size() const { return base::count(); }
+
+    constexpr auto empty() const { return size() == 0; }
+
+    constexpr auto max_size() const { return static_cast<size_type>(-1); }
+
+    constexpr auto get_allocator() -> Allocator& { return *this; }
+
+    constexpr auto operator[](size_type pos) -> reference
+    {
+        Expects(pos < size());
+        return data()[pos];
+    }
+
+    constexpr auto operator[](size_type pos) const -> const_reference
+    {
+        return const_cast<dyn_array&>(*this)[pos];
+    }
+
+    constexpr auto data() { return base::data(); }
+    constexpr auto data() const -> const T* { return const_cast<dyn_array&>(*this).data(); }
+
+    constexpr auto begin() { return iterator{data(), 0, size()}; }
+    constexpr auto begin() const { return const_iterator{data(), 0, size()}; }
+    constexpr auto cbegin() const { return begin(); }
+
+    constexpr auto rbegin() { return reverse_iterator{end()}; }
+    constexpr auto rbegin() const { return const_reverse_iterator{end()}; }
+    constexpr auto crbegin() const { return rbegin(); }
+
+#ifdef _MSC_VER
+    constexpr auto _Unchecked_begin() { return data(); }
+    constexpr auto _Unchecked_begin() const -> const T*
+    {
+        return const_cast<dyn_array&>(*this)._Unchecked_begin();
+    }
+#endif /* _MSC_VER */
+
+    constexpr auto end() { return iterator{data(), size(), size()}; }
+    constexpr auto end() const { return const_iterator{data(), size(), size()}; }
+    constexpr auto cend() const { return end(); }
+
+    constexpr auto rend() { return reverse_iterator{begin()}; }
+    constexpr auto rend() const { return const_reverse_iterator{begin()}; }
+    constexpr auto crend() const { return rend(); }
+
+#ifdef _MSC_VER
+    constexpr auto _Unchecked_end() { return data() + size(); }
+    constexpr auto _Unchecked_end() const -> const T*
+    {
+        return const_cast<dyn_array&>(*this)._Unchecked_end();
+    }
+#endif /* _MSC_VER */
+};
+
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
+
+template <class InputIt,
+          class Alloc = std::allocator<typename std::iterator_traits<InputIt>::value_type>>
+dyn_array(InputIt, InputIt, Alloc = {})
+    -> dyn_array<typename std::iterator_traits<InputIt>::value_type, Alloc>;
+
+#if defined(__cpp_lib_containers_ranges) && (__cpp_lib_containers_ranges >= 202202L)
+template <std::ranges::input_range InputRg,
+          class Alloc = std::allocator<std::ranges::range_value_t<InputRg>>>
+dyn_array(std::from_range_t, InputRg&&, Alloc = {})
+    -> dyn_array<std::ranges::range_value_t<InputRg>, Alloc>;
+#endif /* __cpp_lib_containers_ranges >= 202202L */
+
+#endif /* __cpp_deduction_guides >= 201703L */
+} // namespace gsl
+
+#endif /* defined(GSL_DYN_ARRAY_H) */

include/gsl/gsl

@@ -18,16 +18,16 @@
 #define GSL_GSL_H
 
 // IWYU pragma: begin_exports
-#include "algorithm"   // copy
-#include "assert"      // Ensures/Expects
-#include "byte"        // byte
-#include "pointers"    // owner, not_null
-#include "span"        // span
-#include "zstring"     // zstring
-#include "util"        // finally()/narrow_cast()...
+#include "./algorithm" // copy
+#include "./assert"    // Ensures/Expects
+#include "./byte"      // byte
+#include "./pointers"  // owner, not_null
+#include "./span"      // span
+#include "./util"      // finally()/narrow_cast()...
+#include "./zstring"   // zstring
 
 #ifdef __cpp_exceptions
-#include "narrow" // narrow()
+#include "./narrow" // narrow()
 #endif
 // IWYU pragma: end_exports
 

include/gsl/narrow

@@ -16,8 +16,8 @@
 
 #ifndef GSL_NARROW_H
 #define GSL_NARROW_H
-#include "assert"    // for GSL_SUPPRESS
-#include "util"      // for narrow_cast
+#include "./assert"  // for GSL_SUPPRESS
+#include "./util"    // for narrow_cast
 #include <exception> // for std::exception
 namespace gsl
 {
@@ -28,28 +28,28 @@ struct narrowing_error : public std::exception
 
 // narrow() : a checked version of narrow_cast() that throws if the cast changed the value
 template <class T, class U, typename std::enable_if<std::is_arithmetic<T>::value>::type* = nullptr>
-// clang-format off
-GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-GSL_SUPPRESS(es.46) // NO-FORMAT: attribute // The warning suggests that a floating->unsigned conversion can occur
-                                            // in the static_cast below, and that gsl::narrow should be used instead.
-                                            // Suppress this warning, since gsl::narrow is defined in terms of
-                                            // static_cast
-    // clang-format on
+GSL_SUPPRESS(type.1)
+    GSL_SUPPRESS(es.46) // The warning suggests that a floating->unsigned conversion can occur
+                        // in the static_cast below, and that gsl::narrow should be used instead.
+                        // Suppress this warning, since gsl::narrow is defined in terms of
+                        // static_cast
     constexpr T narrow(U u)
 {
     constexpr const bool is_different_signedness =
         (std::is_signed<T>::value != std::is_signed<U>::value);
 
-GSL_SUPPRESS(es.103) // NO-FORMAT: attribute // don't overflow
-GSL_SUPPRESS(es.104) // NO-FORMAT: attribute // don't underflow
-GSL_SUPPRESS(p.2) // NO-FORMAT: attribute // don't rely on undefined behavior
-    const T t = narrow_cast<T>(u); // While this is technically undefined behavior in some cases (i.e., if the source value is of floating-point type
-                                   // and cannot fit into the destination integral type), the resultant behavior is benign on the platforms
-                                   // that we target (i.e., no hardware trap representations are hit).
+    GSL_SUPPRESS(es.103) // don't overflow
+    GSL_SUPPRESS(es.104) // don't underflow
+    GSL_SUPPRESS(p.2)    // don't rely on undefined behavior
+    const T t = narrow_cast<T>(
+        u); // While this is technically undefined behavior in some cases (i.e., if the source value
+            // is of floating-point type and cannot fit into the destination integral type), the
+            // resultant behavior is benign on the platforms that we target (i.e., no hardware trap
+            // representations are hit).
 
 #if defined(__clang__) || defined(__GNUC__)
-    #pragma GCC diagnostic push
-    #pragma GCC diagnostic ignored "-Wfloat-equal"
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wfloat-equal"
 #endif
     // Note: NaN will always throw, since NaN != NaN
     if (static_cast<U>(t) != u || (is_different_signedness && ((t < T{}) != (u < U{}))))
@@ -57,24 +57,18 @@ GSL_SUPPRESS(p.2) // NO-FORMAT: attribute // don't rely on undefined behavior
         throw narrowing_error{};
     }
 #if defined(__clang__) || defined(__GNUC__)
-    #pragma GCC diagnostic pop
+#pragma GCC diagnostic pop
 #endif
 
     return t;
 }
 
 template <class T, class U, typename std::enable_if<!std::is_arithmetic<T>::value>::type* = nullptr>
-// clang-format off
-GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
-    constexpr T narrow(U u)
+GSL_SUPPRESS(type.1) constexpr T narrow(U u)
 {
     const T t = narrow_cast<T>(u);
 
-    if (static_cast<U>(t) != u)
-    {
-        throw narrowing_error{};
-    }
+    if (static_cast<U>(t) != u) { throw narrowing_error{}; }
 
     return t;
 }

include/gsl/pointers

@@ -1,3 +1,4 @@
+// -*- C++ -*-
 ///////////////////////////////////////////////////////////////////////////////
 //
 // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
@@ -17,13 +18,14 @@
 #ifndef GSL_POINTERS_H
 #define GSL_POINTERS_H
 
-#include "assert" // for Ensures, Expects
+#include "./assert" // for Ensures, Expects
+#include "./util"   // for GSL_DEPRECATED
 
-#include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
-#include <functional>   // for less, greater
-#include <memory>       // for shared_ptr, unique_ptr, hash
-#include <type_traits>  // for enable_if_t, is_convertible, is_assignable
-#include <utility>      // for declval, forward
+#include <cstddef>     // for ptrdiff_t, nullptr_t, size_t
+#include <functional>  // for less, greater
+#include <memory>      // for shared_ptr, unique_ptr, hash
+#include <type_traits> // for enable_if_t, is_convertible, is_assignable
+#include <utility>     // for declval, forward
 
 #if !defined(GSL_NO_IOSTREAMS)
 #include <iosfwd> // for ostream
@@ -47,35 +49,40 @@ namespace details
     {
     };
 
-    // Resolves to the more efficient of `const T` or `const T&`, in the context of returning a const-qualified value
-    // of type T.
+    // Resolves to the more efficient of `const T` or `const T&`, in the context of returning a
+    // const-qualified value of type T.
     //
-    // Copied from cppfront's implementation of the CppCoreGuidelines F.16 (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-in)
-    template<typename T>
-    using value_or_reference_return_t = std::conditional_t<
-                                            sizeof(T) < 2*sizeof(void*) && std::is_trivially_copy_constructible<T>::value,
-                                            const T,
-                                            const T&>;
+    // Copied from cppfront's implementation of the CppCoreGuidelines F.16
+    // (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#rf-in)
+    template <typename T>
+    using value_or_reference_return_t =
+        std::conditional_t<sizeof(T) <= 2 * sizeof(void*) &&
+                               std::is_trivially_copy_constructible<T>::value,
+                           const T, const T&>;
 
 } // namespace details
 
 //
 // GSL.owner: ownership pointers
 //
-using std::shared_ptr;
-using std::unique_ptr;
+template <typename... Ts>
+using shared_ptr GSL_DEPRECATED("Use std::shared_ptr instead") = std::shared_ptr<Ts...>;
+
+template <typename... Ts>
+using unique_ptr GSL_DEPRECATED("Use std::unique_ptr instead") = std::unique_ptr<Ts...>;
 
 //
 // owner
 //
-// `gsl::owner<T>` is designed as a safety mechanism for code that must deal directly with raw pointers that own memory.
-// Ideally such code should be restricted to the implementation of low-level abstractions. `gsl::owner` can also be used
-// as a stepping point in converting legacy code to use more modern RAII constructs, such as smart pointers.
+// `gsl::owner<T>` is designed as a safety mechanism for code that must deal directly with raw
+// pointers that own memory. Ideally such code should be restricted to the implementation of
+// low-level abstractions. `gsl::owner` can also be used as a stepping point in converting legacy
+// code to use more modern RAII constructs, such as smart pointers.
 //
 // T must be a pointer type
 // - disallow construction from any type other than pointer type
 //
-template <class T, class = std::enable_if_t<std::is_pointer<T>::value>>
+template <class T, std::enable_if_t<std::is_pointer<T>::value, bool> = true>
 using owner = T;
 
 //
@@ -98,26 +105,32 @@ class not_null
 public:
     static_assert(details::is_comparable_to_nullptr<T>::value, "T cannot be compared to nullptr.");
 
+    using element_type = T;
+
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value) : ptr_(std::forward<U>(u))
+    constexpr not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value)
+        : ptr_(std::forward<U>(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
-    constexpr not_null(T u) noexcept(std::is_nothrow_move_constructible<T>::value) : ptr_(std::move(u))
+    constexpr not_null(T u) noexcept(std::is_nothrow_move_constructible<T>::value)
+        : ptr_(std::move(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr not_null(const not_null<U>& other) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null(other.get())
+    constexpr not_null(const not_null<U>& other) noexcept(
+        std::is_nothrow_move_constructible<T>::value)
+        : not_null(other.get())
     {}
 
     not_null(const not_null& other) = default;
     not_null& operator=(const not_null& other) = default;
     constexpr details::value_or_reference_return_t<T> get() const
-        noexcept(noexcept(details::value_or_reference_return_t<T>{std::declval<T&>()}))
+        noexcept(noexcept(details::value_or_reference_return_t<T>(std::declval<T&>())))
     {
         return ptr_;
     }
@@ -139,10 +152,20 @@ public:
     not_null& operator-=(std::ptrdiff_t) = delete;
     void operator[](std::ptrdiff_t) const = delete;
 
+    void swap(not_null<T>& other) noexcept { std::swap(ptr_, other.ptr_); }
+
 private:
     T ptr_;
 };
 
+template <typename T, std::enable_if_t<std::is_move_assignable<T>::value &&
+                                           std::is_move_constructible<T>::value,
+                                       bool> = true>
+void swap(not_null<T>& a, not_null<T>& b) noexcept
+{
+    a.swap(b);
+}
+
 template <class T>
 auto make_not_null(T&& t) noexcept
 {
@@ -159,48 +182,50 @@ std::ostream& operator<<(std::ostream& os, const not_null<T>& val)
 #endif // !defined(GSL_NO_IOSTREAMS)
 
 template <class T, class U>
-auto operator==(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(lhs.get() == rhs.get()))
+constexpr auto operator==(const not_null<T>& lhs,
+                          const not_null<U>& rhs) noexcept(noexcept(lhs.get() == rhs.get()))
     -> decltype(lhs.get() == rhs.get())
 {
     return lhs.get() == rhs.get();
 }
 
 template <class T, class U>
-auto operator!=(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(lhs.get() != rhs.get()))
+constexpr auto operator!=(const not_null<T>& lhs,
+                          const not_null<U>& rhs) noexcept(noexcept(lhs.get() != rhs.get()))
     -> decltype(lhs.get() != rhs.get())
 {
     return lhs.get() != rhs.get();
 }
 
 template <class T, class U>
-auto operator<(const not_null<T>& lhs,
-               const not_null<U>& rhs) noexcept(noexcept(std::less<>{}(lhs.get(), rhs.get())))
-    -> decltype(std::less<>{}(lhs.get(), rhs.get()))
+constexpr auto operator<(const not_null<T>& lhs, const not_null<U>& rhs) noexcept(
+    noexcept(std::less<>{}(lhs.get(), rhs.get()))) -> decltype(std::less<>{}(lhs.get(), rhs.get()))
 {
     return std::less<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
-auto operator<=(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(std::less_equal<>{}(lhs.get(), rhs.get())))
+constexpr auto
+operator<=(const not_null<T>& lhs,
+           const not_null<U>& rhs) noexcept(noexcept(std::less_equal<>{}(lhs.get(), rhs.get())))
     -> decltype(std::less_equal<>{}(lhs.get(), rhs.get()))
 {
     return std::less_equal<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
-auto operator>(const not_null<T>& lhs,
-               const not_null<U>& rhs) noexcept(noexcept(std::greater<>{}(lhs.get(), rhs.get())))
+constexpr auto
+operator>(const not_null<T>& lhs,
+          const not_null<U>& rhs) noexcept(noexcept(std::greater<>{}(lhs.get(), rhs.get())))
     -> decltype(std::greater<>{}(lhs.get(), rhs.get()))
 {
     return std::greater<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
-auto operator>=(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(std::greater_equal<>{}(lhs.get(), rhs.get())))
+constexpr auto
+operator>=(const not_null<T>& lhs,
+           const not_null<U>& rhs) noexcept(noexcept(std::greater_equal<>{}(lhs.get(), rhs.get())))
     -> decltype(std::greater_equal<>{}(lhs.get(), rhs.get()))
 {
     return std::greater_equal<>{}(lhs.get(), rhs.get());
@@ -216,11 +241,13 @@ not_null<T> operator+(const not_null<T>&, std::ptrdiff_t) = delete;
 template <class T>
 not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 
-
-template <class T, class U = decltype(std::declval<const T&>().get()), bool = std::is_default_constructible<std::hash<U>>::value>
+// T is conceptually a pointer so we don't have to worry about it being a reference and violating
+// std::hash requirements
+template <class T, class U = typename T::element_type,
+          bool = std::is_default_constructible<std::hash<U>>::value>
 struct not_null_hash
 {
-    std::size_t operator()(const T& value) const { return std::hash<U>{}(value.get()); }
+    std::size_t operator()(const T& value) const noexcept { return std::hash<U>{}(value.get()); }
 };
 
 template <class T, class U>
@@ -267,24 +294,32 @@ class strict_not_null : public not_null<T>
 {
 public:
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr explicit strict_not_null(U&& u) : not_null<T>(std::forward<U>(u))
+    constexpr explicit strict_not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value)
+        : not_null<T>(std::forward<U>(u))
     {}
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
-    constexpr explicit strict_not_null(T u) : not_null<T>(u)
+    constexpr explicit strict_not_null(T u) noexcept(std::is_nothrow_move_constructible<T>::value)
+        : not_null<T>(std::move(u))
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr strict_not_null(const not_null<U>& other) : not_null<T>(other)
+    constexpr strict_not_null(const not_null<U>& other) noexcept(
+        std::is_nothrow_move_constructible<T>::value)
+        : not_null<T>(other)
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr strict_not_null(const strict_not_null<U>& other) : not_null<T>(other)
+    constexpr strict_not_null(const strict_not_null<U>& other) noexcept(
+        std::is_nothrow_move_constructible<T>::value)
+        : not_null<T>(other)
     {}
 
     // To avoid invalidating the "not null" invariant, the contained pointer is actually copied
-    // instead of moved. If it is a custom pointer, its constructor could in theory throw exceptions.
-    strict_not_null(strict_not_null&& other) noexcept(std::is_nothrow_copy_constructible<T>::value) = default;
+    // instead of moved. If it is a custom pointer, its constructor could in theory throw
+    // exceptions.
+    strict_not_null(strict_not_null&& other) noexcept(
+        std::is_nothrow_copy_constructible<T>::value) = default;
     strict_not_null(const strict_not_null& other) = default;
     strict_not_null& operator=(const strict_not_null& other) = default;
     strict_not_null& operator=(const not_null<T>& other)
@@ -323,7 +358,7 @@ auto make_strict_not_null(T&& t) noexcept
     return strict_not_null<std::remove_cv_t<std::remove_reference_t<T>>>{std::forward<T>(t)};
 }
 
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
 
 // deduction guides to prevent the ctad-maybe-unsupported warning
 template <class T>
@@ -331,7 +366,7 @@ not_null(T) -> not_null<T>;
 template <class T>
 strict_not_null(T) -> strict_not_null<T>;
 
-#endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
+#endif // defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
 
 } // namespace gsl
 

include/gsl/span

@@ -1,3 +1,4 @@
+// -*- C++ -*-
 ///////////////////////////////////////////////////////////////////////////////
 //
 // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
@@ -17,10 +18,10 @@
 #ifndef GSL_SPAN_H
 #define GSL_SPAN_H
 
-#include "assert"   // for Expects
-#include "byte"     // for byte
-#include "span_ext" // for span specialization of gsl::at and other span-related extensions
-#include "util"     // for narrow_cast
+#include "./assert"   // for Expects
+#include "./byte"     // for gsl::impl::byte
+#include "./span_ext" // for span specialization of gsl::at and other span-related extensions
+#include "./util"     // for narrow_cast
 
 #include <array>       // for array
 #include <cstddef>     // for ptrdiff_t, size_t, nullptr_t
@@ -140,7 +141,9 @@ namespace details
 
         constexpr span_iterator(pointer begin, pointer end, pointer current)
             : begin_(begin), end_(end), current_(current)
-        {}
+        {
+            Expects(begin_ <= current_ && current <= end_);
+        }
 
         constexpr operator span_iterator<const Type>() const noexcept
         {
@@ -149,24 +152,19 @@ namespace details
 
         constexpr reference operator*() const noexcept
         {
-            Expects(begin_ && end_);
-            Expects(begin_ <= current_ && current_ < end_);
+            Expects(current_ != end_);
             return *current_;
         }
 
         constexpr pointer operator->() const noexcept
         {
-            Expects(begin_ && end_);
-            Expects(begin_ <= current_ && current_ < end_);
+            Expects(current_ != end_);
             return current_;
         }
         constexpr span_iterator& operator++() noexcept
         {
-            Expects(begin_ && current_ && end_);
-            Expects(current_ < end_);
-            // clang-format off
-            GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-            // clang-format on
+            Expects(current_ != end_);
+            GSL_SUPPRESS(bounds.1)
             ++current_;
             return *this;
         }
@@ -180,8 +178,7 @@ namespace details
 
         constexpr span_iterator& operator--() noexcept
         {
-            Expects(begin_ && end_);
-            Expects(begin_ < current_);
+            Expects(begin_ != current_);
             --current_;
             return *this;
         }
@@ -198,9 +195,7 @@ namespace details
             if (n != 0) Expects(begin_ && current_ && end_);
             if (n > 0) Expects(end_ - current_ >= n);
             if (n < 0) Expects(current_ - begin_ >= -n);
-            // clang-format off
-            GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-            // clang-format on
+            GSL_SUPPRESS(bounds.1)
             current_ += n;
             return *this;
         }
@@ -223,7 +218,7 @@ namespace details
             if (n != 0) Expects(begin_ && current_ && end_);
             if (n > 0) Expects(current_ - begin_ >= n);
             if (n < 0) Expects(end_ - current_ >= -n);
-            GSL_SUPPRESS(bounds .1)
+            GSL_SUPPRESS(bounds.1)
             current_ -= n;
             return *this;
         }
@@ -317,9 +312,7 @@ namespace details
             if (n < 0) Expects(current_ - begin_ >= -n);
         }
 
-        // clang-format off
-        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-        // clang-format on
+        GSL_SUPPRESS(bounds.1)
         constexpr pointer _Unwrapped() const noexcept
         { // after seeking *this to a high water mark, or using one of the
             // _Verify_xxx functions above, unwrap this span_iterator to a raw
@@ -334,9 +327,7 @@ namespace details
 #else
         static constexpr bool _Unwrap_when_unverified = false;
 #endif
-        // clang-format off
-        GSL_SUPPRESS(con.3) // NO-FORMAT: attribute // TODO: false positive
-        // clang-format on
+        GSL_SUPPRESS(con.3) // TODO: false positive
         constexpr void _Seek_to(const pointer p) noexcept
         { // adjust the position of *this to previously verified location p
             // after _Unwrapped
@@ -351,7 +342,8 @@ namespace details
         template <typename Ptr>
         friend struct std::pointer_traits;
     };
-}} // namespace gsl::details
+} // namespace details
+} // namespace gsl
 
 namespace std
 {
@@ -366,7 +358,10 @@ struct pointer_traits<::gsl::details::span_iterator<Type>>
 };
 } // namespace std
 
-namespace gsl { namespace details {
+namespace gsl
+{
+namespace details
+{
     template <std::size_t Ext>
     class extent_type
     {
@@ -584,27 +579,28 @@ public:
     template <std::size_t Count>
     constexpr span<element_type, Count> first() const noexcept
     {
+        static_assert(Extent == dynamic_extent || Count <= Extent,
+                      "first() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data(), Count};
     }
 
     template <std::size_t Count>
-    // clang-format off
-    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-        // clang-format on
-        constexpr span<element_type, Count> last() const noexcept
+    GSL_SUPPRESS(bounds.1) constexpr span<element_type, Count> last() const noexcept
     {
+        static_assert(Extent == dynamic_extent || Count <= Extent,
+                      "last() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data() + (size() - Count), Count};
     }
 
     template <std::size_t Offset, std::size_t Count = dynamic_extent>
-    // clang-format off
-    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-        // clang-format on
-        constexpr auto subspan() const noexcept ->
+    GSL_SUPPRESS(bounds.1) constexpr auto subspan() const noexcept ->
         typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
+        static_assert(Extent == dynamic_extent || (Extent >= Offset && (Count == dynamic_extent ||
+                                                                        Count <= Extent - Offset)),
+                      "subspan() cannot extract more elements from a span than it contains.");
         Expects((size() >= Offset) && (Count == dynamic_extent || (Count <= size() - Offset)));
         using type =
             typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type;
@@ -637,9 +633,7 @@ public:
     constexpr bool empty() const noexcept { return size() == 0; }
 
     // [span.elem], span element access
-    // clang-format off
-    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(bounds.1)
     constexpr reference operator[](size_type idx) const noexcept
     {
         Expects(idx < size());
@@ -664,18 +658,14 @@ public:
     constexpr iterator begin() const noexcept
     {
         const auto data = storage_.data();
-        // clang-format off
-        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-        // clang-format on
+        GSL_SUPPRESS(bounds.1)
         return {data, data + size(), data};
     }
 
     constexpr iterator end() const noexcept
     {
         const auto data = storage_.data();
-        // clang-format off
-        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-        // clang-format on
+        GSL_SUPPRESS(bounds.1)
         const auto endData = data + storage_.size();
         return {data, endData, endData};
     }
@@ -688,9 +678,7 @@ public:
     constexpr pointer _Unchecked_begin() const noexcept { return data(); }
     constexpr pointer _Unchecked_end() const noexcept
     {
-        // clang-format off
-        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-        // clang-format on
+        GSL_SUPPRESS(bounds.1)
         return data() + size();
     }
 #endif // _MSC_VER
@@ -747,9 +735,7 @@ private:
         return tmp.subspan(offset, count);
     }
 
-    // clang-format off
-    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(bounds.1)
     constexpr span<element_type, dynamic_extent>
     make_subspan(size_type offset, size_type count, subspan_selector<dynamic_extent>) const noexcept
     {
@@ -762,7 +748,7 @@ private:
     }
 };
 
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
 
 // Deduction Guides
 template <class Type, std::size_t Extent>
@@ -782,12 +768,14 @@ template <class Container,
           class Element = std::remove_pointer_t<decltype(std::declval<const Container&>().data())>>
 span(const Container&) -> span<Element>;
 
-#endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
+#endif // defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
 
 #if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
 #if defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
 #pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wdeprecated" // Bug in clang-cl.exe which raises a C++17 -Wdeprecated warning about this static constexpr workaround in C++14 mode.
+#pragma clang diagnostic ignored                                                                   \
+    "-Wdeprecated" // Bug in clang-cl.exe which raises a C++17 -Wdeprecated warning about this
+                   // static constexpr workaround in C++14 mode.
 #endif // defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
 template <class ElementType, std::size_t Extent>
 constexpr const typename span<ElementType, Extent>::size_type span<ElementType, Extent>::extent;
@@ -819,28 +807,25 @@ namespace details
 
 // [span.objectrep], views of object representation
 template <class ElementType, std::size_t Extent>
-span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
+span<const gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_bytes(span<ElementType, Extent> s) noexcept
 {
-    using type = span<const byte, details::calculate_byte_size<ElementType, Extent>::value>;
+    using type =
+        span<const gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>;
 
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
-    return type{reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
+    GSL_SUPPRESS(type.1)
+    return type{reinterpret_cast<const gsl::impl::byte*>(s.data()), s.size_bytes()};
 }
 
 template <class ElementType, std::size_t Extent,
           std::enable_if_t<!std::is_const<ElementType>::value, int> = 0>
-span<byte, details::calculate_byte_size<ElementType, Extent>::value>
+span<gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_writable_bytes(span<ElementType, Extent> s) noexcept
 {
-    using type = span<byte, details::calculate_byte_size<ElementType, Extent>::value>;
+    using type = span<gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>;
 
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
-    return type{reinterpret_cast<byte*>(s.data()), s.size_bytes()};
+    GSL_SUPPRESS(type.1)
+    return type{reinterpret_cast<gsl::impl::byte*>(s.data()), s.size_bytes()};
 }
 
 } // namespace gsl

include/gsl/span_ext

@@ -27,8 +27,8 @@
 //
 ///////////////////////////////////////////////////////////////////////////////
 
-#include "assert" // GSL_KERNEL_MODE
-#include "util"   // for narrow_cast, narrow
+#include "./assert" // GSL_KERNEL_MODE
+#include "./util"   // for narrow_cast, narrow
 
 #include <cstddef> // for ptrdiff_t, size_t
 #include <utility>
@@ -123,14 +123,14 @@ constexpr span<const typename Container::value_type> make_span(const Container&
 }
 
 template <class Ptr>
-[[deprecated("This function is deprecated. See GSL issue #1092.")]]
+GSL_DEPRECATED("This function is deprecated. See GSL issue #1092.")
 constexpr span<typename Ptr::element_type> make_span(Ptr& cont, std::size_t count)
 {
     return span<typename Ptr::element_type>(cont, count);
 }
 
 template <class Ptr>
-[[deprecated("This function is deprecated. See GSL issue #1092.")]]
+GSL_DEPRECATED("This function is deprecated. See GSL issue #1092.")
 constexpr span<typename Ptr::element_type> make_span(Ptr& cont)
 {
     return span<typename Ptr::element_type>(cont);
@@ -149,7 +149,7 @@ constexpr ElementType& at(span<ElementType, Extent> s, index i)
 template <class ElementType, std::size_t Extent>
 constexpr std::ptrdiff_t ssize(const span<ElementType, Extent>& s) noexcept
 {
-    return static_cast<std::ptrdiff_t>(s.size());
+    return gsl::narrow_cast<std::ptrdiff_t>(s.size());
 }
 
 // [span.iter] Free functions for begin/end functions

include/gsl/string_span

@@ -1,4 +0,0 @@
-#pragma once
-#pragma message(                                                                                   \
-    "This header will soon be removed. Use <gsl/zstring> instead of <gsl/string_span>")
-#include "zstring"

include/gsl/util

@@ -1,3 +1,4 @@
+// -*- C++ -*-
 ///////////////////////////////////////////////////////////////////////////////
 //
 // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
@@ -17,12 +18,12 @@
 #ifndef GSL_UTIL_H
 #define GSL_UTIL_H
 
-#include "assert" // for Expects
+#include "./assert" // for Expects
 
-#include <array>
 #include <cstddef>          // for ptrdiff_t, size_t
-#include <limits>           // for numeric_limits
 #include <initializer_list> // for initializer_list
+#include <iterator>         // for iterator_traits
+#include <limits>           // for numeric_limits
 #include <type_traits>      // for is_signed, integral_constant
 #include <utility>          // for exchange, forward
 
@@ -60,6 +61,39 @@
 #define GSL_INLINE
 #endif
 
+#if defined(__has_cpp_attribute)
+#if __has_cpp_attribute(deprecated)
+#define GSL_DEPRECATED(msg) [[deprecated(msg)]]
+#endif // __has_cpp_attribute(deprecated)
+#endif // defined(__has_cpp_attribute)
+
+#if !defined(GSL_DEPRECATED)
+#if defined(__cplusplus)
+#if __cplusplus >= 201309L
+#define GSL_DEPRECATED(msg) [[deprecated(msg)]]
+#endif // __cplusplus >= 201309L
+#endif // defined(__cplusplus)
+#endif // !defined(GSL_DEPRECATED)
+
+#if !defined(GSL_DEPRECATED)
+#if defined(_MSC_VER)
+#define GSL_DEPRECATED(msg) __declspec(deprecated(msg))
+#elif defined(__GNUC__)
+#define GSL_DEPRECATED(msg) __attribute__((deprecated(msg)))
+#endif // defined(_MSC_VER)
+#endif // !defined(GSL_DEPRECATED)
+
+#if !defined(GSL_DEPRECATED)
+#define GSL_DEPRECATED(msg)
+#endif // !defined(GSL_DEPRECATED)
+
+#if __cplusplus >= 202002L
+#define GSL_CONSTEXPR_SINCE_CPP20 constexpr
+#else // ^^^ since C++20 /// before C++20 vvv
+#define GSL_BEFORE_CPP20
+#define GSL_CONSTEXPR_SINCE_CPP20
+#endif // __cplusplus >= 202002L
+
 namespace gsl
 {
 //
@@ -69,23 +103,55 @@ namespace gsl
 // index type for all container indexes/subscripts/sizes
 using index = std::ptrdiff_t;
 
+namespace details
+{
+    template <typename...>
+    using void_t = void;
+
+    template <typename T, typename = void>
+    struct is_iterator : std::false_type
+    {
+    };
+
+    template <typename T>
+    struct is_iterator<T, void_t<typename std::iterator_traits<T>::value_type>> : std::true_type
+    {
+    };
+
+    template <typename T, typename = void>
+    struct is_fwd_iterator : std::false_type
+    {
+    };
+
+    template <typename T>
+    struct is_fwd_iterator<T, void_t<typename std::iterator_traits<T>::iterator_category>>
+        : std::integral_constant<
+              bool, std::is_base_of<std::forward_iterator_tag,
+                                    typename std::iterator_traits<T>::iterator_category>::value>
+    {
+    };
+} // namespace details
+
 // final_action allows you to ensure something gets run at the end of a scope
 template <class F>
 class final_action
 {
 public:
-    explicit final_action(const F& ff) noexcept : f{ff} { }
-    explicit final_action(F&& ff) noexcept : f{std::move(ff)} { }
+    explicit final_action(const F& ff) noexcept : f{ff} {}
+    explicit final_action(F&& ff) noexcept : f{std::move(ff)} {}
 
-    ~final_action() noexcept { if (invoke) f(); }
+    ~final_action() noexcept
+    {
+        if (invoke) f();
+    }
 
     final_action(final_action&& other) noexcept
         : f(std::move(other.f)), invoke(std::exchange(other.invoke, false))
-    { }
+    {}
 
-    final_action(const final_action&)   = delete;
+    final_action(const final_action&) = delete;
     void operator=(const final_action&) = delete;
-    void operator=(final_action&&)      = delete;
+    void operator=(final_action&&) = delete;
 
 private:
     F f;
@@ -101,10 +167,7 @@ GSL_NODISCARD auto finally(F&& f) noexcept
 
 // narrow_cast(): a searchable way to do narrowing casts of values
 template <class T, class U>
-// clang-format off
-GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
-    constexpr T narrow_cast(U&& u) noexcept
+GSL_SUPPRESS(type.1) constexpr T narrow_cast(U&& u) noexcept
 {
     return static_cast<T>(std::forward<U>(u));
 }
@@ -113,23 +176,17 @@ GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
 // at() - Bounds-checked way of accessing builtin arrays, std::array, std::vector
 //
 template <class T, std::size_t N>
-// clang-format off
-GSL_SUPPRESS(bounds.4) // NO-FORMAT: attribute
-GSL_SUPPRESS(bounds.2) // NO-FORMAT: attribute
-    // clang-format on
-    constexpr T& at(T (&arr)[N], const index i)
+GSL_SUPPRESS(bounds.4) GSL_SUPPRESS(bounds.2) constexpr T& at(T (&arr)[N], const index i)
 {
-    static_assert(N <= static_cast<std::size_t>((std::numeric_limits<std::ptrdiff_t>::max)()), "We only support arrays up to PTRDIFF_MAX bytes.");
+    static_assert(N <= static_cast<std::size_t>((std::numeric_limits<std::ptrdiff_t>::max)()),
+                  "We only support arrays up to PTRDIFF_MAX bytes.");
     Expects(i >= 0 && i < narrow_cast<index>(N));
     return arr[narrow_cast<std::size_t>(i)];
 }
 
 template <class Cont>
-// clang-format off
-GSL_SUPPRESS(bounds.4) // NO-FORMAT: attribute
-GSL_SUPPRESS(bounds.2) // NO-FORMAT: attribute
-    // clang-format on
-    constexpr auto at(Cont& cont, const index i) -> decltype(cont[cont.size()])
+GSL_SUPPRESS(bounds.4) GSL_SUPPRESS(bounds.2) constexpr auto at(Cont& cont, const index i)
+    -> decltype(cont[cont.size()])
 {
     Expects(i >= 0 && i < narrow_cast<index>(cont.size()));
     using size_type = decltype(cont.size());
@@ -137,15 +194,19 @@ GSL_SUPPRESS(bounds.2) // NO-FORMAT: attribute
 }
 
 template <class T>
-// clang-format off
-GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-    // clang-format on
-    constexpr T at(const std::initializer_list<T> cont, const index i)
+GSL_SUPPRESS(bounds.1) constexpr T at(const std::initializer_list<T> cont, const index i)
 {
     Expects(i >= 0 && i < narrow_cast<index>(cont.size()));
     return *(cont.begin() + i);
 }
 
+template <class T, std::enable_if_t<std::is_move_assignable<T>::value &&
+                                    std::is_move_constructible<T>::value>>
+void swap(T& a, T& b)
+{
+    std::swap(a, b);
+}
+
 #if defined(__cpp_lib_span) && __cpp_lib_span >= 202002L
 template <class T, std::size_t extent = std::dynamic_extent>
 constexpr auto at(std::span<T, extent> sp, const index i) -> decltype(sp[sp.size()])

include/gsl/zstring

@@ -17,9 +17,9 @@
 #ifndef GSL_ZSTRING_H
 #define GSL_ZSTRING_H
 
-#include "span_ext" // for dynamic_extent
+#include "./span_ext" // for dynamic_extent
 
-#include <cstddef>   // for size_t, nullptr_t
+#include <cstddef> // for size_t, nullptr_t
 
 namespace gsl
 {

scripts/apply-formatting.bat

@@ -0,0 +1,13 @@
+@echo off
+setlocal enabledelayedexpansion
+
+"%VCINSTALLDIR%Tools\Llvm\bin\clang-format" -version
+if %errorlevel% neq 0 (
+    echo [ERROR] clang-format not found, script should be called from a visual studio developer command prompt.
+    exit /b %errorlevel%
+)
+
+for %%f in (include\gsl\* tests\*.h tests\*.cpp) do (
+    echo formatting %%f
+    "%VCINSTALLDIR%Tools\Llvm\bin\clang-format" -i --assume-filename x.cpp "%%f"
+)

scripts/apply-formatting.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -euo pipefail
+
+cf=clang-format-$(grep -i 'CLANG_VERSION:' .github/workflows/clang-format.yml | sed -E 's/.*"([^"]+)".*/\1/')
+readonly cf
+
+if ! "${cf}" -version; then
+	echo "[ERROR] clang-format not found. Please install it using: sudo apt install ${cf}"
+	exit 1
+fi
+
+{
+	find include/gsl -type f
+	find tests -type f \( -name '*.cpp' -o -name '*.h' \)
+} | xargs "${cf}" -i --assume-filename=x.cpp --verbose
+
+find scripts -type f -name '*.sh' -print -exec shfmt -w {} \;

tests/CMakeLists.txt

@@ -110,6 +110,7 @@ if(MSVC) # MSVC or simulating MSVC
           -Wno-shift-sign-overflow # GTest gtest-port.h
           -Wno-undef # GTest
           -Wno-used-but-marked-unused # GTest EXPECT_DEATH
+          -Wno-switch-default # GTest EXPECT_DEATH
           $<$<EQUAL:${GSL_CXX_STANDARD},14>: # no support for [[maybe_unused]]
             -Wno-unused-member-function
             -Wno-unused-variable
@@ -150,6 +151,7 @@ else()
           -Wno-unknown-attributes
           -Wno-used-but-marked-unused # GTest EXPECT_DEATH
           -Wno-weak-vtables
+          -Wno-poison-system-directories
           $<$<EQUAL:${GSL_CXX_STANDARD},14>: # no support for [[maybe_unused]]
             -Wno-unused-member-function
             -Wno-unused-variable
@@ -198,27 +200,22 @@ target_include_directories(gsl_tests_config SYSTEM INTERFACE
     googletest/googletest/include
 )
 
-add_executable(gsl_tests
-    algorithm_tests.cpp
-    assertion_tests.cpp
-    at_tests.cpp
-    byte_tests.cpp
-    notnull_tests.cpp
-    owner_tests.cpp
-    span_compatibility_tests.cpp
-    span_ext_tests.cpp
-    span_tests.cpp
-    strict_notnull_tests.cpp
-    
-    utils_tests.cpp
-)
+# Individually build and register each test source (except no_exception_ensure_tests.cpp)
+# no_exception_ensure_tests.cpp is built separately with exceptions disabled
+file(GLOB GSL_TEST_SOURCES CONFIGURE_DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/*.cpp")
+list(FILTER GSL_TEST_SOURCES EXCLUDE REGEX "no_exception_ensure_tests\\.cpp$")
 
-target_link_libraries(gsl_tests
+foreach(src IN LISTS GSL_TEST_SOURCES)
+  get_filename_component(test_name "${src}" NAME_WE)
+  add_executable(${test_name} ${src})
+  target_link_libraries(${test_name}
     Microsoft.GSL::GSL
     gsl_tests_config
     ${GTestMain_LIBRARIES}
-)
-add_test(gsl_tests gsl_tests)
+  )
+  add_test(NAME ${test_name} COMMAND ${test_name})
+  set_target_properties(${test_name} PROPERTIES FOLDER "tests")
+endforeach()
 
 # No exception tests
 
@@ -284,6 +281,7 @@ else()
           -Wno-missing-prototypes
           -Wno-unknown-attributes
           -Wno-weak-vtables
+          -Wno-poison-system-directories
         >
         $<$<CXX_COMPILER_ID:GNU>:
           -Wdouble-promotion # float implicit to double

tests/CMakeLists.txt.in

@@ -1,10 +1,10 @@
-cmake_minimum_required(VERSION 3.0.2)
+cmake_minimum_required(VERSION 3.13)
 project(googletest-download NONE)
 
 include(ExternalProject)
 ExternalProject_Add(googletest
   GIT_REPOSITORY    https://github.com/google/googletest.git
-  GIT_TAG           1b18723e874b256c1e39378c6774a90701d70f7a
+  GIT_TAG           v1.14.0
   SOURCE_DIR        "${CMAKE_CURRENT_BINARY_DIR}/googletest-src"
   BINARY_DIR        "${CMAKE_CURRENT_BINARY_DIR}/googletest-build"
   CONFIGURE_COMMAND ""

tests/algorithm_tests.cpp

@@ -188,7 +188,7 @@ TEST(algorithm_tests, incompatible_type)
     span<int> src_span_dyn(src);
     span<int, 4> src_span_static(src);
     span<int*> dst_span_dyn(dst);
-    span<int*, 4> dst_span_static(dst);
+    span<int*, 4> dst_span_static(gsl::make_span(dst));
 
     // every line should produce a compilation error
     copy(src_span_dyn, dst_span_dyn);

tests/at_tests.cpp

@@ -32,7 +32,7 @@
 TEST(at_tests, static_array)
 {
     int a[4] = {1, 2, 3, 4};
-    const int(&c_a)[4] = a;
+    const int (&c_a)[4] = a;
 
     for (int i = 0; i < 4; ++i)
     {
@@ -152,7 +152,7 @@ TEST(at_tests, std_span)
 static constexpr bool test_constexpr()
 {
     int a1[4] = {1, 2, 3, 4};
-    const int(&c_a1)[4] = a1;
+    const int (&c_a1)[4] = a1;
     std::array<int, 4> a2 = {1, 2, 3, 4};
     const std::array<int, 4>& c_a2 = a2;
 

tests/byte_tests.cpp

@@ -16,8 +16,12 @@
 
 #include <gtest/gtest.h>
 
+#define GSL_USE_STD_BYTE 0
 #include <gsl/byte> // for to_byte, to_integer, byte, operator&, ope...
 
+#include <type_traits>
+#include <utility>
+
 using namespace std;
 using namespace gsl;
 
@@ -33,28 +37,28 @@ int modify_both(gsl::byte& b, int& i)
 TEST(byte_tests, construction)
 {
     {
-        const byte b = static_cast<byte>(4);
+        const gsl::byte b = static_cast<gsl::byte>(4);
         EXPECT_TRUE(static_cast<unsigned char>(b) == 4);
     }
 
     {
-        const byte b = byte(12);
+        const gsl::byte b = gsl::byte(12);
         EXPECT_TRUE(static_cast<unsigned char>(b) == 12);
     }
 
     {
-        const byte b = to_byte<12>();
+        const gsl::byte b = to_byte<12>();
         EXPECT_TRUE(static_cast<unsigned char>(b) == 12);
     }
     {
         const unsigned char uc = 12;
-        const byte b = to_byte(uc);
+        const gsl::byte b = to_byte(uc);
         EXPECT_TRUE(static_cast<unsigned char>(b) == 12);
     }
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
     {
-        const byte b{14};
+        const gsl::byte b{14};
         EXPECT_TRUE(static_cast<unsigned char>(b) == 14);
     }
 #endif
@@ -63,14 +67,16 @@ TEST(byte_tests, construction)
     to_byte(char{});
     to_byte(3);
     to_byte(3u);
+    to_byte<-1>();
+    to_byte<256u>();
 #endif
 }
 
 TEST(byte_tests, bitwise_operations)
 {
-    const byte b = to_byte<0xFF>();
+    const gsl::byte b = to_byte<0xFF>();
 
-    byte a = to_byte<0x00>();
+    gsl::byte a = to_byte<0x00>();
     EXPECT_TRUE((b | a) == to_byte<0xFF>());
     EXPECT_TRUE(a == to_byte<0x00>());
 
@@ -104,7 +110,7 @@ TEST(byte_tests, bitwise_operations)
 
 TEST(byte_tests, to_integer)
 {
-    const byte b = to_byte<0x12>();
+    const gsl::byte b = to_byte<0x12>();
 
     EXPECT_TRUE(0x12 == gsl::to_integer<char>(b));
     EXPECT_TRUE(0x12 == gsl::to_integer<short>(b));
@@ -123,12 +129,50 @@ TEST(byte_tests, to_integer)
 TEST(byte_tests, aliasing)
 {
     int i{0};
-    const int res = modify_both(reinterpret_cast<byte&>(i), i);
+    const int res = modify_both(reinterpret_cast<gsl::byte&>(i), i);
     EXPECT_TRUE(res == i);
 }
 
-} // namespace
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-copy(src_span_static, dst_span_static);
-#endif
+template <typename U, typename = void>
+static constexpr bool LShiftCompilesFor = false;
+template <typename U>
+static constexpr bool LShiftCompilesFor<
+    U, void_t<decltype(gsl::operator<< <float>(declval<gsl::byte>(), declval<U>()))>> = true;
+static_assert(!LShiftCompilesFor<float>, "!LShiftCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool RShiftCompilesFor = false;
+template <typename U>
+static constexpr bool RShiftCompilesFor<
+    U, void_t<decltype(gsl::operator>> <U>(declval<gsl::byte>(), declval<U>()))>> = true;
+static_assert(!RShiftCompilesFor<float>, "!RShiftCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool LShiftAssignCompilesFor = false;
+template <typename U>
+static constexpr bool LShiftAssignCompilesFor<
+    U, void_t<decltype(gsl::operator<<= <U>(declval<gsl::byte&>(), declval<U>()))>> = true;
+static_assert(!LShiftAssignCompilesFor<float>, "!LShiftAssignCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool RShiftAssignCompilesFor = false;
+template <typename U>
+static constexpr bool RShiftAssignCompilesFor<
+    U, void_t<decltype(gsl::operator>>= <U>(declval<gsl::byte&>(), declval<U>()))>> = true;
+static_assert(!RShiftAssignCompilesFor<float>, "!RShiftAssignCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool ToIntegerCompilesFor = false;
+template <typename U>
+static constexpr bool ToIntegerCompilesFor<U, void_t<decltype(gsl::to_integer<U>(gsl::byte{}))>> =
+    true;
+static_assert(!ToIntegerCompilesFor<float>, "!ToIntegerCompilesFor<float>");
+
+} // namespace

tests/constexpr_notnull_tests.cpp

@@ -0,0 +1,75 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2025 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#include <gsl/pointers> // for not_null
+#include <gtest/gtest.h>
+
+#include <type_traits> // for declval
+
+using namespace gsl;
+
+namespace
+{
+constexpr bool comparison_test(const int* ptr1, const int* ptr2)
+{
+    const not_null<const int*> p1(ptr1);
+    const not_null<const int*> p1_same(ptr1);
+    const not_null<const int*> p2(ptr2);
+
+    // Testing operator==
+    const bool eq_result = (p1 == p1_same); // Should be true
+    const bool neq_result = (p1 != p2);     // Should be true
+
+    // Testing operator<= and operator>=
+    const bool le_result = (p1 <= p1_same); // Should be true
+    const bool ge_result = (p1 >= p1_same); // Should be true
+
+    // The exact comparison results will depend on pointer ordering,
+    // but we can verify that the basic equality checks work as expected
+    return eq_result && neq_result && le_result && ge_result;
+}
+
+constexpr bool workaround_test(const int* ptr1, const int* ptr2)
+{
+    const not_null<const int*> p1(ptr1);
+    const not_null<const int*> p1_same(ptr1);
+    const not_null<const int*> p2(ptr2);
+
+    // Using .get() to compare
+    const bool eq_result = (p1.get() == p1_same.get()); // Should be true
+    const bool neq_result = (p1.get() != p2.get());     // Should be true
+
+    return eq_result && neq_result;
+}
+} // namespace
+
+constexpr int test_value1 = 1;
+constexpr int test_value2 = 2;
+
+static_assert(comparison_test(&test_value1, &test_value2),
+              "not_null comparison operators should be constexpr");
+static_assert(workaround_test(&test_value1, &test_value2),
+              "not_null .get() comparison workaround should work");
+
+TEST(notnull_constexpr_tests, TestNotNullConstexprComparison)
+{
+    // This test simply verifies that the constexpr functions compile and run
+    // If we got here, it means the constexpr comparison operators are working
+    static const int value1 = 1;
+    static const int value2 = 2;
+    EXPECT_TRUE(comparison_test(&value1, &value2));
+    EXPECT_TRUE(workaround_test(&value1, &value2));
+}

tests/dyn_array_tests.cpp

@@ -0,0 +1,684 @@
+#include <gtest/gtest.h>
+
+#include "deathTestCommon.h"
+#include "gsl/dyn_array"
+#include <cstdlib>
+#include <exception>
+#include <gsl/dyn_array>
+#include <gsl/util>
+#include <iostream>
+#include <sstream>
+#include <type_traits>
+
+// Despite using <algorithm> and <ranges> utilities in this test, they
+// are not being included directly by this file as a test to ensure
+// transitive inclusion via <gsl/dyn_array>.
+
+static_assert(sizeof(gsl::dyn_array<int>) == 2 * sizeof(void*),
+              "gsl::dyn_array (with the default allocator) should be 16 bytes");
+
+#if defined(__cpp_lib_concepts) && (__cpp_lib_concepts >= 202002L)
+static_assert(std::input_iterator<gsl::dyn_array<int>::iterator>,
+              "gsl::dyn_array should expose a valid input_iterator");
+#endif /* __cpp_lib_concepts >= 202002L */
+
+#if defined(__cpp_lib_ranges) && (__cpp_lib_ranges >= 201911L)
+static_assert(std::ranges::input_range<gsl::dyn_array<int>>,
+              "gsl::dyn_array should be a valid input range");
+#endif /* __cpp_lib_ranges >= 201911L */
+
+TEST(dyn_array_tests, default_ctor)
+{
+    gsl::dyn_array<char> diamondbacks;
+    EXPECT_TRUE(diamondbacks.empty());
+    EXPECT_EQ(diamondbacks.size(), 0);
+    EXPECT_EQ(diamondbacks.data(), nullptr);
+}
+
+TEST(dyn_array_tests, count_ctor)
+{
+    gsl::dyn_array<char> athletics(10);
+    EXPECT_FALSE(athletics.empty());
+    EXPECT_EQ(athletics.size(), 10);
+    EXPECT_NE(athletics.data(), nullptr);
+
+    gsl::dyn_array<char> braves(0);
+    EXPECT_TRUE(braves.empty());
+    EXPECT_EQ(braves.size(), 0);
+    EXPECT_EQ(braves.data(), nullptr);
+    EXPECT_TRUE(std::all_of(braves.begin(), braves.end(), [](char c) { return c == char{}; }));
+}
+
+TEST(dyn_array_tests, count_value_ctor)
+{
+    gsl::dyn_array<char> orioles(10, 'c');
+    EXPECT_FALSE(orioles.empty());
+    EXPECT_EQ(orioles.size(), 10);
+    EXPECT_NE(orioles.data(), nullptr);
+    EXPECT_TRUE(std::all_of(orioles.begin(), orioles.end(), [](char c) { return c == 'c'; }));
+
+    gsl::dyn_array<int> redsox(10, 42);
+    EXPECT_FALSE(redsox.empty());
+    EXPECT_EQ(redsox.size(), 10);
+    EXPECT_NE(redsox.data(), nullptr);
+    EXPECT_TRUE(std::all_of(redsox.begin(), redsox.end(), [](int i) { return i == 42; }));
+}
+
+TEST(dyn_array_tests, inputit_ctor)
+{
+    std::vector<char> cubs(10, 'c');
+    gsl::dyn_array<char> whitesox(cubs.begin(), cubs.end());
+    EXPECT_FALSE(whitesox.empty());
+    EXPECT_EQ(whitesox.size(), cubs.size());
+    EXPECT_NE(whitesox.data(), nullptr);
+    EXPECT_TRUE(std::all_of(whitesox.begin(), whitesox.end(), [](char c) { return c == 'c'; }));
+}
+
+TEST(dyn_array_tests, copy_ctor)
+{
+    gsl::dyn_array<char> reds(10, 'c');
+    gsl::dyn_array<char> guardians(reds);
+    EXPECT_FALSE(guardians.empty());
+    EXPECT_EQ(guardians.size(), reds.size());
+    EXPECT_NE(guardians.data(), nullptr);
+    EXPECT_TRUE(std::all_of(guardians.begin(), guardians.end(), [](char c) { return c == 'c'; }));
+}
+
+TEST(dyn_array_tests, access_operator)
+{
+    gsl::dyn_array<char> rockies(10, 'c');
+    using ST = typename decltype(rockies)::size_type;
+    for (int i = 0; i < gsl::narrow<int>(rockies.size()); i++)
+        EXPECT_EQ(rockies[gsl::narrow<ST>(i)], 'c');
+    for (int i = 0; i < gsl::narrow<int>(rockies.size()); i++) rockies[gsl::narrow<ST>(i)] = 'r';
+    for (int i = 0; i < gsl::narrow<int>(rockies.size()); i++)
+        EXPECT_EQ(rockies[gsl::narrow<ST>(i)], 'r');
+    gsl::dyn_array<int> tigers(10);
+    for (int i = 0; i < gsl::narrow<int>(tigers.size()); i++) tigers[gsl::narrow<ST>(i)] = i;
+    for (int i = 0; i < gsl::narrow<int>(tigers.size()); i++)
+        EXPECT_EQ(tigers[gsl::narrow<ST>(i)], i);
+}
+
+TEST(dyn_array_tests, iterators)
+{
+    gsl::dyn_array<char> astros(10, 'c');
+    for (auto it = astros.begin(); it != astros.end(); it++) EXPECT_EQ(*it, 'c');
+    for (auto it = astros.begin(); it != astros.end(); it++) *it = 'r';
+    for (auto it = astros.begin(); it != astros.end(); it++) EXPECT_EQ(*it, 'r');
+    EXPECT_TRUE(std::all_of(astros.begin(), astros.end(), [](char c) { return c == 'r'; }));
+
+    gsl::dyn_array<char> royals(10, 'c');
+    for (auto it = royals.begin(); it != royals.end(); ++it) EXPECT_EQ(*it, 'c');
+    for (auto it = royals.begin(); it != royals.end(); ++it) *it = 'r';
+    for (auto it = royals.begin(); it != royals.end(); ++it) EXPECT_EQ(*it, 'r');
+    EXPECT_TRUE(std::all_of(royals.begin(), royals.end(), [](char c) { return c == 'r'; }));
+}
+
+TEST(dyn_array_tests, range_for)
+{
+    gsl::dyn_array<char> angels(10, 'c');
+    for (auto x : angels) EXPECT_EQ(x, 'c');
+    for (auto& x : angels) x = 'r';
+    for (auto x : angels) EXPECT_EQ(x, 'r');
+    EXPECT_TRUE(std::all_of(angels.begin(), angels.end(), [](char c) { return c == 'r'; }));
+}
+
+TEST(dyn_array_tests, use_std_algorithms)
+{
+    gsl::dyn_array<char> dodgers(26);
+    std::generate(dodgers.begin(), dodgers.end(), [i = 0]() mutable { return 'a' + i++; });
+    char ch = 'a';
+    for (auto x : dodgers) EXPECT_EQ(x, ch++);
+    EXPECT_EQ(std::find(dodgers.begin(), dodgers.end(), 'a'), dodgers.begin());
+    {
+        auto it = std::find(dodgers.begin(), dodgers.end(), 'c');
+        EXPECT_EQ(std::distance(dodgers.begin(), it), 'c' - 'a');
+        EXPECT_EQ(std::distance(it, dodgers.begin()), 'a' - 'c');
+    }
+    {
+        auto it = std::lower_bound(dodgers.begin(), dodgers.end(), 'j');
+        EXPECT_EQ(*it, 'j');
+        EXPECT_EQ(std::distance(dodgers.begin(), it), 'j' - 'a');
+        EXPECT_EQ(std::distance(it, dodgers.begin()), 'a' - 'j');
+    }
+    EXPECT_EQ(dodgers.begin(), std::begin(dodgers));
+    EXPECT_EQ(dodgers.end(), std::end(dodgers));
+}
+
+#if defined(__cpp_lib_constexpr_dynamic_alloc) && (__cpp_lib_constexpr_dynamic_alloc >= 201907L)
+constexpr auto default_constructed_count_dyn_array_is_constexpr()
+{
+    gsl::dyn_array<int> values(3);
+    return values.size() == 3 && values[0] == 0 && values[1] == 0 && values[2] == 0;
+}
+
+TEST(dyn_array_tests, constexprness)
+{
+    constexpr gsl::dyn_array<char> marlins;
+    static_assert(marlins == marlins);
+    static_assert(marlins.empty());
+    static_assert(marlins.size() == 0);
+    static_assert(marlins.data() == nullptr);
+    static_assert(marlins.begin() == marlins.end());
+    static_assert(std::distance(marlins.begin(), marlins.end()) == 0);
+    static_assert(default_constructed_count_dyn_array_is_constexpr());
+}
+#endif /* __cpp_lib_constexpr_dynamic_alloc >= 201907L */
+
+#if defined(__cpp_lib_ranges) && (__cpp_lib_ranges >= 201911L)
+TEST(dyn_array_tests, ranges)
+{
+    gsl::dyn_array<char> brewers(26);
+    std::ranges::generate(brewers, [c = 'a']() mutable { return c++; });
+    char ch = 'a';
+    for (auto x : brewers) EXPECT_EQ(x, ch++);
+    EXPECT_EQ(std::ranges::find(brewers, 'a'), std::ranges::begin(brewers));
+    {
+        auto it = std::ranges::find(brewers, 'c');
+        EXPECT_EQ(std::ranges::distance(std::ranges::begin(brewers), it), 'c' - 'a');
+        EXPECT_EQ(std::ranges::distance(it, std::ranges::begin(brewers)), 'a' - 'c');
+    }
+
+#if defined(__cpp_lib_containers_ranges) && (__cpp_lib_containers_ranges >= 202202L)
+    std::vector<char> twins(10, 'c');
+    gsl::dyn_array<char> mets(std::from_range, twins);
+    EXPECT_EQ(twins.size(), mets.size());
+    EXPECT_TRUE(std::ranges::all_of(mets, [](char c) { return c == 'c'; }));
+#endif /* __cpp_lib_containers_ranges >= 202202L */
+}
+#endif /* __cpp_lib_ranges >= 201911L */
+
+#if defined(__cpp_lib_constexpr_dynamic_alloc) && (__cpp_lib_constexpr_dynamic_alloc >= 201907L)
+template <typename T, unsigned N>
+struct ConstexprAllocator
+{
+    using value_type = T;
+
+    T buf[N]{};
+    std::size_t sz{};
+
+    constexpr ConstexprAllocator() = default;
+
+    template <typename U>
+    constexpr ConstexprAllocator(const ConstexprAllocator<U, N>&) noexcept : buf{}, sz{}
+    {}
+
+    template <typename U>
+    struct rebind
+    {
+        using other = ConstexprAllocator<U, N>;
+    };
+
+    constexpr auto allocate(std::size_t n) -> value_type*
+    {
+        auto addr = &buf[sz];
+        sz += n;
+        return addr;
+    }
+
+    constexpr void deallocate(value_type*, std::size_t) noexcept {}
+};
+
+template <typename T1, unsigned N1, typename T2, unsigned N2>
+constexpr auto operator==(const ConstexprAllocator<T1, N1>& lhs,
+                          const ConstexprAllocator<T2, N2>& rhs) noexcept
+{
+    return std::addressof(lhs) == std::addressof(rhs);
+}
+
+template <typename T1, unsigned N1, typename T2, unsigned N2>
+constexpr auto operator!=(const ConstexprAllocator<T1, N1>& lhs,
+                          const ConstexprAllocator<T2, N2>& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+#endif /* __cpp_lib_constexpr_dynamic_alloc >= 201907L */
+
+template <typename T>
+static int AllocCounter = 0;
+
+template <typename T>
+static int DeallocCounter = 0;
+
+struct LifetimeCounter
+{
+    static int alive_count;
+
+    int value{};
+
+    explicit LifetimeCounter(int v = 0) : value(v) { ++alive_count; }
+
+    LifetimeCounter(const LifetimeCounter& other) : value(other.value) { ++alive_count; }
+
+    ~LifetimeCounter() { --alive_count; }
+};
+
+int LifetimeCounter::alive_count = 0;
+
+struct DefaultConstructionCounter
+{
+    static int default_constructor_count;
+    static int copy_constructor_count;
+
+    int value{};
+
+    DefaultConstructionCounter() { ++default_constructor_count; }
+
+    DefaultConstructionCounter(const DefaultConstructionCounter& other) : value(other.value)
+    {
+        ++copy_constructor_count;
+    }
+
+    static void reset()
+    {
+        default_constructor_count = 0;
+        copy_constructor_count = 0;
+    }
+};
+
+int DefaultConstructionCounter::default_constructor_count = 0;
+int DefaultConstructionCounter::copy_constructor_count = 0;
+
+struct DefaultOnlyElement
+{
+    DefaultOnlyElement() = default;
+    DefaultOnlyElement(const DefaultOnlyElement&) = delete;
+    DefaultOnlyElement& operator=(const DefaultOnlyElement&) = delete;
+    DefaultOnlyElement(DefaultOnlyElement&&) = delete;
+    DefaultOnlyElement& operator=(DefaultOnlyElement&&) = delete;
+};
+
+struct ThrowOnCopy
+{
+    static int alive_count;
+    static int copy_count;
+    static int throw_on_copy_index;
+
+    int value{};
+
+    explicit ThrowOnCopy(int v = 0) : value(v) { ++alive_count; }
+
+    ThrowOnCopy(const ThrowOnCopy& other) : value(other.value)
+    {
+        if (copy_count == throw_on_copy_index)
+        {
+            ++copy_count;
+            throw 42;
+        }
+        ++copy_count;
+        ++alive_count;
+    }
+
+    ~ThrowOnCopy() { --alive_count; }
+};
+
+int ThrowOnCopy::alive_count = 0;
+int ThrowOnCopy::copy_count = 0;
+int ThrowOnCopy::throw_on_copy_index = -1;
+
+template <typename T>
+class Newocator
+{
+public:
+    using value_type = T;
+
+    Newocator() = default;
+
+    template <typename U>
+    Newocator(const Newocator<U>&) noexcept
+    {}
+
+    static void init()
+    {
+        AllocCounter<Newocator<T>> = 0;
+        DeallocCounter<Newocator<T>> = 0;
+    }
+
+    static void check() { EXPECT_EQ(AllocCounter<Newocator<T>>, DeallocCounter<Newocator<T>>); }
+
+    auto allocate(std::size_t n) -> value_type*
+    {
+        AllocCounter<Newocator<T>> ++;
+        return static_cast<value_type*>(::operator new(n * sizeof(value_type)));
+    }
+
+    void deallocate(value_type* p, std::size_t) noexcept
+    {
+        DeallocCounter<Newocator<T>> ++;
+        ::operator delete(p);
+    }
+
+    template <typename U>
+    struct rebind
+    {
+        using other = Newocator<U>;
+    };
+};
+
+template <typename T, typename U>
+constexpr auto operator==(const Newocator<T>&, const Newocator<U>&) noexcept
+{
+    return true;
+}
+
+template <typename T, typename U>
+constexpr auto operator!=(const Newocator<T>& lhs, const Newocator<U>& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+template <typename T>
+class OwnershipTrackingAllocator
+{
+public:
+    using value_type = T;
+
+    OwnershipTrackingAllocator() noexcept : owner_id(next_owner_id()) { ++next_owner_id(); }
+
+    explicit OwnershipTrackingAllocator(int owner) noexcept : owner_id(owner) {}
+
+    template <typename U>
+    OwnershipTrackingAllocator(const OwnershipTrackingAllocator<U>& other) noexcept
+        : owner_id(other.owner())
+    {}
+
+    auto allocate(std::size_t count) -> value_type*
+    {
+        static_assert(alignof(value_type) <= alignof(int),
+                      "test allocator only supports types with int-or-smaller alignment");
+        auto raw =
+            static_cast<unsigned char*>(::operator new(sizeof(int) + count * sizeof(value_type)));
+        *reinterpret_cast<int*>(raw) = owner_id;
+        ++allocation_count();
+        return reinterpret_cast<value_type*>(raw + sizeof(int));
+    }
+
+    void deallocate(value_type* pointer, std::size_t) noexcept
+    {
+        auto raw = reinterpret_cast<unsigned char*>(pointer) - sizeof(int);
+        if (*reinterpret_cast<int*>(raw) != owner_id) { ++mismatched_deallocation_count(); }
+        ++deallocation_count();
+        ::operator delete(raw);
+    }
+
+    auto owner() const noexcept { return owner_id; }
+
+    static void reset()
+    {
+        next_owner_id() = 1;
+        allocation_count() = 0;
+        deallocation_count() = 0;
+        mismatched_deallocation_count() = 0;
+    }
+
+    static auto mismatched_deallocations() { return mismatched_deallocation_count(); }
+
+private:
+    int owner_id;
+
+    static auto next_owner_id() -> int&
+    {
+        static int value = 1;
+        return value;
+    }
+
+    static auto allocation_count() -> int&
+    {
+        static int value = 0;
+        return value;
+    }
+
+    static auto deallocation_count() -> int&
+    {
+        static int value = 0;
+        return value;
+    }
+
+    static auto mismatched_deallocation_count() -> int&
+    {
+        static int value = 0;
+        return value;
+    }
+};
+
+template <typename T, typename U>
+constexpr auto operator==(const OwnershipTrackingAllocator<T>& lhs,
+                          const OwnershipTrackingAllocator<U>& rhs) noexcept
+{
+    return lhs.owner() == rhs.owner();
+}
+
+template <typename T, typename U>
+constexpr auto operator!=(const OwnershipTrackingAllocator<T>& lhs,
+                          const OwnershipTrackingAllocator<U>& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+TEST(dyn_array_tests, custom_allocator_models_allocator)
+{
+    using traits = std::allocator_traits<Newocator<char>>;
+    using ptr = traits::pointer;
+
+    static_assert(std::is_same<traits::value_type, char>::value, "allocator trait type mismatch");
+    static_assert(std::is_same<ptr, char*>::value, "allocator trait type mismatch");
+
+    Newocator<char> alloc;
+    auto p = traits::allocate(alloc, 1);
+    traits::deallocate(alloc, p, 1);
+
+#if defined(__cpp_lib_constexpr_dynamic_alloc) && (__cpp_lib_constexpr_dynamic_alloc >= 201907L)
+    using constexpr_traits = std::allocator_traits<ConstexprAllocator<char, 10>>;
+    static_assert(std::is_same<constexpr_traits::value_type, char>::value,
+                  "allocator trait type mismatch");
+#endif /* __cpp_lib_constexpr_dynamic_alloc >= 201907L */
+}
+
+TEST(dyn_array_tests, custom_allocator)
+{
+#if defined(__cpp_lib_constexpr_dynamic_alloc) && (__cpp_lib_constexpr_dynamic_alloc >= 201907L)
+    static constexpr gsl::dyn_array<char, ConstexprAllocator<char, 10>> mets(10, 'c');
+    static_assert(mets.size() == 10);
+    static_assert(mets[0] == 'c');
+    static_assert(std::all_of(std::begin(mets), std::end(mets), [](char c) { return c == 'c'; }));
+#endif /* __cpp_lib_constexpr_dynamic_alloc >= 201907L */
+
+    Newocator<char>::init();
+    {
+        gsl::dyn_array<char, Newocator<char>> yankees(10, 'c');
+        EXPECT_EQ(yankees.size(), 10);
+        EXPECT_TRUE(
+            std::all_of(std::begin(yankees), std::end(yankees), [](char c) { return c == 'c'; }));
+        yankees[0] = 'a';
+        yankees[1] = 'b';
+        EXPECT_EQ(yankees[0], 'a');
+        EXPECT_EQ(yankees[1], 'b');
+        EXPECT_EQ(yankees[2], 'c');
+        yankees.get_allocator().deallocate(yankees.get_allocator().allocate(1), 1);
+    }
+    Newocator<char>::check();
+}
+
+TEST(dyn_array_tests, non_trivial_elements_are_destroyed)
+{
+    LifetimeCounter::alive_count = 0;
+
+    {
+        gsl::dyn_array<LifetimeCounter> values(5, LifetimeCounter{7});
+        EXPECT_EQ(values.size(), 5);
+        EXPECT_EQ(LifetimeCounter::alive_count, 5);
+    }
+
+    EXPECT_EQ(LifetimeCounter::alive_count, 0);
+}
+
+TEST(dyn_array_tests, count_constructor_default_constructs_each_element)
+{
+    DefaultConstructionCounter::reset();
+
+    {
+        gsl::dyn_array<DefaultConstructionCounter> values(4);
+        EXPECT_EQ(values.size(), 4);
+    }
+
+    EXPECT_EQ(DefaultConstructionCounter::default_constructor_count, 4);
+    EXPECT_EQ(DefaultConstructionCounter::copy_constructor_count, 0);
+}
+
+#ifdef GSL_DYN_ARRAY_COMPILE_FAILURE_TESTS
+TEST(dyn_array_compile_failure_tests, count_constructor_accepts_default_constructible_only_elements)
+{
+    gsl::dyn_array<DefaultOnlyElement> values(4);
+    EXPECT_EQ(values.size(), 4);
+}
+#endif /* GSL_DYN_ARRAY_COMPILE_FAILURE_TESTS */
+
+TEST(dyn_array_tests, failed_element_construction_rolls_back)
+{
+    ThrowOnCopy::alive_count = 0;
+    ThrowOnCopy::copy_count = 0;
+    ThrowOnCopy::throw_on_copy_index = 2;
+
+    EXPECT_THROW((gsl::dyn_array<ThrowOnCopy>(5, ThrowOnCopy{1})), int);
+    EXPECT_EQ(ThrowOnCopy::alive_count, 0);
+
+    ThrowOnCopy::throw_on_copy_index = -1;
+}
+
+TEST(dyn_array_tests, init_list)
+{
+    gsl::dyn_array<char> phillies = {'a', 'b', 'c'};
+    EXPECT_EQ(phillies.size(), 3);
+    EXPECT_EQ(phillies[0], 'a');
+    EXPECT_EQ(phillies[1], 'b');
+    EXPECT_EQ(phillies[2], 'c');
+}
+
+TEST(dyn_array_tests, const_operations)
+{
+    const gsl::dyn_array<char> pirates{'a', 'b', 'c', 'd'};
+    EXPECT_EQ(pirates.size(), 4);
+    EXPECT_EQ(pirates[0], 'a');
+}
+
+TEST(dyn_array_tests, reverse_iterator)
+{
+    const gsl::dyn_array<char> padres{'a', 'b', 'c'};
+    auto it = std::rbegin(padres);
+    EXPECT_EQ(*it++, 'c');
+    EXPECT_EQ(*it++, 'b');
+    EXPECT_EQ(*it++, 'a');
+}
+
+TEST(dyn_array_tests, random_access_iterator_arithmetic)
+{
+    gsl::dyn_array<char> bluejays{'a', 'b', 'c', 'd'};
+
+    auto first = bluejays.begin();
+    auto third = first + 2;
+
+    EXPECT_EQ(*third, 'c');
+    EXPECT_EQ(third - first, 2);
+    EXPECT_EQ(*(third - 1), 'b');
+    EXPECT_EQ(*std::prev(third), 'b');
+}
+
+TEST(dyn_array_tests, random_access_iterator_arithmetic_accepts_negative_offsets)
+{
+    gsl::dyn_array<char> bluejays{'a', 'b', 'c', 'd'};
+
+    auto third = bluejays.begin() + 2;
+    char previous{};
+    char next{};
+
+    EXPECT_NO_THROW(previous = *(third + -1));
+    EXPECT_EQ(previous, 'b');
+
+    EXPECT_NO_THROW(next = *(third - -1));
+    EXPECT_EQ(next, 'd');
+}
+
+TEST(dyn_array_tests, input_iterator_constructor)
+{
+    std::istringstream stream{"n a t s"};
+    std::istream_iterator<char> first{stream};
+    const std::istream_iterator<char> last{};
+
+    gsl::dyn_array<char> nationals(first, last);
+
+    ASSERT_EQ(nationals.size(), 4);
+    EXPECT_EQ(nationals[0], 'n');
+    EXPECT_EQ(nationals[1], 'a');
+    EXPECT_EQ(nationals[2], 't');
+    EXPECT_EQ(nationals[3], 's');
+}
+
+TEST(dyn_array_tests, contract_violations)
+{
+    const auto terminateHandler = std::set_terminate([] {
+        std::cerr << "Expected Death. dyn_array_contract_violations";
+        std::abort();
+    });
+    const auto expected = GetExpectedDeathString(terminateHandler);
+
+    gsl::dyn_array<char> values(3, 'v');
+    gsl::dyn_array<char> other(3, 'o');
+
+    EXPECT_DEATH(values[values.size()], expected);
+    EXPECT_DEATH((void) *values.end(), expected);
+    EXPECT_DEATH(++values.end(), expected);
+    EXPECT_DEATH(--values.begin(), expected);
+    EXPECT_DEATH((void) (values.begin() == other.begin()), expected);
+}
+
+#ifdef _MSC_VER
+TEST(dyn_array_tests, unchecked_iterators)
+{
+    gsl::dyn_array<char> values;
+    const gsl::dyn_array<char> const_values(3, 'v');
+
+    EXPECT_TRUE((std::is_same<decltype(const_values._Unchecked_begin()), const char*>::value));
+    EXPECT_TRUE((std::is_same<decltype(const_values._Unchecked_end()), const char*>::value));
+
+    std::size_t count = 0;
+    for (const auto value : const_values)
+    {
+        EXPECT_EQ(value, 'v');
+        ++count;
+    }
+    EXPECT_EQ(count, const_values.size());
+
+    EXPECT_EQ(values._Unchecked_begin(), nullptr);
+    EXPECT_EQ(values._Unchecked_end(), nullptr);
+}
+#endif /* _MSC_VER */
+
+TEST(DynArrayTests, TypeConsistency)
+{
+    static_assert(std::is_same<gsl::dyn_array<int>::value_type, int>::value, "Value type mismatch");
+    static_assert(std::is_same<gsl::dyn_array<int>::reference, int&>::value,
+                  "Reference type mismatch");
+    static_assert(std::is_same<gsl::dyn_array<int>::const_reference, const int&>::value,
+                  "Const reference type mismatch");
+    static_assert(std::is_same<gsl::dyn_array<int>::iterator::value_type, int>::value,
+                  "Iterator value type mismatch");
+    static_assert(std::is_same<gsl::dyn_array<int>::iterator::reference, int&>::value,
+                  "Iterator reference type mismatch");
+    static_assert(std::is_same<gsl::dyn_array<int>::iterator::const_reference, const int&>::value,
+                  "Iterator const reference type mismatch");
+    static_assert(std::is_same<gsl::dyn_array<int>::size_type, std::size_t>::value,
+                  "Size type mismatch");
+    static_assert(std::is_same<gsl::dyn_array<int>::difference_type, std::ptrdiff_t>::value,
+                  "Difference type mismatch");
+}
+
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
+TEST(dyn_array_tests, deduction_guides)
+{
+    std::vector<char> giants{10};
+#if defined(__cpp_lib_containers_ranges) && (__cpp_lib_containers_ranges >= 202202L)
+    gsl::dyn_array mariners(std::from_range, giants);
+#endif /* __cpp_lib_containers_ranges >= 202202L */
+    gsl::dyn_array cardinals(std::begin(giants), std::end(giants));
+}
+#endif /* __cpp_deduction_guides >= 201703L */

tests/notnull_tests.cpp

@@ -18,17 +18,25 @@
 
 #include <gsl/pointers> // for not_null, operator<, operator<=, operator>
 
-#include <algorithm> // for addressof
-#include <cstdint>   // for uint16_t
-#include <memory>    // for shared_ptr, make_shared, operator<, opera...
-#include <sstream>   // for operator<<, ostringstream, basic_ostream:...
-#include <string>    // for basic_string, operator==, string, operator<<
-#include <typeinfo>  // for type_info
-#include <variant>   // for variant, monostate, get
+#include <algorithm>   // for addressof
+#include <cstdint>     // for uint16_t
+#include <memory>      // for shared_ptr, make_shared, operator<, opera...
+#include <sstream>     // for operator<<, ostringstream, basic_ostream:...
+#include <string>      // for basic_string, operator==, string, operator<<
+#include <type_traits> // for declval
+#include <typeinfo>    // for type_info
+#include <variant>     // for variant, monostate, get
 
 #include "deathTestCommon.h"
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 struct MyBase
 {
 };
@@ -61,9 +69,7 @@ struct CustomPtr
 template <typename T, typename U>
 std::string operator==(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 {
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(type.1)
     return reinterpret_cast<const void*>(lhs.p_) == reinterpret_cast<const void*>(rhs.p_) ? "true"
                                                                                           : "false";
 }
@@ -71,9 +77,7 @@ std::string operator==(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 template <typename T, typename U>
 std::string operator!=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 {
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(type.1)
     return reinterpret_cast<const void*>(lhs.p_) != reinterpret_cast<const void*>(rhs.p_) ? "true"
                                                                                           : "false";
 }
@@ -81,9 +85,7 @@ std::string operator!=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 template <typename T, typename U>
 std::string operator<(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 {
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(type.1)
     return reinterpret_cast<const void*>(lhs.p_) < reinterpret_cast<const void*>(rhs.p_) ? "true"
                                                                                          : "false";
 }
@@ -91,9 +93,7 @@ std::string operator<(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 template <typename T, typename U>
 std::string operator>(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 {
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(type.1)
     return reinterpret_cast<const void*>(lhs.p_) > reinterpret_cast<const void*>(rhs.p_) ? "true"
                                                                                          : "false";
 }
@@ -101,9 +101,7 @@ std::string operator>(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 template <typename T, typename U>
 std::string operator<=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 {
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(type.1)
     return reinterpret_cast<const void*>(lhs.p_) <= reinterpret_cast<const void*>(rhs.p_) ? "true"
                                                                                           : "false";
 }
@@ -111,9 +109,7 @@ std::string operator<=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 template <typename T, typename U>
 std::string operator>=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
 {
-    // clang-format off
-    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-    // clang-format on
+    GSL_SUPPRESS(type.1)
     return reinterpret_cast<const void*>(lhs.p_) >= reinterpret_cast<const void*>(rhs.p_) ? "true"
                                                                                           : "false";
 }
@@ -129,28 +125,47 @@ struct NonCopyableNonMovable
 
 namespace
 {
-// clang-format off
-GSL_SUPPRESS(f .4) // NO-FORMAT: attribute
-// clang-format on
+GSL_SUPPRESS(f.4)
 bool helper(not_null<int*> p) { return *p == 12; }
-// clang-format off
-GSL_SUPPRESS(f .4) // NO-FORMAT: attribute
-// clang-format on
+GSL_SUPPRESS(f.4)
 bool helper_const(not_null<const int*> p) { return *p == 12; }
 
 int* return_pointer() { return nullptr; }
 } // namespace
 
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_A = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_A<U, void_t<decltype(gsl::not_null<void*>{std::declval<U>()})>> = true;
+
+template <typename U, int N, typename = void>
+static constexpr bool CtorCompilesFor_B = false;
+template <typename U, int N>
+static constexpr bool CtorCompilesFor_B<U, N, void_t<decltype(gsl::not_null<U>{N})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool DefaultCtorCompilesFor = false;
+template <typename U>
+static constexpr bool DefaultCtorCompilesFor<U, void_t<decltype(gsl::not_null<U>{})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_C = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_C<U, void_t<decltype(gsl::not_null<U*>{std::declval<std::unique_ptr<U>>()})>> =
+        true;
+
 TEST(notnull_tests, TestNotNullConstructors)
 {
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        not_null<int*> p = nullptr;          // yay...does not compile!
-        not_null<std::vector<char>*> p1 = 0; // yay...does not compile!
-        not_null<int*> p2;                   // yay...does not compile!
-        std::unique_ptr<int> up = std::make_unique<int>(120);
-        not_null<int*> p3 = up;
+        static_assert(CtorCompilesFor_A<void*>, "CtorCompilesFor_A<void*>");
+        static_assert(!CtorCompilesFor_A<std::nullptr_t>, "!CtorCompilesFor_A<std::nullptr_t>");
+        static_assert(!CtorCompilesFor_B<void*, 0>, "!CtorCompilesFor_B<void*, 0>");
+        static_assert(!DefaultCtorCompilesFor<void*>, "!DefaultCtorCompilesFor<void*>");
+        static_assert(!CtorCompilesFor_C<int>, "CtorCompilesFor_C<int>");
 
+#ifdef CONFIRM_COMPILATION_ERRORS
         // Forbid non-nullptr assignable types
         not_null<std::vector<int>> f(std::vector<int>{1});
         not_null<int> z(10);
@@ -178,6 +193,14 @@ TEST(notnull_tests, TestNotNullConstructors)
         EXPECT_DEATH((not_null<decltype(pi)>(pi)), expected);
     }
 
+    {
+        // from unique pointer
+        not_null<std::unique_ptr<int>> x(
+            std::make_unique<int>(10)); // unique_ptr<int> is nullptr assignable
+
+        EXPECT_DEATH((not_null<std::unique_ptr<int>>(std::unique_ptr<int>{})), expected);
+    }
+
     {
         // from pointer to local
         int t = 42;
@@ -268,6 +291,27 @@ TEST(notnull_tests, TestNotNullostream)
     ostream_helper<std::string>("string");
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool AssignmentCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    AssignmentCompilesFor<U, V,
+                          void_t<decltype(std::declval<gsl::not_null<U*>&>().operator=(
+                              std::declval<gsl::not_null<V*>&>()))>> = true;
+
+template <typename U, typename V, typename = void>
+static constexpr bool SCastCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    SCastCompilesFor<U, V, void_t<decltype(static_cast<U*>(std::declval<gsl::not_null<V*>&>()))>> =
+        true;
+
+template <typename U, typename V, typename = void>
+static constexpr bool RCastCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool RCastCompilesFor<
+    U, V, void_t<decltype(reinterpret_cast<U*>(std::declval<gsl::not_null<V*>&>()))>> = true;
+
 TEST(notnull_tests, TestNotNullCasting)
 {
     MyBase base;
@@ -280,15 +324,30 @@ TEST(notnull_tests, TestNotNullCasting)
     q = p; // allowed with heterogeneous copy ctor
     EXPECT_TRUE(q == p);
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    q = u; // no viable conversion possible between MyBase* and Unrelated*
-    p = q; // not possible to implicitly convert MyBase* to MyDerived*
+    static_assert(AssignmentCompilesFor<MyBase, MyDerived>,
+                  "AssignmentCompilesFor<MyBase, MyDerived>");
+    static_assert(!AssignmentCompilesFor<MyBase, Unrelated>,
+                  "!AssignmentCompilesFor<MyBase, Unrelated>");
+    static_assert(!AssignmentCompilesFor<Unrelated, MyDerived>,
+                  "!AssignmentCompilesFor<Unrelated, MyDerived>");
+    static_assert(!AssignmentCompilesFor<MyDerived, MyBase>,
+                  "!AssignmentCompilesFor<MyDerived, MyBase>");
+
+    static_assert(SCastCompilesFor<MyDerived, MyDerived>, "SCastCompilesFor<MyDerived, MyDerived>");
+    static_assert(SCastCompilesFor<MyBase, MyDerived>, "SCastCompilesFor<MyBase, MyDerived>");
+    static_assert(!SCastCompilesFor<MyDerived, MyBase>, "!SCastCompilesFor<MyDerived, MyBase>");
+    static_assert(!SCastCompilesFor<Unrelated, MyDerived>,
+                  "!SCastCompilesFor<Unrelated, MyDerived>");
+    static_assert(!RCastCompilesFor<MyDerived, MyDerived>,
+                  "!SCastCompilesFor<MyDerived, MyDerived>");
+    static_assert(!RCastCompilesFor<Unrelated, MyDerived>,
+                  "!SCastCompilesFor<Unrelated, MyDerived>");
 
-    not_null<Unrelated*> r = p;
-    not_null<Unrelated*> s = reinterpret_cast<Unrelated*>(p);
-#endif
     not_null<Unrelated*> t(reinterpret_cast<Unrelated*>(p.get()));
     EXPECT_TRUE(reinterpret_cast<void*>(p.get()) == reinterpret_cast<void*>(t.get()));
+
+    (void) static_cast<MyDerived*>(p);
+    (void) static_cast<MyBase*>(p);
 }
 
 TEST(notnull_tests, TestNotNullAssignment)
@@ -430,6 +489,18 @@ TEST(notnull_tests, TestNotNullCustomPtrComparison)
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
+template <typename U, typename = void>
+static constexpr bool TypeDeductionCtorCompilesFor = false;
+template <typename U>
+static constexpr bool
+    TypeDeductionCtorCompilesFor<U, void_t<decltype(not_null{std::declval<U>()})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool TypeDeductionHelperCompilesFor = false;
+template <typename U>
+static constexpr bool
+    TypeDeductionHelperCompilesFor<U, void_t<decltype(helper(not_null{std::declval<U>()}))>> = true;
+
 TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
 {
     {
@@ -446,9 +517,9 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         const int i = 42;
 
         not_null x{&i};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(not_null{&i});
-#endif
+        static_assert(TypeDeductionHelperCompilesFor<int*>, "TypeDeductionHelperCompilesFor<int*>");
+        static_assert(!TypeDeductionHelperCompilesFor<const int*>,
+                      "!TypeDeductionHelperCompilesFor<const int*>");
         helper_const(not_null{&i});
 
         EXPECT_TRUE(*x == 42);
@@ -470,9 +541,6 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         const int* p = &i;
 
         not_null x{p};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(not_null{p});
-#endif
         helper_const(not_null{p});
 
         EXPECT_TRUE(*x == 42);
@@ -507,12 +575,15 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         EXPECT_DEATH(helper_const(not_null{p}), expected);
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        not_null x{nullptr};
-        helper(not_null{nullptr});
-        helper_const(not_null{nullptr});
-    }
+    static_assert(TypeDeductionCtorCompilesFor<void*>, "TypeDeductionCtorCompilesFor<void*>");
+#if defined(_MSC_VER) && !defined(__clang__)
+    // Fails on gcc, clang, xcode, VS clang with
+    // "error : no type named 'type' in 'std::enable_if<false>'; 'enable_if' cannot be used to
+    // disable this declaration"
+    static_assert(!TypeDeductionCtorCompilesFor<std::nullptr_t>,
+                  "!TypeDeductionCtorCompilesFor<std::nullptr_t>");
+    static_assert(!TypeDeductionHelperCompilesFor<std::nullptr_t>,
+                  "!TypeDeductionHelperCompilesFor<std::nullptr_t>");
 #endif
 }
 
@@ -528,6 +599,11 @@ TEST(notnull_tests, TestVariantEmplace)
 }
 #endif // #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
+template <typename U, typename = void>
+static constexpr bool HelperCompilesFor = false;
+template <typename U>
+static constexpr bool HelperCompilesFor<U, void_t<decltype(helper(std::declval<U>()))>> = true;
+
 TEST(notnull_tests, TestMakeNotNull)
 {
     {
@@ -544,9 +620,8 @@ TEST(notnull_tests, TestMakeNotNull)
         const int i = 42;
 
         const auto x = make_not_null(&i);
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(make_not_null(&i));
-#endif
+        static_assert(HelperCompilesFor<gsl::not_null<int*>>,
+                      "HelperCompilesFor<gsl::not_null<int*>>");
         helper_const(make_not_null(&i));
 
         EXPECT_TRUE(*x == 42);
@@ -568,9 +643,8 @@ TEST(notnull_tests, TestMakeNotNull)
         const int* p = &i;
 
         const auto x = make_not_null(p);
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(make_not_null(p));
-#endif
+        static_assert(!HelperCompilesFor<gsl::not_null<const int*>>,
+                      "!HelperCompilesFor<gsl::not_null<const int*>>");
         helper_const(make_not_null(p));
 
         EXPECT_TRUE(*x == 42);
@@ -645,4 +719,18 @@ TEST(notnull_tests, TestStdHash)
         EXPECT_FALSE(hash_nn(nn) == hash_intptr(&y));
         EXPECT_FALSE(hash_nn(nn) == hash_intptr(nullptr));
     }
+
+    {
+        auto x = std::make_shared<int>(42);
+        auto y = std::make_shared<int>(99);
+        not_null<std::shared_ptr<int>> nn{x};
+        const not_null<std::shared_ptr<int>> cnn{x};
+
+        std::hash<not_null<std::shared_ptr<int>>> hash_nn;
+        std::hash<std::shared_ptr<int>> hash_sharedptr;
+
+        EXPECT_TRUE(hash_nn(nn) == hash_sharedptr(x));
+        EXPECT_FALSE(hash_nn(nn) == hash_sharedptr(y));
+        EXPECT_TRUE(hash_nn(cnn) == hash_sharedptr(x));
+    }
 }

tests/owner_tests.cpp

@@ -17,10 +17,11 @@
 #include <gtest/gtest.h>
 
 #include <gsl/pointers> // for owner
+#include <type_traits>  // for declval
 
 using namespace gsl;
 
-GSL_SUPPRESS(f .23) // NO-FORMAT: attribute
+GSL_SUPPRESS(f.23)
 void f(int* i) { *i += 1; }
 
 TEST(owner_tests, basic_test)
@@ -32,12 +33,17 @@ TEST(owner_tests, basic_test)
     delete p;
 }
 
-TEST(owner_tests, check_pointer_constraint)
-{
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        owner<int> integerTest = 10;
-        owner<std::shared_ptr<int>> sharedPtrTest(new int(10));
-    }
-#endif
-}
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
+template <typename U, typename = void>
+static constexpr bool OwnerCompilesFor = false;
+template <typename U>
+static constexpr bool OwnerCompilesFor<U, void_t<decltype(gsl::owner<U>{})>> = true;
+static_assert(OwnerCompilesFor<int*>, "OwnerCompilesFor<int*>");
+static_assert(!OwnerCompilesFor<int>, "!OwnerCompilesFor<int>");
+static_assert(!OwnerCompilesFor<std::shared_ptr<int>>, "!OwnerCompilesFor<std::shared_ptr<int>>");

tests/pointers_tests.cpp

@@ -0,0 +1,119 @@
+#include <gtest/gtest.h>
+
+#include <gsl/pointers>
+
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
+namespace
+{
+// Custom pointer type that can be used for gsl::not_null, but for which these cannot be swapped.
+struct NotMoveAssignableCustomPtr
+{
+    NotMoveAssignableCustomPtr() = default;
+    NotMoveAssignableCustomPtr(const NotMoveAssignableCustomPtr&) = default;
+    NotMoveAssignableCustomPtr& operator=(const NotMoveAssignableCustomPtr&) = default;
+    NotMoveAssignableCustomPtr(NotMoveAssignableCustomPtr&&) = default;
+    NotMoveAssignableCustomPtr& operator=(NotMoveAssignableCustomPtr&&) = delete;
+
+    bool operator!=(std::nullptr_t) const { return true; }
+
+    int dummy{}; // Without this clang warns, that NotMoveAssignableCustomPtr() is unneeded
+};
+
+template <typename U, typename = void>
+static constexpr bool SwapCompilesFor = false;
+template <typename U>
+static constexpr bool
+    SwapCompilesFor<U, void_t<decltype(gsl::swap<U>(std::declval<gsl::not_null<U>&>(),
+                                                    std::declval<gsl::not_null<U>&>()))>> = true;
+
+TEST(pointers_test, swap)
+{
+    // taken from gh-1129:
+    {
+        gsl::not_null<std::unique_ptr<int>> a(std::make_unique<int>(0));
+        gsl::not_null<std::unique_ptr<int>> b(std::make_unique<int>(1));
+
+        static_assert(noexcept(gsl::swap(a, b)),
+                      "not null unique_ptr should be noexcept-swappable");
+
+        EXPECT_TRUE(*a == 0);
+        EXPECT_TRUE(*b == 1);
+
+        gsl::swap(a, b);
+
+        EXPECT_TRUE(*a == 1);
+        EXPECT_TRUE(*b == 0);
+
+        // Make sure our custom ptr can be used with not_null. The shared_pr is to prevent "unused"
+        // compiler warnings.
+        const auto shared_custom_ptr{std::make_shared<NotMoveAssignableCustomPtr>()};
+        gsl::not_null<NotMoveAssignableCustomPtr> c{*shared_custom_ptr};
+        EXPECT_TRUE(c.get() != nullptr);
+    }
+
+    {
+        gsl::strict_not_null<std::unique_ptr<int>> a{std::make_unique<int>(0)};
+        gsl::strict_not_null<std::unique_ptr<int>> b{std::make_unique<int>(1)};
+
+        static_assert(noexcept(gsl::swap(a, b)),
+                      "strict not null unique_ptr should be noexcept-swappable");
+
+        EXPECT_TRUE(*a == 0);
+        EXPECT_TRUE(*b == 1);
+
+        gsl::swap(a, b);
+
+        EXPECT_TRUE(*a == 1);
+        EXPECT_TRUE(*b == 0);
+    }
+
+    {
+        gsl::not_null<std::unique_ptr<int>> a{std::make_unique<int>(0)};
+        gsl::strict_not_null<std::unique_ptr<int>> b{std::make_unique<int>(1)};
+
+        EXPECT_TRUE(*a == 0);
+        EXPECT_TRUE(*b == 1);
+
+        gsl::swap(a, b);
+
+        EXPECT_TRUE(*a == 1);
+        EXPECT_TRUE(*b == 0);
+    }
+
+    static_assert(!SwapCompilesFor<NotMoveAssignableCustomPtr>,
+                  "!SwapCompilesFor<NotMoveAssignableCustomPtr>");
+}
+
+TEST(pointers_test, member_types)
+{
+    static_assert(std::is_same<gsl::not_null<int*>::element_type, int*>::value,
+                  "check member type: element_type");
+}
+
+TEST(pointers_test, hash_noexcept_compiles)
+{
+    {
+        using Key = gsl::not_null<std::shared_ptr<int>>;
+        static_assert(noexcept(std::hash<Key>{}(std::declval<Key>())),
+                      "gsl::not_null hash operator must be noexcept");
+    }
+
+    {
+        using Key = gsl::strict_not_null<std::shared_ptr<int>>;
+        static_assert(noexcept(std::hash<Key>{}(std::declval<Key>())),
+                      "gsl::strict_not_null hash operator must be noexcept");
+    }
+}
+
+} // namespace

tests/span_compatibility_tests.cpp

@@ -61,10 +61,10 @@ void ArrayConvertibilityCheck()
 
         static_assert(std::is_same<decltype(gsl::span{stl_nullptr}), gsl::span<T*, 3>>::value,
                       "std::is_same< decltype(span{stl_nullptr}), span<T*, 3>>::value");
-        static_assert(
-            std::is_same<decltype(gsl::span{std::as_const(stl_nullptr)}), gsl::span<T* const, 3>>::value,
-            "std::is_same< decltype(span{std::as_const(stl_nullptr)}), span<T* const, "
-            "3>>::value");
+        static_assert(std::is_same<decltype(gsl::span{std::as_const(stl_nullptr)}),
+                                   gsl::span<T* const, 3>>::value,
+                      "std::is_same< decltype(span{std::as_const(stl_nullptr)}), span<T* const, "
+                      "3>>::value");
     }
 #endif
 }
@@ -1005,12 +1005,18 @@ static_assert(std::is_convertible<const std::array<int, 3>&, gsl::span<const int
               "std::is_convertible<const std::array<int, 3>&, gsl::span<const int>>");
 
 #if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 template <typename U, typename = void>
 static constexpr bool AsWritableBytesCompilesFor = false;
 
 template <typename U>
 static constexpr bool
-    AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(declval<U>()))>> = true;
+    AsWritableBytesCompilesFor<U, ::void_t<decltype(as_writable_bytes(declval<U>()))>> = true;
 
 static_assert(AsWritableBytesCompilesFor<gsl::span<int>>,
               "AsWritableBytesCompilesFor<gsl::span<int>>");
@@ -1020,4 +1026,3 @@ static_assert(!AsWritableBytesCompilesFor<gsl::span<const int>>,
               "!AsWritableBytesCompilesFor<gsl::span<const int>>");
 static_assert(!AsWritableBytesCompilesFor<gsl::span<const int, 9>>,
               "!AsWritableBytesCompilesFor<gsl::span<const int, 9>>");
-#endif // __cplusplus >= 201703l

tests/span_ext_tests.cpp

@@ -114,7 +114,7 @@ TEST(span_ext_test, make_span_from_array_constructor)
 
 TEST(span_ext_test, make_span_from_dynamic_array_constructor)
 {
-    double(*arr)[3][4] = new double[100][3][4];
+    double (*arr)[3][4] = new double[100][3][4];
 
     {
         auto s = make_span(&arr[0][0][0], 10);

tests/span_tests.cpp

@@ -32,14 +32,14 @@
 #include <vector> // for vector
 
 // the string_view include and macro are used in the deduction guide verification
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
 #ifdef __has_include
 #if __has_include(<string_view>)
 #include <string_view>
 #define HAS_STRING_VIEW
 #endif // __has_include(<string_view>)
 #endif // __has_include
-#endif // (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#endif // defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
 #if defined(__cplusplus) && __cplusplus >= 202002L
 #include <span>
 #endif // __cplusplus >= 202002L
@@ -48,6 +48,13 @@
 
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 namespace
 {
 
@@ -62,8 +69,7 @@ struct AddressOverloaded
 #if (__cplusplus > 201402L)
     [[maybe_unused]]
 #endif
-    AddressOverloaded
-    operator&() const
+    AddressOverloaded operator&() const
     {
         return {};
     }
@@ -216,6 +222,12 @@ TEST(span_test, from_pointer_length_constructor)
 
 TEST(span_test, from_pointer_pointer_construction)
 {
+    // const auto terminateHandler = std::set_terminate([] {
+    //     std::cerr << "Expected Death. from_pointer_pointer_construction";
+    //     std::abort();
+    // });
+    // const auto expected = GetExpectedDeathString(terminateHandler);
+
     int arr[4] = {1, 2, 3, 4};
 
     {
@@ -245,19 +257,13 @@ TEST(span_test, from_pointer_pointer_construction)
         EXPECT_TRUE(s.data() == &arr[0]);
     }
 
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
+    // this test succeeds on all platforms, gsl::span is more relaxed than std::span where this
+    // would be UB
     //{
     //    auto workaround_macro = [&]() { span<int> s{&arr[1], &arr[0]}; };
     //    EXPECT_DEATH(workaround_macro(), expected);
     //}
 
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
-    //    int* p = nullptr;
-    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
-    //    EXPECT_DEATH(workaround_macro(), expected);
-    //}
-
     {
         int* p = nullptr;
         span<int> s{p, p};
@@ -271,19 +277,21 @@ TEST(span_test, from_pointer_pointer_construction)
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
     }
-
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
-    //    int* p = nullptr;
-    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
-    //    EXPECT_DEATH(workaround_macro(), expected);
-    //}
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool CtorCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool CtorCompilesFor<U, V, void_t<decltype(U{std::declval<V>()})>> = true;
+
 TEST(span_test, from_array_constructor)
 {
     int arr[5] = {1, 2, 3, 4, 5};
 
+    static_assert(!CtorCompilesFor<span<int, 6>, int[5]>, "!CtorCompilesFor<span<int, 6>, int[5]>");
+    static_assert(!CtorCompilesFor<span<int, 0>, int[5]>, "!CtorCompilesFor<span<int, 0>, int[5]>");
+    static_assert(!CtorCompilesFor<span<int>, int[2][3]>, "!CtorCompilesFor<span<int>, int[2][3]>");
+
     {
         const span<int> s{arr};
         EXPECT_TRUE(s.size() == 5);
@@ -298,70 +306,28 @@ TEST(span_test, from_array_constructor)
 
     int arr2d[2][3] = {1, 2, 3, 4, 5, 6};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 6> s{arr};
-    }
+    static_assert(!CtorCompilesFor<span<int, 0>, int[2][3]>,
+                  "!CtorCompilesFor<span<int, 0>, int[2][3]>");
+    static_assert(!CtorCompilesFor<span<int, 6>, int[2][3]>,
+                  "!CtorCompilesFor<span<int, 6>, int[2][3]>");
 
-    {
-        span<int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr[0]);
-    }
-
-    {
-        span<int> s{arr2d};
-        EXPECT_TRUE(s.size() == 6);
-        EXPECT_TRUE(s.data() == &arr2d[0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[5] == 6);
-    }
-
-    {
-        span<int, 0> s{arr2d};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr2d[0][0]);
-    }
-
-    {
-        span<int, 6> s{arr2d};
-    }
-#endif
     {
         const span<int[3]> s{std::addressof(arr2d[0]), 1};
         EXPECT_TRUE(s.size() == 1);
         EXPECT_TRUE(s.data() == std::addressof(arr2d[0]));
     }
 
-    int arr3d[2][3][2] = { { {1, 2}, {3, 4}, {5, 6} }, { {7, 8}, {9, 10}, {11, 12} } };
+    int arr3d[2][3][2] = {{{1, 2}, {3, 4}, {5, 6}}, {{7, 8}, {9, 10}, {11, 12}}};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int> s{arr3d};
-        EXPECT_TRUE(s.size() == 12);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[11] == 12);
-    }
+    static_assert(!CtorCompilesFor<span<int>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 0>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 0>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 11>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 11>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 12>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 12>, int[2][3][2]>");
 
-    {
-        span<int, 0> s{arr3d};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-    }
-
-    {
-        span<int, 11> s{arr3d};
-    }
-
-    {
-        span<int, 12> s{arr3d};
-        EXPECT_TRUE(s.size() == 12);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[5] == 6);
-    }
-#endif
     {
         const span<int[3][2]> s{std::addressof(arr3d[0]), 1};
         EXPECT_TRUE(s.size() == 1);
@@ -378,7 +344,7 @@ TEST(span_test, from_array_constructor)
 
 TEST(span_test, from_dynamic_array_constructor)
 {
-    double(*arr)[3][4] = new double[100][3][4];
+    double (*arr)[3][4] = new double[100][3][4];
 
     {
         span<double> s(&arr[0][0][0], 10);
@@ -389,6 +355,13 @@ TEST(span_test, from_dynamic_array_constructor)
     delete[] arr;
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool ConversionCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    ConversionCompilesFor<U, V, void_t<decltype(std::declval<void (*)(U)>()(std::declval<V>()))>> =
+        true;
+
 TEST(span_test, from_std_array_constructor)
 {
     std::array<int, 4> arr = {1, 2, 3, 4};
@@ -428,43 +401,31 @@ TEST(span_test, from_std_array_constructor)
         EXPECT_TRUE(ao_arr.data() == fs.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
+    static_assert(!CtorCompilesFor<span<int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 2>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>");
 
-        span<const int, 2> cs{arr};
-        EXPECT_TRUE(cs.size() == 2);
-        EXPECT_TRUE(cs.data() == arr.data());
-    }
+    static_assert(!CtorCompilesFor<span<int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 0>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>");
 
-    {
-        span<int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
+    static_assert(!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>");
 
-        span<const int, 0> cs{arr};
-        EXPECT_TRUE(cs.size() == 0);
-        EXPECT_TRUE(cs.data() == arr.data());
-    }
-
-    {
-        span<int, 5> s{arr};
-    }
-
-    {
-        auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
-        auto take_a_span = [](span<int> s) { static_cast<void>(s); };
-        // try to take a temporary std::array
-        take_a_span(get_an_array());
-    }
+#if !defined(_MSC_VER) || (__cplusplus >= 201703L)
+    // Fails on MSVC. TODO: report a feedback bug.
+    static_assert(!ConversionCompilesFor<span<int>, std::array<int, 4>>,
+                  "!ConversionCompilesFor<span<int>, std::array<int, 4>>");
 #endif
 
     {
         auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
-        auto take_a_span = [](span<const int> s) { static_cast<void>(s); };
+        auto take_a_span = [](span<const int>) {};
         // try to take a temporary std::array
+        static_assert(ConversionCompilesFor<span<const int>, std::array<int, 4>>,
+                      "ConversionCompilesFor<span<const int>, std::array<int, 4>>");
         take_a_span(get_an_array());
     }
 }
@@ -493,23 +454,12 @@ TEST(span_test, from_const_std_array_constructor)
         EXPECT_TRUE(s.data() == ao_arr.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<const int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 5> s{arr};
-    }
-#endif
+    static_assert(!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>");
 
     {
         auto get_an_array = []() -> const std::array<int, 4> { return {1, 2, 3, 4}; };
@@ -535,27 +485,14 @@ TEST(span_test, from_std_array_const_constructor)
         EXPECT_TRUE(s.data() == arr.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<const int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 5> s{arr};
-    }
-
-    {
-        span<int, 4> s{arr};
-    }
-#endif
+    static_assert(!CtorCompilesFor<span<const int, 2>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 5>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 5>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<int, 5>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, const std::array<int, 4>&>");
 }
 
 TEST(span_test, from_container_constructor)
@@ -577,32 +514,28 @@ TEST(span_test, from_container_constructor)
     const std::string cstr = "hello";
 
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        span<char> s{str};
-        EXPECT_TRUE(s.size() == str.size());
-         EXPECT_TRUE(s.data() == str.data()));
-#endif
-         span<const char> cs{str};
-         EXPECT_TRUE(cs.size() == str.size());
-         EXPECT_TRUE(cs.data() == str.data());
+        static_assert(CtorCompilesFor<span<char>, std::string&> == (__cplusplus >= 201703L),
+                      "CtorCompilesFor<span<char>, std::string&> == (__cplusplus >= 201703L)");
+
+        span<const char> cs{str};
+        EXPECT_TRUE(cs.size() == str.size());
+        EXPECT_TRUE(cs.data() == str.data());
     }
 
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        span<char> s{cstr};
-#endif
+        static_assert(!CtorCompilesFor<span<char>, const std::string&>,
+                      "!CtorCompilesFor<span<char>, const std::string&>");
+
         span<const char> cs{cstr};
         EXPECT_TRUE(cs.size() == cstr.size());
         EXPECT_TRUE(cs.data() == cstr.data());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_vector = []() -> std::vector<int> { return {}; };
-        auto use_span = [](span<int> s) { static_cast<void>(s); };
-        use_span(get_temp_vector());
-#endif
-    }
+#if !defined(_MSC_VER) || (__cplusplus >= 201703L)
+    // Fails on MSVC. TODO: report a feedback bug.
+    static_assert(!ConversionCompilesFor<span<int>, std::vector<int>>,
+                  "!ConversionCompilesFor<span<int>, std::vector<int>>");
+#endif // !defined(_MSC_VER) || (_MSC_VER > 1942) || (__cplusplus >= 201703L)
 
     {
         auto get_temp_vector = []() -> std::vector<int> { return {}; };
@@ -610,13 +543,8 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_vector());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_string = []() -> std::string { return {}; };
-        auto use_span = [](span<char> s) { static_cast<void>(s); };
-        use_span(get_temp_string());
-#endif
-    }
+    static_assert(!ConversionCompilesFor<span<char>, std::string>,
+                  "!ConversionCompilesFor<span<char>, std::string>");
 
     {
         auto get_temp_string = []() -> std::string { return {}; };
@@ -624,13 +552,10 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_string());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_vector = []() -> const std::vector<int> { return {}; };
-        auto use_span = [](span<const char> s) { static_cast<void>(s); };
-        use_span(get_temp_vector());
-#endif
-    }
+    static_assert(!ConversionCompilesFor<span<const char>, const std::vector<int>>,
+                  "!ConversionCompilesFor<span<const char>, const std::vector<int>>");
+    static_assert(!ConversionCompilesFor<span<char>, const std::string>,
+                  "!ConversionCompilesFor<span<char>, const std::string>");
 
     {
         auto get_temp_string = []() -> const std::string { return {}; };
@@ -638,12 +563,8 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_string());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        std::map<int, int> m;
-        span<int> s{m};
-#endif
-    }
+    static_assert(!CtorCompilesFor<span<int>, std::map<int, int>&>,
+                  "!CtorCompilesFor<span<int>, std::map<int, int>&>");
 }
 
 TEST(span_test, from_convertible_span_constructor)
@@ -695,52 +616,20 @@ TEST(span_test, from_convertible_span_constructor)
         EXPECT_DEATH(T{avd}, expected);
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        std::array<DerivedClass, 2> arr{};
-        span<DerivedClass> avd{arr};
-        span<const DerivedClass, 2> avcd = avd;
-        static_cast<void>(avcd);
-    }
-
-    {
-        std::array<DerivedClass, 2> arr{};
-        span<DerivedClass, 2> avd{arr};
-        span<const DerivedClass, 1> avcd = avd;
-        static_cast<void>(avcd);
-    }
-
-    {
-        std::array<DerivedClass, 2> arr{};
-        span<DerivedClass, 2> avd{arr};
-        span<const DerivedClass, 3> avcd = avd;
-        static_cast<void>(avcd);
-    }
-
-    {
-        span<DerivedClass> avd;
-        span<BaseClass> avb = avd;
-        static_cast<void>(avb);
-    }
-
-    {
-        span<int> s;
-        span<unsigned int> s2 = s;
-        static_cast<void>(s2);
-    }
-
-    {
-        span<int> s;
-        span<const unsigned int> s2 = s;
-        static_cast<void>(s2);
-    }
-
-    {
-        span<int> s;
-        span<short> s2 = s;
-        static_cast<void>(s2);
-    }
-#endif
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 2>, span<DerivedClass>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 2>, span<DerivedClass>&>");
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 1>, span<DerivedClass, 2>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 1>, span<DerivedClass, 2>&>");
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 3>, span<DerivedClass, 2>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 3>, span<DerivedClass, 2>&>");
+    static_assert(!ConversionCompilesFor<span<BaseClass, 3>, span<DerivedClass>&>,
+                  "!ConversionCompilesFor<span<BaseClass, 3>, span<DerivedClass>&>");
+    static_assert(!ConversionCompilesFor<span<unsigned int>, span<int>&>,
+                  "!ConversionCompilesFor<span<unsigned int>, span<int>&>");
+    static_assert(!ConversionCompilesFor<span<const unsigned int>, span<int>&>,
+                  "!ConversionCompilesFor<span<const unsigned int>, span<int>&>");
+    static_assert(!ConversionCompilesFor<span<short>, span<int>&>,
+                  "!ConversionCompilesFor<span<short>, span<int>&>");
 }
 
 TEST(span_test, copy_move_and_assignment)
@@ -800,10 +689,9 @@ TEST(span_test, first)
     {
         span<int, 5> av = arr;
 #ifdef CONFIRM_COMPILATION_ERRORS
-        EXPECT_TRUE(av.first<6>().size() == 6);
-        EXPECT_TRUE(av.first<-1>().size() == -1);
+        (void) av.first<6>();
 #endif
-        EXPECT_DEATH(av.first(6).size(), expected);
+        EXPECT_DEATH(av.first(6), expected);
     }
 
     {
@@ -844,9 +732,9 @@ TEST(span_test, last)
     {
         span<int, 5> av = arr;
 #ifdef CONFIRM_COMPILATION_ERRORS
-        EXPECT_TRUE(av.last<6>().size() == 6);
+        (void) av.last<6>();
 #endif
-        EXPECT_DEATH(av.last(6).size(), expected);
+        EXPECT_DEATH(av.last(6), expected);
     }
 
     {
@@ -871,6 +759,9 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<2, 2>().size()) == 2);
         EXPECT_TRUE(decltype(av.subspan<2, 2>())::extent == 2);
         EXPECT_TRUE(av.subspan(2, 2).size() == 2);
+
+        EXPECT_TRUE((av.subspan<2, 3>().size()) == 3);
+        EXPECT_TRUE(decltype(av.subspan<2, 3>())::extent == 3);
         EXPECT_TRUE(av.subspan(2, 3).size() == 3);
     }
 
@@ -887,8 +778,12 @@ TEST(span_test, subspan)
         EXPECT_TRUE(decltype(av.subspan<0, 5>())::extent == 5);
         EXPECT_TRUE(av.subspan(0, 5).size() == 5);
 
-        EXPECT_DEATH(av.subspan(0, 6).size(), expected);
-        EXPECT_DEATH(av.subspan(1, 5).size(), expected);
+#ifdef CONFIRM_COMPILATION_ERRORS
+        (void) av.subspan<0, 6>();
+        (void) av.subspan<1, 5>();
+#endif
+        EXPECT_DEATH(av.subspan(0, 6), expected);
+        EXPECT_DEATH(av.subspan(1, 5), expected);
     }
 
     {
@@ -896,14 +791,22 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<4, 0>().size()) == 0);
         EXPECT_TRUE(decltype(av.subspan<4, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(4, 0).size() == 0);
+
+        EXPECT_TRUE((av.subspan<5, 0>().size()) == 0);
+        EXPECT_TRUE(decltype(av.subspan<5, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(5, 0).size() == 0);
-        EXPECT_DEATH(av.subspan(6, 0).size(), expected);
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+        (void) av.subspan<6, 0>();
+#endif
+        EXPECT_DEATH(av.subspan(6, 0), expected);
     }
 
     {
         span<int, 5> av = arr;
         EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(decltype(av.subspan<1>())::extent == 4);
+        EXPECT_TRUE(av.subspan(1).size() == 4);
     }
 
     {
@@ -911,35 +814,58 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<0, 0>().size()) == 0);
         EXPECT_TRUE(decltype(av.subspan<0, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(0, 0).size() == 0);
-        EXPECT_DEATH((av.subspan<1, 0>().size()), expected);
+
+        EXPECT_DEATH((av.subspan<1, 0>()), expected);
+        EXPECT_DEATH((av.subspan(1, 0)), expected);
     }
 
     {
         span<int> av;
+        EXPECT_TRUE((av.subspan<0>().size()) == 0);
+        EXPECT_TRUE(decltype(av.subspan<0>())::extent == dynamic_extent);
         EXPECT_TRUE(av.subspan(0).size() == 0);
-        EXPECT_DEATH(av.subspan(1).size(), expected);
+
+        EXPECT_DEATH(av.subspan<1>(), expected);
+        EXPECT_TRUE(decltype(av.subspan<1>())::extent == dynamic_extent);
+        EXPECT_DEATH(av.subspan(1), expected);
     }
 
     {
         span<int> av = arr;
         EXPECT_TRUE(av.subspan(0).size() == 5);
+        EXPECT_TRUE(av.subspan<0>().size() == 5);
         EXPECT_TRUE(av.subspan(1).size() == 4);
+        EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(av.subspan(4).size() == 1);
+        EXPECT_TRUE(av.subspan<4>().size() == 1);
         EXPECT_TRUE(av.subspan(5).size() == 0);
-        EXPECT_DEATH(av.subspan(6).size(), expected);
+        EXPECT_TRUE(av.subspan<5>().size() == 0);
+        EXPECT_DEATH(av.subspan(6), expected);
+        EXPECT_DEATH(av.subspan<6>(), expected);
         const auto av2 = av.subspan(1);
         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
+        const auto av3 = av.subspan<1>();
+        for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av3[i] == static_cast<int>(i) + 2);
     }
 
     {
         span<int, 5> av = arr;
         EXPECT_TRUE(av.subspan(0).size() == 5);
+        EXPECT_TRUE(av.subspan<0>().size() == 5);
         EXPECT_TRUE(av.subspan(1).size() == 4);
+        EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(av.subspan(4).size() == 1);
+        EXPECT_TRUE(av.subspan<4>().size() == 1);
         EXPECT_TRUE(av.subspan(5).size() == 0);
-        EXPECT_DEATH(av.subspan(6).size(), expected);
+        EXPECT_TRUE(av.subspan<5>().size() == 0);
+        EXPECT_DEATH(av.subspan(6), expected);
+#ifdef CONFIRM_COMPILATION_ERRORS
+        EXPECT_DEATH(av.subspan<6>(), expected);
+#endif
         const auto av2 = av.subspan(1);
         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
+        const auto av3 = av.subspan<1>();
+        for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av3[i] == static_cast<int>(i) + 2);
     }
 }
 
@@ -1114,9 +1040,21 @@ TEST(span_test, rbegin_rend)
     }
 }
 
+template <typename U, typename = void>
+static constexpr bool AsWritableBytesCompilesFor = false;
+template <typename U>
+static constexpr bool
+    AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(std::declval<U>()))>> = true;
+
 TEST(span_test, as_bytes)
 {
     int a[] = {1, 2, 3, 4};
+
+    static_assert(AsWritableBytesCompilesFor<span<int>>, "AsWriteableBytesCompilesFor<span<int>>");
+    // you should not be able to get writeable bytes for const objects
+    static_assert(!AsWritableBytesCompilesFor<span<const int>>,
+                  "!AsWriteableBytesCompilesFor<span<const int>>");
+
     {
         const span<const int> s = a;
         EXPECT_TRUE(s.size() == 4);
@@ -1147,17 +1085,6 @@ TEST(span_test, as_writable_bytes)
 {
     int a[] = {1, 2, 3, 4};
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        // you should not be able to get writeable bytes for const objects
-        span<const int> s = a;
-        EXPECT_TRUE(s.size() == 4);
-        span<const byte> bs = as_writable_bytes(s);
-        EXPECT_TRUE(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
-        EXPECT_TRUE(bs.size() == s.size_bytes());
-#endif
-    }
-
     {
         span<int> s;
         const auto bs = as_writable_bytes(s);
@@ -1197,30 +1124,15 @@ TEST(span_test, fixed_size_conversions)
         static_cast<void>(s);
     }
 
-// initialization or assignment to static span that REDUCES size is NOT ok
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 2> s = arr;
-    }
-    {
-        span<int, 2> s2 = s4;
-        static_cast<void>(s2);
-    }
-#endif
+    // initialization or assignment to static span that REDUCES size is NOT ok
+    static_assert(!ConversionCompilesFor<span<int, 2>, int[4]>,
+                  "!ConversionCompilesFor<span<int, 2>, int[4]>");
+    static_assert(!ConversionCompilesFor<span<int, 2>, span<int, 4>>,
+                  "!ConversionCompilesFor<span<int, 2>, span<int, 4>>");
 
     // even when done dynamically
-    {
-        /*
-        // this now results in a compile-time error, rather than runtime.
-        // There is no suitable conversion from dynamic span to fixed span.
-        span<int> s = arr;
-        auto f = [&]() {
-            const span<int, 2> s2 = s;
-            static_cast<void>(s2);
-        };
-        EXPECT_DEATH(f(), expected);
-        */
-    }
+    static_assert(!ConversionCompilesFor<span<int, 2>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 2>, span<int>>");
 
     // but doing so explicitly is ok
 
@@ -1234,32 +1146,24 @@ TEST(span_test, fixed_size_conversions)
         static_cast<void>(s1);
     }
 
-    /*
-     // this is not a legal operation in std::span, so we are no longer supporting it
-     // conversion from span<int, 4> to span<int, dynamic_extent> via call to `first`
-     // then convert from span<int, dynamic_extent> to span<int, 1>
-     // The dynamic to fixed extents are not supported in the standard
-     // to make this work, span<int, 1> would need to be span<int>.
-     {
-
-         // NB: implicit conversion to span<int,1> from span<int>
-         span<int, 1> s1 = s4.first(1);
-         static_cast<void>(s1);
-     }
-     */
+    // this is not a legal operation in std::span, so we are no longer supporting it
+    // conversion from span<int, 4> to span<int, dynamic_extent> via call to `first`
+    // then convert from span<int, dynamic_extent> to span<int, 1>
+    // The dynamic to fixed extents are not supported in the standard
+    // to make this work, span<int, 1> would need to be span<int>.
+    static_assert(!ConversionCompilesFor<span<int, 1>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 1>, span<int>>");
 
     // initialization or assignment to static span that requires size INCREASE is not ok.
     int arr2[2] = {1, 2};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 4> s3 = arr2;
-    }
-    {
-        span<int, 2> s2 = arr2;
-        span<int, 4> s4a = s2;
-    }
-#endif
+    static_assert(!ConversionCompilesFor<span<int, 4>, int[2]>,
+                  "!ConversionCompilesFor<span<int, 4>, int[2]>");
+    static_assert(!ConversionCompilesFor<span<int, 2>, int[2]>,
+                  "!ConversionCompilesFor<span<int, 2>, int[2]>");
+    static_assert(!ConversionCompilesFor<span<int, 4>, span<int, 2>>,
+                  "!ConversionCompilesFor<span<int, 4>, span<int, 2>>");
+
     {
         auto f = [&]() {
             const span<int, 4> _s4{arr2, 2};
@@ -1268,16 +1172,11 @@ TEST(span_test, fixed_size_conversions)
         EXPECT_DEATH(f(), expected);
     }
 
-    /*
-     // This no longer compiles. There is no suitable conversion from dynamic span to a fixed size
-     span.
-     // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
-     span<int> av = arr2; auto f = [&]() {
-         const span<int, 4> _s4 = av;
-         static_cast<void>(_s4);
-     };
-     EXPECT_DEATH(f(), expected);
-     */
+    // This no longer compiles. There is no suitable conversion from dynamic span to a fixed size
+    // span.
+    // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
+    static_assert(!ConversionCompilesFor<span<int, 4>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 4>, span<int>>");
 }
 
 TEST(span_test, interop_with_std_regex)
@@ -1312,7 +1211,7 @@ TEST(span_test, default_constructible)
 
 TEST(span_test, std_container_ctad)
 {
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
     // this test is just to verify that these compile
     {
         std::vector<int> v{1, 2, 3, 4};
@@ -1331,7 +1230,7 @@ TEST(span_test, std_container_ctad)
         static_assert(std::is_same<decltype(sp), gsl::span<const char>>::value);
     }
 #endif
-#endif
+#endif // defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
 }
 
 TEST(span_test, front_back)
@@ -1376,8 +1275,154 @@ TEST(span_test, msvc_compile_error_PR1100)
     int arr[]{1, 7, 2, 9};
     gsl::span sp{arr, std::size(arr)};
     std::ranges::sort(sp);
-    for (const auto& e : sp) {
-        (void)e;
-    }
+    for (const auto& e : sp) { (void) e; }
 }
 #endif // defined(__cpp_lib_span) && defined(__cpp_lib_ranges)
+
+TEST(span_test, empty_span)
+{
+    span<int> s{};
+    EXPECT_TRUE(s.empty());
+    EXPECT_TRUE(s.size() == 0);
+    EXPECT_TRUE(s.data() == nullptr);
+
+    span<const int> cs{};
+    EXPECT_TRUE(cs.empty());
+    EXPECT_TRUE(cs.size() == 0);
+    EXPECT_TRUE(cs.data() == nullptr);
+}
+
+TEST(span_test, conversions)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
+    span s = arr;
+    span cs = s;
+#else  // ^^^ deduction guides /// no deduction guides vvv
+    span<int, 5> s = arr;
+    span<int, 5> cs = s;
+#endif // defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
+
+    EXPECT_TRUE(cs.size() == s.size());
+    EXPECT_TRUE(cs.data() == s.data());
+
+    span<int, 5> fs = s;
+    EXPECT_TRUE(fs.size() == s.size());
+    EXPECT_TRUE(fs.data() == s.data());
+
+    span<const int, 5> cfs = s;
+    EXPECT_TRUE(cfs.size() == s.size());
+    EXPECT_TRUE(cfs.data() == s.data());
+}
+
+TEST(span_test, comparison_operators)
+{
+    int arr1[3] = {1, 2, 3};
+    int arr2[3] = {1, 2, 3};
+    int arr3[3] = {4, 5, 6};
+
+    span<int> s1 = arr1;
+    span<int> s2 = arr2;
+    span<int> s3 = arr3;
+
+    EXPECT_TRUE(s1 == s2);
+    EXPECT_FALSE(s1 != s2);
+    EXPECT_FALSE(s1 == s3);
+    EXPECT_TRUE(s1 != s3);
+    EXPECT_TRUE(s1 < s3);
+    EXPECT_FALSE(s3 < s1);
+    EXPECT_TRUE(s1 <= s2);
+    EXPECT_TRUE(s1 <= s3);
+    EXPECT_FALSE(s3 <= s1);
+    EXPECT_TRUE(s3 > s1);
+    EXPECT_FALSE(s1 > s3);
+    EXPECT_TRUE(s3 >= s1);
+    EXPECT_TRUE(s1 >= s2);
+    EXPECT_FALSE(s1 >= s3);
+}
+
+// ...existing code...
+
+#if defined(__cpp_lib_span) && __cpp_lib_span >= 202002L
+
+#include <span> // for std::span
+
+TEST(span_test, compare_empty_span)
+{
+    gsl::span<int> gsl_s{};
+    std::span<int> std_s{};
+
+    EXPECT_TRUE(gsl_s.empty());
+    EXPECT_TRUE(std_s.empty());
+    EXPECT_EQ(gsl_s.size(), std_s.size());
+    EXPECT_EQ(gsl_s.data(), std_s.data());
+}
+
+TEST(span_test, compare_subspan)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+    gsl::span gsl_s = arr;
+    std::span std_s = arr;
+
+    auto gsl_sub1 = gsl_s.subspan(1);
+    auto std_sub1 = std_s.subspan(1);
+    EXPECT_EQ(gsl_sub1.size(), std_sub1.size());
+    EXPECT_EQ(gsl_sub1.data(), std_sub1.data());
+
+    auto gsl_sub2 = gsl_s.subspan(1, 2);
+    auto std_sub2 = std_s.subspan(1, 2);
+    EXPECT_EQ(gsl_sub2.size(), std_sub2.size());
+    EXPECT_EQ(gsl_sub2.data(), std_sub2.data());
+}
+
+TEST(span_test, compare_conversions)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+    gsl::span gsl_s = arr;
+    std::span std_s = arr;
+
+    gsl::span gsl_cs = gsl_s;
+    std::span std_cs = std_s;
+    EXPECT_EQ(gsl_cs.size(), std_cs.size());
+    EXPECT_EQ(gsl_cs.data(), std_cs.data());
+
+    gsl::span<int, 5> gsl_fs = gsl_s;
+    std::span<int, 5> std_fs = std_s;
+    EXPECT_EQ(gsl_fs.size(), std_fs.size());
+    EXPECT_EQ(gsl_fs.data(), std_fs.data());
+
+    gsl::span<const int, 5> gsl_cfs = gsl_s;
+    std::span<const int, 5> std_cfs = std_s;
+    EXPECT_EQ(gsl_cfs.size(), std_cfs.size());
+    EXPECT_EQ(gsl_cfs.data(), std_cfs.data());
+}
+
+TEST(span_test, deduction_guides)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+    std::array<int, 5> std_arr = {1, 2, 3, 4, 5};
+    std::vector<int> vec = {1, 2, 3, 4, 5};
+
+    // Test deduction guides for gsl::span
+    gsl::span gsl_s1 = arr;
+    gsl::span gsl_s2 = std_arr;
+    gsl::span gsl_s3 = vec;
+
+    // Test deduction guides for std::span (for sanity checks)
+    std::span std_s1 = arr;
+    std::span std_s2 = std_arr;
+    std::span std_s3 = vec;
+
+    // Compare sizes
+    EXPECT_EQ(gsl_s1.size(), std_s1.size());
+    EXPECT_EQ(gsl_s2.size(), std_s2.size());
+    EXPECT_EQ(gsl_s3.size(), std_s3.size());
+
+    // Compare data pointers
+    EXPECT_EQ(gsl_s1.data(), std_s1.data());
+    EXPECT_EQ(gsl_s2.data(), std_s2.data());
+    EXPECT_EQ(gsl_s3.data(), std_s3.data());
+}
+
+#endif // defined(__cpp_lib_span) && __cpp_lib_span >= 202002L

tests/strict_notnull_tests.cpp

@@ -17,38 +17,188 @@
 #include <gsl/pointers> // for not_null, operator<, operator<=, operator>
 #include <gtest/gtest.h>
 
+#include <type_traits> // for declval
+
 #include "deathTestCommon.h"
 
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
+// stand-in for a user-defined ref-counted class
+template <typename T>
+struct RefCounted
+{
+    RefCounted(T* p) : p_(p) {}
+    operator T*() { return p_; }
+    T* p_;
+};
+
 namespace
 {
-// clang-format off
-GSL_SUPPRESS(f.4) // NO-FORMAT: attribute
-// clang-format on
+GSL_SUPPRESS(f.4)
 bool helper(not_null<int*> p) { return *p == 12; }
 
-// clang-format off
-GSL_SUPPRESS(f.4) // NO-FORMAT: attribute
-// clang-format on
+GSL_SUPPRESS(f.4)
 bool helper_const(not_null<const int*> p) { return *p == 12; }
 
-// clang-format off
-GSL_SUPPRESS(f.4) // NO-FORMAT: attribute
-// clang-format on
+GSL_SUPPRESS(f.4)
 bool strict_helper(strict_not_null<int*> p) { return *p == 12; }
 
-// clang-format off
-GSL_SUPPRESS(f.4) // NO-FORMAT: attribute
-// clang-format on
+GSL_SUPPRESS(f.4)
 bool strict_helper_const(strict_not_null<const int*> p) { return *p == 12; }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
 int* return_pointer() { return nullptr; }
-const int* return_pointer_const() { return nullptr; }
-#endif
 } // namespace
 
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_A = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_A<U, void_t<decltype(gsl::strict_not_null<void*>{std::declval<U>()})>> = true;
+
+template <typename U, int N, typename = void>
+static constexpr bool CtorCompilesFor_B = false;
+template <typename U, int N>
+static constexpr bool CtorCompilesFor_B<U, N, void_t<decltype(gsl::strict_not_null<U>{N})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool DefaultCtorCompilesFor = false;
+template <typename U>
+static constexpr bool DefaultCtorCompilesFor<U, void_t<decltype(gsl::strict_not_null<U>{})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_C = false;
+template <typename U>
+static constexpr bool CtorCompilesFor_C<
+    U, void_t<decltype(gsl::strict_not_null<U*>{std::declval<std::unique_ptr<U>>()})>> = true;
+
+TEST(strict_notnull_tests, TestStrictNotNullConstructors)
+{
+    {
+        static_assert(CtorCompilesFor_A<void*>, "CtorCompilesFor_A<void*>");
+        static_assert(!CtorCompilesFor_A<std::nullptr_t>, "!CtorCompilesFor_A<std::nullptr_t>");
+        static_assert(!CtorCompilesFor_B<void*, 0>, "!CtorCompilesFor_B<void*, 0>");
+        static_assert(!DefaultCtorCompilesFor<void*>, "!DefaultCtorCompilesFor<void*>");
+        static_assert(!CtorCompilesFor_C<int>, "CtorCompilesFor_C<int>");
+#ifdef CONFIRM_COMPILATION_ERRORS
+        // Forbid non-nullptr assignable types
+        strict_not_null<std::vector<int>> f(std::vector<int>{1});
+        strict_not_null<int> z(10);
+        strict_not_null<std::vector<int>> y({1, 2});
+#endif
+    }
+
+    const auto terminateHandler = std::set_terminate([] {
+        std::cerr << "Expected Death. TestNotNullConstructors";
+        std::abort();
+    });
+    const auto expected = GetExpectedDeathString(terminateHandler);
+
+    {
+        // from shared pointer
+        int i = 12;
+        auto rp = RefCounted<int>(&i);
+        strict_not_null<int*> p(rp);
+        EXPECT_TRUE(p.get() == &i);
+
+        strict_not_null<std::shared_ptr<int>> x(
+            std::make_shared<int>(10)); // shared_ptr<int> is nullptr assignable
+
+        int* pi = nullptr;
+        EXPECT_DEATH((strict_not_null<decltype(pi)>(pi)), expected);
+    }
+
+    {
+        // from unique pointer
+        strict_not_null<std::unique_ptr<int>> x(
+            std::make_unique<int>(10)); // unique_ptr<int> is nullptr assignable
+
+        EXPECT_DEATH((strict_not_null<std::unique_ptr<int>>(std::unique_ptr<int>{})), expected);
+    }
+
+    {
+        // from pointer to local
+        int t = 42;
+
+        strict_not_null<int*> x{&t};
+        helper(&t);
+        helper_const(&t);
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from raw pointer
+        // from strict_not_null pointer
+
+        int t = 42;
+        int* p = &t;
+
+        strict_not_null<int*> x{p};
+        helper(p);
+        helper_const(p);
+        helper(x);
+        helper_const(x);
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from raw const pointer
+        // from strict_not_null const pointer
+
+        int t = 42;
+        const int* cp = &t;
+
+        strict_not_null<const int*> x{cp};
+        helper_const(cp);
+        helper_const(x);
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from strict_not_null const pointer, using auto
+        int t = 42;
+        const int* cp = &t;
+
+        auto x = strict_not_null<const int*>{cp};
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from returned pointer
+
+        EXPECT_DEATH(helper(return_pointer()), expected);
+        EXPECT_DEATH(helper_const(return_pointer()), expected);
+    }
+}
+
+template <typename U, typename = void>
+static constexpr bool StrictHelperCompilesFor = false;
+template <typename U>
+static constexpr bool
+    StrictHelperCompilesFor<U, void_t<decltype(strict_helper(std::declval<U>()))>> = true;
+
+template <typename U, typename = void>
+static constexpr bool StrictHelperConstCompilesFor = false;
+template <typename U>
+static constexpr bool
+    StrictHelperConstCompilesFor<U, void_t<decltype(strict_helper_const(std::declval<U>()))>> =
+        true;
+
+template <typename U, typename = void>
+static constexpr bool HelperCompilesFor = false;
+template <typename U>
+static constexpr bool HelperCompilesFor<U, void_t<decltype(helper(std::declval<U>()))>> = true;
+
 TEST(strict_notnull_tests, TestStrictNotNull)
 {
     {
@@ -57,14 +207,14 @@ TEST(strict_notnull_tests, TestStrictNotNull)
 
 #ifdef CONFIRM_COMPILATION_ERRORS
         strict_not_null<int*> snn = &x;
-        strict_helper(&x);
-        strict_helper_const(&x);
-        strict_helper(return_pointer());
-        strict_helper_const(return_pointer_const());
 #endif
+        static_assert(!StrictHelperCompilesFor<int*>, "!StrictHelperCompilesFor<int*>");
+        static_assert(!StrictHelperConstCompilesFor<int*>, "!StrictHelperCompilesFor<int*>");
 
         const strict_not_null<int*> snn1{&x};
 
+        static_assert(StrictHelperCompilesFor<const strict_not_null<int*>>,
+                      "StrictHelperCompilesFor<const strict_not_null<int*>>");
         helper(snn1);
         helper_const(snn1);
 
@@ -77,17 +227,17 @@ TEST(strict_notnull_tests, TestStrictNotNull)
 
 #ifdef CONFIRM_COMPILATION_ERRORS
         strict_not_null<int*> snn = &x;
-        strict_helper(&x);
-        strict_helper_const(&x);
-        strict_helper(return_pointer());
-        strict_helper_const(return_pointer_const());
 #endif
+        static_assert(!StrictHelperCompilesFor<const int*>, "!StrictHelperFor<const int*>");
+        static_assert(!StrictHelperConstCompilesFor<const int*>,
+                      "!StrictHelperCompilesFor<const int*>");
 
         const strict_not_null<const int*> snn1{&x};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(snn1);
-#endif
+        static_assert(!HelperCompilesFor<const strict_not_null<const int*>>,
+                      "!HelperCompilesFor<const strict_not_null<const int*>>");
+        static_assert(StrictHelperConstCompilesFor<const strict_not_null<const int*>>,
+                      "StrictHelperCompilesFor<const strict_not_null<const int*>>");
         helper_const(snn1);
 
         EXPECT_TRUE(*snn1 == 42);
@@ -114,9 +264,8 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         strict_not_null<const int*> snn1{&x};
         const strict_not_null<const int*> snn2{&x};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        strict_helper(snn1);
-#endif
+        static_assert(!StrictHelperCompilesFor<strict_not_null<const int*>>,
+                      "!StrictHelperCompilesFor<strict_not_null<const int*>>");
         strict_helper_const(snn1);
         strict_helper_const(snn2);
 
@@ -148,9 +297,8 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         const not_null<const int*> nn1 = snn;
         const not_null<const int*> nn2{snn};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(snn);
-#endif
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
         helper_const(snn);
 
         EXPECT_TRUE(snn == nn1);
@@ -190,9 +338,8 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         const strict_not_null<const int*> snn1{nn};
         const strict_not_null<const int*> snn2{nn};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        strict_helper(nn);
-#endif
+        static_assert(!StrictHelperCompilesFor<not_null<const int*>>,
+                      "!StrictHelperCompilesFor<not_null<const int*>>");
         strict_helper_const(nn);
 
         EXPECT_TRUE(snn1 == nn);
@@ -206,12 +353,13 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         EXPECT_TRUE(hash_nn(snn1) == hash_nn(snn2));
         EXPECT_TRUE(hash_snn(snn1) == hash_snn(nn));
     }
+}
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        strict_not_null<int*> p{nullptr};
-    }
-#endif
+TEST(pointers_test, member_types)
+{
+    // make sure `element_type` is inherited from `gsl::not_null`
+    static_assert(std::is_same<gsl::strict_not_null<int*>::element_type, int*>::value,
+                  "check member type: element_type");
 }
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
@@ -238,9 +386,8 @@ TEST(strict_notnull_tests, TestStrictNotNullConstructorTypeDeduction)
         const int i = 42;
 
         strict_not_null x{&i};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(strict_not_null{&i});
-#endif
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
         helper_const(strict_not_null{&i});
 
         EXPECT_TRUE(*x == 42);
@@ -262,9 +409,8 @@ TEST(strict_notnull_tests, TestStrictNotNullConstructorTypeDeduction)
         const int* p = &i;
 
         strict_not_null x{p};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(strict_not_null{p});
-#endif
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
         helper_const(strict_not_null{p});
 
         EXPECT_TRUE(*x == 42);

tests/utils_tests.cpp

@@ -16,7 +16,7 @@
 
 #include <gtest/gtest.h>
 
-#include <algorithm>   // for move
+#include <algorithm> // for move
 #include <complex>
 #include <cstddef>     // for std::ptrdiff_t
 #include <cstdint>     // for uint32_t, int32_t
@@ -156,8 +156,8 @@ TEST(utils_tests, narrow)
     n = -42;
     EXPECT_THROW(narrow<unsigned>(n), narrowing_error);
 
-    EXPECT_TRUE(
-        narrow<std::complex<float>>(std::complex<double>(4, 2)) == std::complex<float>(4, 2));
+    EXPECT_TRUE(narrow<std::complex<float>>(std::complex<double>(4, 2)) ==
+                std::complex<float>(4, 2));
     EXPECT_THROW(narrow<std::complex<float>>(std::complex<double>(4.2)), narrowing_error);
 
     EXPECT_TRUE(narrow<int>(float(1)) == 1);