Update documentation

Add variant_gcc_false_positive.cpp to reproduce GCC 12+ spurious
-Wmaybe-uninitialized warnings when copying/moving variant containing
non-trivially-copyable types like std::exception_ptr and std::string.

Tests use boost::system::result which internally uses variant2, and
include coroutine-based tests when C++20 coroutine support is available.

The test uses -O3 optimization to trigger the aggressive inlining that
exposes the false positive where GCC's dataflow analysis cannot prove
the discriminator ensures only initialized alternatives are accessed.

See https://github.com/boostorg/variant2/issues/33

.drone.jsonnet

@@ -0,0 +1,465 @@
+# Copyright 2022 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+local library = "variant2";
+
+local triggers =
+{
+    branch: [ "master", "develop", "feature/*" ]
+};
+
+local ubsan = { UBSAN: '1', UBSAN_OPTIONS: 'print_stacktrace=1' };
+local asan = { ASAN: '1' };
+
+local linux_pipeline(name, image, environment, packages = "", sources = [], arch = "amd64") =
+{
+    name: name,
+    kind: "pipeline",
+    type: "docker",
+    trigger: triggers,
+    platform:
+    {
+        os: "linux",
+        arch: arch
+    },
+    steps:
+    [
+        {
+            name: "everything",
+            image: image,
+            environment: environment,
+            commands:
+            [
+                'set -e',
+                'uname -a',
+                'echo $DRONE_STAGE_MACHINE',
+            ] +
+            (if sources != [] then [ ('apt-add-repository "' + source + '"') for source in sources ] else []) +
+            (if packages != "" then [ 'apt-get update', 'apt-get -y install ' + packages ] else []) +
+            [
+                'export LIBRARY=' + library,
+                './.drone/drone.sh',
+            ]
+        }
+    ]
+};
+
+local macos_pipeline(name, environment, xcode_version = "12.2", osx_version = "catalina", arch = "amd64") =
+{
+    name: name,
+    kind: "pipeline",
+    type: "exec",
+    trigger: triggers,
+    platform: {
+        "os": "darwin",
+        "arch": arch
+    },
+    node: {
+        "os": osx_version
+    },
+    steps: [
+        {
+            name: "everything",
+            environment: environment + { "DEVELOPER_DIR": "/Applications/Xcode-" + xcode_version + ".app/Contents/Developer" },
+            commands:
+            [
+                'export LIBRARY=' + library,
+                './.drone/drone.sh',
+            ]
+        }
+    ]
+};
+
+local windows_pipeline(name, image, environment, arch = "amd64") =
+{
+    name: name,
+    kind: "pipeline",
+    type: "docker",
+    trigger: triggers,
+    platform:
+    {
+        os: "windows",
+        arch: arch
+    },
+    "steps":
+    [
+        {
+            name: "everything",
+            image: image,
+            environment: environment,
+            commands:
+            [
+                'cmd /C .drone\\\\drone.bat ' + library,
+            ]
+        }
+    ]
+};
+
+[
+    linux_pipeline(
+        "Linux 16.04 GCC 4.8 32/64",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-4.8', CXXSTD: '11', ADDRMD: '32,64' },
+        "g++-4.8-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 16.04 GCC 4.9 32/64",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-4.9', CXXSTD: '11', ADDRMD: '32,64' },
+        "g++-4.9-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 16.04 GCC 5* 32/64",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '11,14', ADDRMD: '32,64' },
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 GCC 6 32/64",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-6', CXXSTD: '11,14', ADDRMD: '32,64' },
+        "g++-6-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 GCC 7* 32/64",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '11,14,17', ADDRMD: '32,64' },
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 GCC 8 32/64",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-8', CXXSTD: '11,14,17', ADDRMD: '32,64' },
+        "g++-8-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 9* 32/64",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '11,14,17,2a', ADDRMD: '32,64' },
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 9* ARM64",
+        "cppalliance/droneubuntu2004:multiarch",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '11,14,17,2a' },
+        arch="arm64",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 9* S390x",
+        "cppalliance/droneubuntu2004:multiarch",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '11,14,17,2a' },
+        arch="s390x",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 10 32/64",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-10', CXXSTD: '11,14,17,20', ADDRMD: '32,64' },
+        "g++-10-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 GCC 11* 32/64",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '11,14,17,2a', ADDRMD: '32,64' },
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 GCC 12 32/64",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-12', CXXSTD: '11,14,17,20,2b', ADDRMD: '32,64' },
+        "g++-12-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 GCC 13 32/64",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-13', CXXSTD: '11,14,17,20,2b', ADDRMD: '32,64' },
+        "g++-13-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 GCC 14 32 ASAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-14', CXXSTD: '11,14,17,20,2b', ADDRMD: '32' } + asan,
+        "g++-14-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 GCC 14 64 ASAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-14', CXXSTD: '11,14,17,20,2b', ADDRMD: '64' } + asan,
+        "g++-14-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 GCC 14 32 UBSAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-14', CXXSTD: '11,14,17,20,2b', ADDRMD: '32' } + ubsan,
+        "g++-14-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 GCC 14 64 UBSAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-14', CXXSTD: '11,14,17,20,2b', ADDRMD: '64' } + ubsan,
+        "g++-14-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 25.04 GCC 15 32/64",
+        "cppalliance/droneubuntu2504:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-15', CXXSTD: '11,14,17,20,23,2c', ADDRMD: '32,64' },
+        "g++-15-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 16.04 Clang 3.5",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-3.5', CXXSTD: '11' },
+        "clang-3.5",
+    ),
+
+    linux_pipeline(
+        "Linux 16.04 Clang 3.6",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-3.6', CXXSTD: '11,14' },
+        "clang-3.6",
+    ),
+
+    linux_pipeline(
+        "Linux 16.04 Clang 3.7",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-3.7', CXXSTD: '11,14' },
+        "clang-3.7",
+    ),
+
+    linux_pipeline(
+        "Linux 16.04 Clang 3.8",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-3.8', CXXSTD: '11,14' },
+        "clang-3.8",
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 Clang 3.9",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-3.9', CXXSTD: '11,14' },
+        "clang-3.9",
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 Clang 4.0",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-4.0', CXXSTD: '11,14' },
+        "clang-4.0",
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 Clang 5.0",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-5.0', CXXSTD: '11,14' },
+        "clang-5.0",
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 Clang 6.0",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-6.0', CXXSTD: '11,14,17' },
+        "clang-6.0",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 7",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-7', CXXSTD: '11,14,17' },
+        "clang-7",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 8",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-8', CXXSTD: '11,14,17' },
+        "clang-8",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 9",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-9', CXXSTD: '11,14,17,2a' },
+        "clang-9",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 10",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-10', CXXSTD: '11,14,17,2a' },
+        "clang-10",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 11",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-11', CXXSTD: '11,14,17,2a' },
+        "clang-11",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 12",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-12', CXXSTD: '11,14,17,2a' },
+        "clang-12",
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 Clang 13",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-13', CXXSTD: '11,14,17,20,2b' },
+        "clang-13",
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 Clang 14",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-14', CXXSTD: '11,14,17,20,2b' },
+        "clang-14",
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 Clang 15",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-15', CXXSTD: '11,14,17,20,2b' },
+        "clang-15",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 16",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-16', CXXSTD: '11,14,17,20,2b' },
+        "clang-16",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 17",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-17', CXXSTD: '11,14,17,20,2b' },
+        "clang-17",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 18",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-18', CXXSTD: '11,14,17,20,2b' },
+        "clang-18",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 19",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-19', CXXSTD: '11,14,17,20,2b' },
+        "clang-19",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 20 UBSAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-20', CXXSTD: '11,14,17,20,23,2c' } + ubsan,
+        "clang-20",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 20 ASAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-20', CXXSTD: '11,14,17,20,23,2c' } + asan,
+        "clang-20",
+    ),
+
+    linux_pipeline(
+        "Linux 25.10 Clang 21",
+        "cppalliance/droneubuntu2510:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-21', CXXSTD: '11,14,17,20,23,2c' },
+        "clang-21",
+    ),
+
+    macos_pipeline(
+        "MacOS 10.15 Xcode 12.2 UBSAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '11,14,1z' } + ubsan,
+    ),
+
+    macos_pipeline(
+        "MacOS 10.15 Xcode 12.2 ASAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '11,14,1z' } + asan,
+    ),
+
+    macos_pipeline(
+        "MacOS 12.4 Xcode 13.4.1 UBSAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '11,14,17,20,2b' } + ubsan,
+        xcode_version = "13.4.1", osx_version = "monterey", arch = "arm64",
+    ),
+
+    macos_pipeline(
+        "MacOS 12.4 Xcode 13.4.1 ASAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '11,14,17,20,2b' } + asan,
+        xcode_version = "13.4.1", osx_version = "monterey", arch = "arm64",
+    ),
+
+    windows_pipeline(
+        "Windows VS2015 msvc-14.0",
+        "cppalliance/dronevs2015",
+        { TOOLSET: 'msvc-14.0', CXXSTD: '14,latest', B2_DONT_EMBED_MANIFEST: '1' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2017 msvc-14.1",
+        "cppalliance/dronevs2017",
+        { TOOLSET: 'msvc-14.1', CXXSTD: '14,17,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2017 msvc-14.1 permissive-",
+        "cppalliance/dronevs2017",
+        { TOOLSET: 'msvc-14.1', CXXSTD: '14,17', CXXFLAGS: '/permissive-' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2019 msvc-14.2",
+        "cppalliance/dronevs2019",
+        { TOOLSET: 'msvc-14.2', CXXSTD: '14,17,20,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2019 msvc-14.2 permissive-",
+        "cppalliance/dronevs2019",
+        { TOOLSET: 'msvc-14.2', CXXSTD: '14,17', CXXFLAGS: '/permissive-' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2022 msvc-14.3",
+        "cppalliance/dronevs2022:1",
+        { TOOLSET: 'msvc-14.3', CXXSTD: '14,17,20,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2022 msvc-14.3 permissive-",
+        "cppalliance/dronevs2022:1",
+        { TOOLSET: 'msvc-14.3', CXXSTD: '14,17', CXXFLAGS: '/permissive-' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2026 msvc-14.5",
+        "cppalliance/dronevs2026:1",
+        { TOOLSET: 'msvc-14.5', CXXSTD: '14,17,20,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2026 msvc-14.5 permissive-",
+        "cppalliance/dronevs2026:1",
+        { TOOLSET: 'msvc-14.5', CXXSTD: '14,17', CXXFLAGS: '/permissive-' },
+    ),
+]

.drone/drone.bat

@@ -0,0 +1,24 @@
+@REM Copyright 2022 Peter Dimov
+@REM Distributed under the Boost Software License, Version 1.0.
+@REM https://www.boost.org/LICENSE_1_0.txt
+
+@ECHO ON
+
+set LIBRARY=%1
+set DRONE_BUILD_DIR=%CD%
+
+set BOOST_BRANCH=develop
+if "%DRONE_BRANCH%" == "master" set BOOST_BRANCH=master
+cd ..
+git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
+cd boost-root
+git submodule update --init tools/boostdep
+xcopy /s /e /q %DRONE_BUILD_DIR% libs\%LIBRARY%\
+python tools/boostdep/depinst/depinst.py %LIBRARY%
+cmd /c bootstrap
+b2 -d0 headers
+
+if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
+if not "%ADDRMD%" == "" set ADDRMD=address-model=%ADDRMD%
+if not "%CXXFLAGS%" == "" set CXXFLAGS=cxxflags=%CXXFLAGS%
+b2 -j3 libs/%LIBRARY%/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% %CXXFLAGS% variant=debug,release embed-manifest-via=linker

.drone/drone.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+# Copyright 2022 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+set -ex
+export PATH=~/.local/bin:/usr/local/bin:$PATH
+
+DRONE_BUILD_DIR=$(pwd)
+
+BOOST_BRANCH=develop
+if [ "$DRONE_BRANCH" = "master" ]; then BOOST_BRANCH=master; fi
+
+cd ..
+git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
+cd boost-root
+git submodule update --init tools/boostdep
+cp -r $DRONE_BUILD_DIR/* libs/$LIBRARY
+python tools/boostdep/depinst/depinst.py $LIBRARY
+./bootstrap.sh
+./b2 -d0 headers
+
+echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
+./b2 -j3 libs/$LIBRARY/test toolset=$TOOLSET cxxstd=$CXXSTD variant=debug,release ${ADDRMD:+address-model=$ADDRMD} ${UBSAN:+undefined-sanitizer=norecover debug-symbols=on} ${ASAN:+address-sanitizer=norecover debug-symbols=on} ${LINKFLAGS:+linkflags=$LINKFLAGS}

.github/workflows/ci.yml

@@ -18,97 +18,225 @@ jobs:
       matrix:
         include:
           - toolset: gcc-4.8
-            cxxstd: "03,11"
-            os: ubuntu-18.04
+            cxxstd: "11"
+            container: ubuntu:16.04
+            os: ubuntu-latest
             install: g++-4.8
+          - toolset: gcc-4.9
+            cxxstd: "11"
+            container: ubuntu:16.04
+            os: ubuntu-latest
+            install: g++-4.9
           - toolset: gcc-5
-            cxxstd: "03,11,14,1z"
-            os: ubuntu-18.04
+            cxxstd: "11,14,1z"
+            container: ubuntu:18.04
+            os: ubuntu-latest
             install: g++-5
           - toolset: gcc-6
-            cxxstd: "03,11,14,1z"
-            os: ubuntu-18.04
+            cxxstd: "11,14,1z"
+            container: ubuntu:18.04
+            os: ubuntu-latest
             install: g++-6
           - toolset: gcc-7
-            cxxstd: "03,11,14,17"
-            os: ubuntu-18.04
+            cxxstd: "11,14,17"
+            container: ubuntu:18.04
+            os: ubuntu-latest
           - toolset: gcc-8
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: g++-8
           - toolset: gcc-9
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:20.04
+            os: ubuntu-latest
           - toolset: gcc-10
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:22.04
+            os: ubuntu-latest
             install: g++-10
           - toolset: gcc-11
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:22.04
+            os: ubuntu-latest
             install: g++-11
+          - toolset: gcc-12
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:22.04
+            os: ubuntu-latest
+            install: g++-12
+          - toolset: gcc-13
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: g++-13
+          - toolset: gcc-14
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: g++-14
+          - toolset: gcc-15
+            cxxstd: "11,14,17,20,23,2c"
+            container: ubuntu:25.04
+            os: ubuntu-latest
+            install: g++-15
           - toolset: clang
             compiler: clang++-3.9
-            cxxstd: "03,11,14"
-            os: ubuntu-18.04
+            cxxstd: "11,14"
+            container: ubuntu:18.04
+            os: ubuntu-latest
             install: clang-3.9
           - toolset: clang
             compiler: clang++-4.0
-            cxxstd: "03,11,14"
-            os: ubuntu-18.04
+            cxxstd: "11,14"
+            container: ubuntu:18.04
+            os: ubuntu-latest
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
-            cxxstd: "03,11,14,1z"
-            os: ubuntu-18.04
+            cxxstd: "11,14"
+            container: ubuntu:18.04
+            os: ubuntu-latest
             install: clang-5.0
           - toolset: clang
             compiler: clang++-6.0
-            cxxstd: "03,11,14,17"
-            os: ubuntu-18.04
+            cxxstd: "11,14,17"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-6.0
           - toolset: clang
             compiler: clang++-7
-            cxxstd: "03,11,14,17"
-            os: ubuntu-18.04
+            cxxstd: "11,14,17"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-7
           - toolset: clang
             compiler: clang++-8
-            cxxstd: "03,11,14,17"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-8
           - toolset: clang
             compiler: clang++-9
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-9
           - toolset: clang
             compiler: clang++-10
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-10
           - toolset: clang
             compiler: clang++-11
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-11
           - toolset: clang
             compiler: clang++-12
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+            cxxstd: "11,14,17,2a"
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-12
           - toolset: clang
-            cxxstd: "03,11,14,17,2a"
-            os: macos-10.15
+            compiler: clang++-13
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:22.04
+            os: ubuntu-latest
+            install: clang-13
+          - toolset: clang
+            compiler: clang++-14
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:22.04
+            os: ubuntu-latest
+            install: clang-14
+          - toolset: clang
+            compiler: clang++-15
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:22.04
+            os: ubuntu-latest
+            install: clang-15
+          - toolset: clang
+            compiler: clang++-16
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-16
+          - toolset: clang
+            compiler: clang++-17
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-17
+          - toolset: clang
+            compiler: clang++-18
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-18
+          - toolset: clang
+            compiler: clang++-19
+            cxxstd: "11,14,17,20,2b"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-19
+          - toolset: clang
+            compiler: clang++-20
+            cxxstd: "11,14,17,20,23,2c"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-20
+          - toolset: clang
+            compiler: clang++-21
+            cxxstd: "11,14,17,20,23,2c"
+            container: ubuntu:25.10
+            os: ubuntu-latest
+            install: clang-21
+          - toolset: clang
+            cxxstd: "11,14,17,20,2b"
+            os: macos-14
+          - toolset: clang
+            cxxstd: "11,14,17,20,23,2c"
+            os: macos-15
+          - toolset: clang
+            cxxstd: "11,14,17,20,23,2c"
+            os: macos-26
 
     runs-on: ${{matrix.os}}
 
+    container:
+      image: ${{matrix.container}}
+      volumes:
+        - /node20217:/node20217:rw,rshared
+        - ${{ startsWith(matrix.container, 'ubuntu:1') && '/node20217:/__e/node20:ro,rshared' || ' ' }}
+
+    defaults:
+      run:
+        shell: bash
+
     steps:
-      - uses: actions/checkout@v2
+      - name: Setup container environment
+        if: matrix.container
+        run: |
+          apt-get update
+          apt-get -y install sudo python3 git g++ curl xz-utils
+
+      - name: Install nodejs20glibc2.17
+        if: ${{ startsWith( matrix.container, 'ubuntu:1' ) }}
+        run: |
+          curl -LO https://archives.boost.io/misc/node/node-v20.9.0-linux-x64-glibc-217.tar.xz
+          tar -xf node-v20.9.0-linux-x64-glibc-217.tar.xz --strip-components 1 -C /node20217
+          ldd /__e/node20/bin/node
+
+      - uses: actions/checkout@v4
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: |
+          sudo apt-get update
+          sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |
@@ -128,7 +256,7 @@ jobs:
           cd boost-root
           cp -r $GITHUB_WORKSPACE/* libs/$LIBRARY
           git submodule update --init tools/boostdep
-          python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
+          python3 tools/boostdep/depinst/depinst.py $LIBRARY
           ./bootstrap.sh
           ./b2 -d0 headers
 
@@ -147,31 +275,23 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - toolset: msvc-14.0
-            cxxstd: "14"
-            addrmd: 32,64
-            os: windows-2019
-          - toolset: msvc-14.1
-            cxxstd: "14,17,latest"
-            addrmd: 32,64
-            os: windows-2016
-          - toolset: msvc-14.2
-            cxxstd: "14,17,latest"
-            addrmd: 32,64
-            os: windows-2019
           - toolset: msvc-14.3
+            cxxstd: "14,17,20,latest"
+            addrmd: 32,64
+            os: windows-2022
+          - toolset: clang-win
             cxxstd: "14,17,latest"
             addrmd: 32,64
             os: windows-2022
           - toolset: gcc
-            cxxstd: "03,11,14,17,2a"
+            cxxstd: "11,14,17,2a"
             addrmd: 64
-            os: windows-2019
+            os: windows-2022
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Setup Boost
         shell: cmd
@@ -199,25 +319,27 @@ jobs:
         shell: cmd
         run: |
           cd ../boost-root
-          b2 -j3 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release
+          b2 -j2 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release embed-manifest-via=linker
 
   posix-cmake-subdir:
     strategy:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-18.04
-          - os: ubuntu-20.04
-          - os: macos-10.15
+          - os: ubuntu-22.04
+          - os: ubuntu-24.04
+          - os: macos-14
+          - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |
@@ -252,18 +374,20 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-18.04
-          - os: ubuntu-20.04
-          - os: macos-10.15
+          - os: ubuntu-22.04
+          - os: ubuntu-24.04
+          - os: macos-14
+          - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |
@@ -308,18 +432,20 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-18.04
-          - os: ubuntu-20.04
-          - os: macos-10.15
+          - os: ubuntu-22.04
+          - os: ubuntu-24.04
+          - os: macos-14
+          - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |
@@ -356,3 +482,180 @@ jobs:
         run: |
           cd ../boost-root/__build__
           ctest --output-on-failure --no-tests=error
+
+  windows-cmake-subdir:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: windows-latest
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Boost
+        shell: cmd
+        run: |
+          echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
+          for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
+          echo LIBRARY: %LIBRARY%
+          echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
+          echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
+          echo GITHUB_REF: %GITHUB_REF%
+          if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
+          set BOOST_BRANCH=develop
+          for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
+          echo BOOST_BRANCH: %BOOST_BRANCH%
+          cd ..
+          git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
+          cd boost-root
+          xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
+
+      - name: Use library with add_subdirectory (Debug)
+        shell: cmd
+        run: |
+          cd ../boost-root/libs/%LIBRARY%/test/cmake_subdir_test
+          mkdir __build__ && cd __build__
+          cmake ..
+          cmake --build . --config Debug
+          ctest --output-on-failure --no-tests=error -C Debug
+
+      - name: Use library with add_subdirectory (Release)
+        shell: cmd
+        run: |
+          cd ../boost-root/libs/%LIBRARY%/test/cmake_subdir_test/__build__
+          cmake --build . --config Release
+          ctest --output-on-failure --no-tests=error -C Release
+
+  windows-cmake-install:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: windows-latest
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Boost
+        shell: cmd
+        run: |
+          echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
+          for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
+          echo LIBRARY: %LIBRARY%
+          echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
+          echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
+          echo GITHUB_REF: %GITHUB_REF%
+          if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
+          set BOOST_BRANCH=develop
+          for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
+          echo BOOST_BRANCH: %BOOST_BRANCH%
+          cd ..
+          git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
+          cd boost-root
+          xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
+
+      - name: Configure
+        shell: cmd
+        run: |
+          cd ../boost-root
+          mkdir __build__ && cd __build__
+          cmake -DBOOST_INCLUDE_LIBRARIES=%LIBRARY% -DCMAKE_INSTALL_PREFIX=C:/cmake-prefix ..
+
+      - name: Install (Debug)
+        shell: cmd
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target install --config Debug
+
+      - name: Install (Release)
+        shell: cmd
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target install --config Release
+
+      - name: Use the installed library (Debug)
+        shell: cmd
+        run: |
+          cd ../boost-root/libs/%LIBRARY%/test/cmake_install_test && mkdir __build__ && cd __build__
+          cmake -DCMAKE_INSTALL_PREFIX=C:/cmake-prefix ..
+          cmake --build . --config Debug
+          ctest --output-on-failure --no-tests=error -C Debug
+
+      - name: Use the installed library (Release)
+        shell: cmd
+        run: |
+          cd ../boost-root/libs/%LIBRARY%/test/cmake_install_test/__build__
+          cmake --build . --config Release
+          ctest --output-on-failure --no-tests=error -C Release
+
+  windows-cmake-test:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: windows-latest
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Boost
+        shell: cmd
+        run: |
+          echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
+          for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
+          echo LIBRARY: %LIBRARY%
+          echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
+          echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
+          echo GITHUB_REF: %GITHUB_REF%
+          if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
+          set BOOST_BRANCH=develop
+          for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
+          echo BOOST_BRANCH: %BOOST_BRANCH%
+          cd ..
+          git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
+          cd boost-root
+          xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
+
+      - name: Configure
+        shell: cmd
+        run: |
+          cd ../boost-root
+          mkdir __build__ && cd __build__
+          cmake -DBOOST_INCLUDE_LIBRARIES=%LIBRARY% -DBUILD_TESTING=ON ..
+
+      - name: Build tests (Debug)
+        shell: cmd
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target tests --config Debug
+
+      - name: Run tests (Debug)
+        shell: cmd
+        run: |
+          cd ../boost-root/__build__
+          ctest --output-on-failure --no-tests=error -C Debug
+
+      - name: Build tests (Release)
+        shell: cmd
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target tests --config Release
+
+      - name: Run tests (Release)
+        shell: cmd
+        run: |
+          cd ../boost-root/__build__
+          ctest --output-on-failure --no-tests=error -C Release

CMakeLists.txt

@@ -19,6 +19,15 @@ target_link_libraries(boost_variant2
     Boost::mp11
 )
 
+if(NOT CMAKE_VERSION VERSION_LESS 3.19 AND CMAKE_GENERATOR MATCHES "Visual Studio")
+
+  file(GLOB_RECURSE boost_variant2_IDEFILES CONFIGURE_DEPENDS include/*.hpp)
+  source_group(TREE ${PROJECT_SOURCE_DIR}/include FILES ${boost_variant2_IDEFILES} PREFIX "Header Files")
+  list(APPEND boost_variant2_IDEFILES extra/boost_variant2.natvis)
+  target_sources(boost_variant2 PRIVATE ${boost_variant2_IDEFILES})
+
+endif()
+
 target_compile_features(boost_variant2 INTERFACE cxx_std_11)
 
 if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")

README.md

@@ -12,7 +12,7 @@ Supported compilers:
 
 * g++ 4.8 or later with `-std=c++11` or above
 * clang++ 3.9 or later with `-std=c++11` or above
-* Visual Studio 2015, 2017, 2019
+* Visual Studio 2015 or later
 
 Tested on [Github Actions](https://github.com/boostorg/variant2/actions) and
 [Appveyor](https://ci.appveyor.com/project/pdimov/variant2-fkab9).

appveyor.yml

@@ -22,17 +22,24 @@ environment:
       ADDRMD: 32,64
       CXXSTD: 14,17,latest
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
-      TOOLSET: clang-win
-      ADDRMD: 64
-      CXXSTD: 14,17,latest
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
-      TOOLSET: msvc-14.2
+      TOOLSET: msvc-14.1
       ADDRMD: 32,64
       CXXSTD: 14,17,latest
+      CXXFLAGS: /permissive-
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      TOOLSET: clang-win
+      ADDRMD: 64
+      CXXSTD: 14,17,latest
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: clang-win
       ADDRMD: 64
       CXXSTD: 14,17,latest
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      CMAKE: 1
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      CMAKE_SUBDIR: 1
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      CMAKE_INSTALL: 1
 
 install:
   - set BOOST_BRANCH=develop
@@ -51,4 +58,28 @@ build: off
 test_script:
   - if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
   - if not "%ADDRMD%" == "" set ADDRMD=address-model=%ADDRMD%
-  - b2 -j3 libs/variant2/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release embed-manifest-via=linker
+  - if not "%CXXFLAGS%" == "" set CXXFLAGS=cxxflags=%CXXFLAGS%
+  - if "%CMAKE%%CMAKE_SUBDIR%%CMAKE_INSTALL%" == ""  b2 -j2 libs/variant2/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% %CXXFLAGS% variant=debug,release embed-manifest-via=linker
+
+  - if not "%CMAKE%" == ""  mkdir __build__ && cd __build__
+  - if not "%CMAKE%" == ""  cmake -DBUILD_TESTING=ON -DBOOST_INCLUDE_LIBRARIES=variant2 -DBoost_VERBOSE=ON ..
+  - if not "%CMAKE%" == ""  cmake --build . --target tests -j 3 --config Debug & ctest --output-on-failure --no-tests=error -j 3 -C Debug
+  - if not "%CMAKE%" == ""  cmake --build . --target tests -j 3 --config Release & ctest --output-on-failure --no-tests=error -j 3 -C Release
+  - if not "%CMAKE%" == ""  cmake --build . --target tests -j 3 --config MinSizeRel & ctest --output-on-failure --no-tests=error -j 3 -C MinSizeRel
+  - if not "%CMAKE%" == ""  cmake --build . --target tests -j 3 --config RelWithDebInfo & ctest --output-on-failure --no-tests=error -j 3 -C RelWithDebInfo
+
+  - if not "%CMAKE_SUBDIR%" == ""  cd libs/variant2/test/cmake_subdir_test && mkdir __build__ && cd __build__
+  - if not "%CMAKE_SUBDIR%" == ""  cmake ..
+  - if not "%CMAKE_SUBDIR%" == ""  cmake --build . --config Debug && cmake --build . --target check --config Debug
+  - if not "%CMAKE_SUBDIR%" == ""  cmake --build . --config Release && cmake --build . --target check --config Release
+  - if not "%CMAKE_SUBDIR%" == ""  cmake --build . --config MinSizeRel && cmake --build . --target check --config MinSizeRel
+  - if not "%CMAKE_SUBDIR%" == ""  cmake --build . --config RelWithDebInfo && cmake --build . --target check --config RelWithDebInfo
+
+  - if not "%CMAKE_INSTALL%" == ""  mkdir __build__ && cd __build__
+  - if not "%CMAKE_INSTALL%" == ""  cmake -DBOOST_INCLUDE_LIBRARIES=variant2 -DCMAKE_INSTALL_PREFIX=C:/cmake-prefix ..
+  - if not "%CMAKE_INSTALL%" == ""  cmake --build . --target install --config Debug
+  - if not "%CMAKE_INSTALL%" == ""  cmake --build . --target install --config Release
+  - if not "%CMAKE_INSTALL%" == ""  cd ../libs/variant2/test/cmake_install_test && mkdir __build__ && cd __build__
+  - if not "%CMAKE_INSTALL%" == ""  cmake -DCMAKE_INSTALL_PREFIX=C:/cmake-prefix ..
+  - if not "%CMAKE_INSTALL%" == ""  cmake --build . --config Debug && cmake --build . --target check --config Debug
+  - if not "%CMAKE_INSTALL%" == ""  cmake --build . --config Release && cmake --build . --target check --config Release

build.jam

@@ -0,0 +1,22 @@
+# Copyright 2023-2024 René Ferdinand Rivera Morell
+# Copyright 2024 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+require-b2 5.2 ;
+
+constant boost_dependencies :
+    /boost/assert//boost_assert
+    /boost/config//boost_config
+    /boost/mp11//boost_mp11
+    ;
+
+project /boost/variant2 ;
+
+explicit
+    [ alias boost_variant2 : : : : <include>include <library>$(boost_dependencies) ]
+    [ alias all : boost_variant2 test ]
+    ;
+
+call-if : boost-library variant2
+    ;

doc/variant2/changelog.adoc

@@ -1,5 +1,5 @@
 ////
-Copyright 2019-2021 Peter Dimov
+Copyright 2019-2025 Peter Dimov
 Distributed under the Boost Software License, Version 1.0.
 https://www.boost.org/LICENSE_1_0.txt
 ////
@@ -8,6 +8,27 @@ https://www.boost.org/LICENSE_1_0.txt
 # Revision History
 :idprefix: changelog_
 
+## Changes in 1.91.0
+
+* `holds_alternative<T>` and `get<T>` have been relaxed to no longer require `T` to occur exactly once in the list of alternatives. It now must occur at least once.
+
+## Changes in 1.90.0
+
+* More functions have been marked as `constexpr`, including `~variant`.
+  This didn't matter before {cpp}20, but does now.
+
+## Changes in 1.88.0
+
+* Use the smallest appropriate unsigned type for the index.
+
+## Changes in 1.83.0
+
+* Added `uses_double_storage()`.
+
+## Changes in 1.81.0
+
+* Added support for `boost::json::value_from` and `boost::json::value_to`.
+
 ## Changes in 1.79.0
 
 * Added `operator<<` for `monostate`.

doc/variant2/implementation.adoc

@@ -16,7 +16,7 @@ This implementation only depends on Boost.Config, Boost.Assert, and Boost.Mp11.
 
 * GCC 4.8 or later with `-std=c++11` or above
 * Clang 3.9 or later with `-std=c++11` or above
-* Visual Studio 2015, 2017, 2019
+* Visual Studio 2015 or later
 
 Tested on https://github.com/boostorg/variant2/actions[Github Actions] and
 https://ci.appveyor.com/project/pdimov/variant2-fkab9[Appveyor].

doc/variant2/reference.adoc

@@ -157,7 +157,7 @@ template<class R = /*unspecified*/, class F, class... V>
 // visit_by_index (extension)
 
 template<class R = /*unspecified*/, class V, class... F>
-  constexpr /*see below*/ visit_by_index(V&& v, F&&.. f);
+  constexpr /*see below*/ visit_by_index(V&& v, F&&... f);
 
 // monostate
 
@@ -250,6 +250,8 @@ public:
   constexpr bool valueless_by_exception() const noexcept;
   constexpr size_t index() const noexcept;
 
+  static constexpr bool uses_double_storage() noexcept;
+
   // swap
 
   void swap( variant& r ) noexcept( /*see below*/ );
@@ -570,6 +572,18 @@ constexpr size_t index() const noexcept;
 Returns:  ::
   The zero-based index of the active alternative.
 
+```
+static constexpr bool uses_double_storage() noexcept;
+```
+[none]
+* {blank}
++
+Returns:  ::
+  `true` if the variant uses double storage to meet the never valueless
+  guarantee because one of the alternatives is not nothrow move constructible,
+  and `false` otherwise.
+
+
 #### Swap
 
 ```
@@ -705,10 +719,8 @@ template<class U, class... T>
 [none]
 * {blank}
 +
-Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
-  program is ill-formed.
-Returns:  :: `true` if `index()` is equal to the zero-based index of `U`
-  in `T...`.
+Requires: :: The type `U` must be present in `T...`. Otherwise, the program is ill-formed.
+Returns:  :: `true` if `v` holds a value of type `U`, otherwise `false`.
 
 ### get
 
@@ -759,10 +771,8 @@ template<class U, class... T>
 [none]
 * {blank}
 +
-Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
-  program is ill-formed.
-Effects:  :: If `v` holds a value of type `U`, returns a reference to that value.
-  Otherwise, throws `bad_variant_access`.
+Requires: :: The type `U` must be present in `T...`. Otherwise, the program is ill-formed.
+Effects:  :: If `v` holds a value of type `U`, returns a reference to that value. Otherwise, throws `bad_variant_access`.
 
 ### get_if
 
@@ -779,10 +789,8 @@ template<size_t I, class... T>
 [none]
 * {blank}
 +
-Effects:  :: A pointer to the value stored in the variant, if
-  `v != nullptr && v\->index() == I`. Otherwise, `nullptr`.
-Remarks: :: These functions do not participate in overload resolution
-  unless `I` < `sizeof...(T)`.
+Returns:  :: A pointer to the value stored in `*v`, if `v != nullptr && v\->index() == I`. Otherwise, `nullptr`.
+Remarks: :: These functions do not participate in overload resolution unless `I` < `sizeof...(T)`.
 
 ```
 template<class U, class... T>
@@ -797,10 +805,8 @@ template<class U, class... T>
 [none]
 * {blank}
 +
-Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
-  program is ill-formed.
-Effects:  :: Equivalent to: `return get_if<I>(v);` with `I` being
-  the zero-based index of `U` in `T...`.
+Requires: :: The type `U` must be present in `T...`. Otherwise, the program is ill-formed.
+Returns:  :: If `v != nullptr` and `*v` holds a value of type `U`, a pointer to that value. Otherwise, `nullptr`.
 
 ### unsafe_get (extension)
 
@@ -922,7 +928,7 @@ Remarks: :: If `R` is given explicitly, as in `visit<int>`, the return
 
 ```
 template<class R = /*unspecified*/, class V, class... F>
-  constexpr /*see below*/ visit_by_index(V&& v, F&&.. f);
+  constexpr /*see below*/ visit_by_index(V&& v, F&&... f);
 ```
 [none]
 * {blank}

extra/boost_variant2.natvis

@@ -0,0 +1,178 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+
+  <Type Name="boost::variant2::detail::variant_base_impl&lt;*,1,*&gt;">
+    <Intrinsic Name="index" Expression="(int)ix_ - 1"/>
+    <DisplayString Condition="index() ==  0" Optional="true">{{ index=0, value={*($T2*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  1" Optional="true">{{ index=1, value={*($T3*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  2" Optional="true">{{ index=2, value={*($T4*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  3" Optional="true">{{ index=3, value={*($T5*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  4" Optional="true">{{ index=4, value={*($T6*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  5" Optional="true">{{ index=5, value={*($T7*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  6" Optional="true">{{ index=6, value={*($T8*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  7" Optional="true">{{ index=7, value={*($T9*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  8" Optional="true">{{ index=8, value={*($T10*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() ==  9" Optional="true">{{ index=9, value={*($T11*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 10" Optional="true">{{ index=10, value={*($T12*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 11" Optional="true">{{ index=11, value={*($T13*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 12" Optional="true">{{ index=12, value={*($T14*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 13" Optional="true">{{ index=13, value={*($T15*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 14" Optional="true">{{ index=14, value={*($T16*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 15" Optional="true">{{ index=15, value={*($T17*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 16" Optional="true">{{ index=16, value={*($T18*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 17" Optional="true">{{ index=17, value={*($T19*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 18" Optional="true">{{ index=18, value={*($T20*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 19" Optional="true">{{ index=19, value={*($T21*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 20" Optional="true">{{ index=10, value={*($T22*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 21" Optional="true">{{ index=21, value={*($T23*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 22" Optional="true">{{ index=22, value={*($T24*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 23" Optional="true">{{ index=23, value={*($T25*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 24" Optional="true">{{ index=24, value={*($T26*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 25" Optional="true">{{ index=25, value={*($T27*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 26" Optional="true">{{ index=26, value={*($T28*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 27" Optional="true">{{ index=27, value={*($T29*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 28" Optional="true">{{ index=28, value={*($T30*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 29" Optional="true">{{ index=29, value={*($T31*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 30" Optional="true">{{ index=30, value={*($T32*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 31" Optional="true">{{ index=31, value={*($T33*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 32" Optional="true">{{ index=32, value={*($T34*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 33" Optional="true">{{ index=33, value={*($T35*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 34" Optional="true">{{ index=34, value={*($T36*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 35" Optional="true">{{ index=35, value={*($T37*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 36" Optional="true">{{ index=36, value={*($T38*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 37" Optional="true">{{ index=37, value={*($T39*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 38" Optional="true">{{ index=38, value={*($T40*)&amp;st_} }}</DisplayString>
+    <DisplayString Condition="index() == 39" Optional="true">{{ index=39, value={*($T41*)&amp;st_} }}</DisplayString>
+    <Expand>
+      <Item Name="index">index()</Item>
+      <Item Name="[value]" Condition="index() ==  0" Optional="true">*($T2*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  1" Optional="true">*($T3*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  2" Optional="true">*($T4*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  3" Optional="true">*($T5*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  4" Optional="true">*($T6*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  5" Optional="true">*($T7*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  6" Optional="true">*($T8*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  7" Optional="true">*($T9*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  8" Optional="true">*($T10*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() ==  9" Optional="true">*($T11*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 10" Optional="true">*($T12*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 11" Optional="true">*($T13*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 12" Optional="true">*($T14*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 13" Optional="true">*($T15*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 14" Optional="true">*($T16*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 15" Optional="true">*($T17*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 16" Optional="true">*($T18*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 17" Optional="true">*($T19*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 18" Optional="true">*($T20*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 19" Optional="true">*($T21*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 20" Optional="true">*($T22*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 21" Optional="true">*($T23*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 22" Optional="true">*($T24*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 23" Optional="true">*($T25*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 24" Optional="true">*($T26*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 25" Optional="true">*($T27*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 26" Optional="true">*($T28*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 27" Optional="true">*($T29*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 28" Optional="true">*($T30*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 29" Optional="true">*($T31*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 30" Optional="true">*($T32*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 31" Optional="true">*($T33*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 32" Optional="true">*($T34*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 33" Optional="true">*($T35*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 34" Optional="true">*($T36*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 35" Optional="true">*($T37*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 36" Optional="true">*($T38*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 37" Optional="true">*($T39*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 38" Optional="true">*($T40*)&amp;st_</Item>
+      <Item Name="[value]" Condition="index() == 39" Optional="true">*($T41*)&amp;st_</Item>
+    </Expand>
+  </Type>
+
+  <Type Name="boost::variant2::detail::variant_base_impl&lt;*,0,*&gt;">
+    <Intrinsic Name="index" Expression="(int)((ix_-2)/2)"/>
+    <DisplayString Condition="index() ==  0" Optional="true">{{ index=0, value={*($T2*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  1" Optional="true">{{ index=1, value={*($T3*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  2" Optional="true">{{ index=2, value={*($T4*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  3" Optional="true">{{ index=3, value={*($T5*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  4" Optional="true">{{ index=4, value={*($T6*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  5" Optional="true">{{ index=5, value={*($T7*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  6" Optional="true">{{ index=6, value={*($T8*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  7" Optional="true">{{ index=7, value={*($T9*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() ==  8" Optional="true">{{ index=8, value={*($T10*)&amp;st_[ix_%2]} }}</DisplayString>
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+    <DisplayString Condition="index() == 10" Optional="true">{{ index=10, value={*($T12*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 11" Optional="true">{{ index=11, value={*($T13*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 12" Optional="true">{{ index=12, value={*($T14*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 13" Optional="true">{{ index=13, value={*($T15*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 14" Optional="true">{{ index=14, value={*($T16*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 15" Optional="true">{{ index=15, value={*($T17*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 16" Optional="true">{{ index=16, value={*($T18*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 17" Optional="true">{{ index=17, value={*($T19*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 18" Optional="true">{{ index=18, value={*($T20*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 19" Optional="true">{{ index=19, value={*($T21*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 20" Optional="true">{{ index=20, value={*($T22*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 21" Optional="true">{{ index=21, value={*($T23*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 22" Optional="true">{{ index=22, value={*($T24*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 23" Optional="true">{{ index=23, value={*($T25*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 24" Optional="true">{{ index=24, value={*($T26*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 25" Optional="true">{{ index=25, value={*($T27*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 26" Optional="true">{{ index=26, value={*($T28*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 27" Optional="true">{{ index=27, value={*($T29*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 28" Optional="true">{{ index=28, value={*($T30*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 29" Optional="true">{{ index=29, value={*($T31*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 30" Optional="true">{{ index=30, value={*($T32*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 31" Optional="true">{{ index=31, value={*($T33*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 32" Optional="true">{{ index=32, value={*($T34*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 33" Optional="true">{{ index=33, value={*($T35*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 34" Optional="true">{{ index=34, value={*($T36*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 35" Optional="true">{{ index=35, value={*($T37*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 36" Optional="true">{{ index=36, value={*($T38*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 37" Optional="true">{{ index=37, value={*($T39*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 38" Optional="true">{{ index=38, value={*($T40*)&amp;st_[ix_%2]} }}</DisplayString>
+    <DisplayString Condition="index() == 39" Optional="true">{{ index=39, value={*($T41*)&amp;st_[ix_%2]} }}</DisplayString>
+    <Expand>
+      <Item Name="index">index()</Item>
+      <Item Name="[value]" Condition="index() ==  0" Optional="true">*($T2*)&amp;st_[ix_%2]</Item>
+      <Item Name="[value]" Condition="index() ==  1" Optional="true">*($T3*)&amp;st_[ix_%2]</Item>
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+      <Item Name="[value]" Condition="index() == 35" Optional="true">*($T37*)&amp;st_[ix_%2]</Item>
+      <Item Name="[value]" Condition="index() == 36" Optional="true">*($T38*)&amp;st_[ix_%2]</Item>
+      <Item Name="[value]" Condition="index() == 37" Optional="true">*($T39*)&amp;st_[ix_%2]</Item>
+      <Item Name="[value]" Condition="index() == 38" Optional="true">*($T40*)&amp;st_[ix_%2]</Item>
+      <Item Name="[value]" Condition="index() == 39" Optional="true">*($T41*)&amp;st_[ix_%2]</Item>
+    </Expand>
+  </Type>
+
+</AutoVisualizer>

test/CMakeLists.txt

@@ -6,6 +6,6 @@ include(BoostTestJamfile OPTIONAL RESULT_VARIABLE HAVE_BOOST_TEST)
 
 if(HAVE_BOOST_TEST)
 
-boost_test_jamfile(FILE Jamfile LINK_LIBRARIES Boost::variant2 Boost::core Boost::container_hash)
+boost_test_jamfile(FILE Jamfile LINK_LIBRARIES Boost::variant2 Boost::core Boost::container_hash Boost::system)
 
 endif()

test/Jamfile

@@ -1,13 +1,8 @@
-#  Boost.Variant2 Library Test Jamfile
-#
-#  Copyright 2015-2019 Peter Dimov
-#
+#  Copyright 2015-2026 Peter Dimov
 #  Distributed under the Boost Software License, Version 1.0.
-#  See accompanying file LICENSE_1_0.txt or copy at
-#  http://www.boost.org/LICENSE_1_0.txt
+#  https://www.boost.org/LICENSE_1_0.txt
 
 import testing ;
-import ../../config/checks/config : requires ;
 
 project
   : default-build
@@ -16,11 +11,14 @@ project
 
   : requirements
 
-    [ requires cxx11_variadic_templates cxx11_template_aliases cxx11_decltype cxx11_constexpr ]
+    <library>/boost/variant2//boost_variant2
+    <library>/boost/core//boost_core
+    <library>/boost/container_hash//boost_container_hash
 
     <toolset>msvc:<warnings-as-errors>on
     <toolset>gcc:<warnings-as-errors>on
     <toolset>clang:<warnings-as-errors>on
+
   ;
 
 run quick.cpp ;
@@ -29,54 +27,54 @@ run variant_size.cpp ;
 run variant_alternative.cpp ;
 
 run variant_holds_alternative.cpp ;
-compile variant_holds_alternative_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_holds_alternative_cx.cpp ;
 
 run variant_get_by_index.cpp ;
-compile variant_get_by_index_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_get_by_index_cx.cpp ;
 
 run variant_get_by_type.cpp ;
-compile variant_get_by_type_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_get_by_type_cx.cpp ;
 
 run variant_default_construct.cpp ;
-compile variant_default_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_default_construct_cx.cpp ;
 
 run variant_copy_construct.cpp ;
-compile variant_copy_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_copy_construct_cx.cpp ;
 
 run variant_move_construct.cpp ;
-compile variant_move_construct_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_move_construct_cx.cpp ;
 
 run variant_value_construct.cpp ;
-compile variant_value_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_value_construct_cx.cpp ;
 
 run variant_in_place_index_construct.cpp ;
-compile variant_in_place_index_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_in_place_index_construct_cx.cpp ;
 
 run variant_in_place_type_construct.cpp ;
-compile variant_in_place_type_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
+compile variant_in_place_type_construct_cx.cpp ;
 
 run variant_copy_assign.cpp ;
-compile variant_copy_assign_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_copy_assign_cx.cpp ;
 
 run variant_move_assign.cpp ;
-compile variant_move_assign_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_move_assign_cx.cpp ;
 
 run variant_value_assign.cpp ;
-compile variant_value_assign_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_value_assign_cx.cpp ;
 
 run variant_emplace_index.cpp ;
-compile variant_emplace_index_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_emplace_index_cx.cpp ;
 
 run variant_emplace_type.cpp ;
-compile variant_emplace_type_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_emplace_type_cx.cpp ;
 
 run variant_swap.cpp ;
 
 run variant_eq_ne.cpp ;
-compile variant_eq_ne_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_eq_ne_cx.cpp ;
 
 run variant_lt_gt.cpp ;
-compile variant_lt_gt_cx.cpp : [ requires cxx14_constexpr ] ;
+compile variant_lt_gt_cx.cpp ;
 
 run variant_destroy.cpp ;
 run variant_visit.cpp ;
@@ -99,10 +97,10 @@ local NX =
     ;
 
 run variant_get_by_index.cpp throw_exception.cpp : : : $(NX) : variant_get_by_index_nx ;
-compile variant_get_by_index_cx.cpp : $(NX) <toolset>msvc-14.0:<build>no : variant_get_by_index_cx_nx ;
+compile variant_get_by_index_cx.cpp : $(NX) : variant_get_by_index_cx_nx ;
 
 run variant_get_by_type.cpp throw_exception.cpp : : : $(NX) : variant_get_by_type_nx ;
-compile variant_get_by_type_cx.cpp : $(NX) <toolset>msvc-14.0:<build>no : variant_get_by_type_cx_nx ;
+compile variant_get_by_type_cx.cpp : $(NX) : variant_get_by_type_cx_nx ;
 
 run variant_subset.cpp throw_exception.cpp : : : $(NX) : variant_subset_nx ;
 
@@ -113,7 +111,12 @@ run variant_special.cpp ;
 
 run variant_visit_derived.cpp ;
 
-run variant_many_types.cpp ;
+# These two tests require /bigobj under GCC/Windows and CMake/VS2022 (for some reason)
+
+run variant_many_types.cpp : : :
+    <toolset>gcc,<target-os>windows:<variant>release
+    <toolset>gcc,<target-os>cygwin:<variant>release
+    ;
 
 run variant_visit_r.cpp : : :
     <toolset>gcc,<target-os>windows:<variant>release
@@ -126,3 +129,54 @@ run variant_visit_by_index.cpp ;
 
 run variant_ostream_insert.cpp ;
 run is_output_streamable.cpp ;
+
+local JSON =
+    <library>/boost/json//boost_json/<warnings>off
+    "<toolset>msvc-14.0:<build>no"
+    "<toolset>msvc-14.2:<cxxflags>-wd5104"
+    "<undefined-sanitizer>norecover:<link>static"
+    "<toolset>gcc-4.9:<build>no"
+    ;
+
+run variant_json_value_from.cpp : : : $(JSON) ;
+run variant_json_value_to.cpp : : : $(JSON) ;
+
+compile variant_uses_double_storage.cpp ;
+
+run variant_derived_construct2.cpp ;
+
+compile variant_visit_cx.cpp ;
+compile variant_visit_cx_2.cpp ;
+compile variant_visit_r_cx.cpp ;
+
+local index-type-reqs =
+
+  <variant>release
+  <toolset>msvc,<address-model>32:<build>no
+  <toolset>clang,<target-os>linux:<cxxflags>-ftemplate-depth=1024 # for Clang 3.x
+;
+
+local list-sizes = 126 127 128 129 254 255 256 257 ;
+
+for local list-size in $(list-sizes)
+{
+    run variant_index_type.cpp : : : $(index-type-reqs) <define>LIST_SIZE=$(list-size) : variant_index_type_$(list-size) ;
+}
+
+compile variant_default_construct_cx_2.cpp ;
+compile variant_default_construct_cx_3.cpp ;
+compile variant_default_construct_cx_4.cpp ;
+compile variant_default_construct_cx_5.cpp ;
+
+compile variant_value_construct_cx_2.cpp ;
+compile variant_value_construct_cx_3.cpp ;
+compile variant_value_construct_cx_4.cpp ;
+
+# GCC 12+ false positive -Wmaybe-uninitialized with non-trivially-copyable types
+run variant_issue_55.cpp
+  : : :
+    <library>/boost/system//boost_system
+
+    # clang-cl 32 bit fails with an assertion in mp_with_index, likely due to a codegen bug
+    "<toolset>clang-win,<address-model>32:<build>no"
+  ;

test/cmake_install_test/CMakeLists.txt

@@ -14,4 +14,4 @@ target_link_libraries(quick Boost::variant2)
 enable_testing()
 add_test(quick quick)
 
-add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure -C $<CONFIG>)
+add_custom_target(check VERBATIM COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure -C $<CONFIG>)

test/cmake_subdir_test/CMakeLists.txt

@@ -18,4 +18,4 @@ target_link_libraries(quick Boost::variant2)
 enable_testing()
 add_test(quick quick)
 
-add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure -C $<CONFIG>)
+add_custom_target(check VERBATIM COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure -C $<CONFIG>)

test/variant_copy_assign_cx.cpp

@@ -1,12 +1,22 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#elif defined(BOOST_GCC) && BOOST_GCC < 70000
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_GCC < 70000" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -33,26 +43,26 @@ enum E
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
-template<class V, class T, class A> constexpr T test( A const& a )
+template<class V, std::size_t I, class A> constexpr variant_alternative_t<I, V> test( A const& a )
 {
     V v;
 
     v = a;
 
-    return get<T>(v);
+    return get<I>(v);
 }
 
 int main()
 {
     {
         constexpr variant<int> v( 1 );
-        constexpr auto w = test<variant<int>, int>( v );
+        constexpr auto w = test<variant<int>, 0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<X> v( 1 );
-        constexpr auto w = test<variant<X>, X>( v );
+        constexpr auto w = test<variant<X>, 0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -61,7 +71,7 @@ int main()
 
     {
         constexpr variant<Y> v( 1 );
-        constexpr auto w = test<variant<Y>, Y>( v );
+        constexpr auto w = test<variant<Y>, 0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -69,31 +79,31 @@ int main()
 
     {
         constexpr variant<int, float> v( 1 );
-        constexpr auto w = test<variant<int, float>, int>( v );
+        constexpr auto w = test<variant<int, float>, 0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<int, float> v( 3.0f );
-        constexpr auto w = test<variant<int, float>, float>( v );
+        constexpr auto w = test<variant<int, float>, 1>( v );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
         constexpr variant<int, int, float> v( 3.0f );
-        constexpr auto w = test<variant<int, int, float>, float>( v );
+        constexpr auto w = test<variant<int, int, float>, 2>( v );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
         constexpr variant<E, E, X> v( 1 );
-        constexpr auto w = test<variant<E, E, X>, X>( v );
+        constexpr auto w = test<variant<E, E, X>, 2>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<int, int, float, float, X> v( X(1) );
-        constexpr auto w = test<variant<int, int, float, float, X>, X>( v );
+        constexpr auto w = test<variant<int, int, float, float, X>, 4>( v );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -102,15 +112,17 @@ int main()
 
     {
         constexpr variant<E, E, Y> v( 1 );
-        constexpr auto w = test<variant<E, E, Y>, Y>( v );
+        constexpr auto w = test<variant<E, E, Y>, 2>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<int, int, float, float, Y> v( Y(1) );
-        constexpr auto w = test<variant<int, int, float, float, Y>, Y>( v );
+        constexpr auto w = test<variant<int, int, float, float, Y>, 4>( v );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 }
+
+#endif

test/variant_copy_assign_throw.cpp

@@ -45,7 +45,7 @@ void test()
     variant<Y1, Y2> v1( in_place_type_t<Y1>{} );
     variant<Y1, Y2> v2( in_place_type_t<Y2>{} );
 
-    BOOST_TEST_THROWS( v1 = v2, std::runtime_error )
+    BOOST_TEST_THROWS( v1 = v2, std::runtime_error );
 }
 
 int main()

test/variant_copy_construct_cx.cpp

@@ -1,12 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1910
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_MSVC < 1910" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -33,22 +38,22 @@ enum E
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
-template<class T, class V> constexpr T test( V const v )
+template<std::size_t I, class V> constexpr variant_alternative_t<I, V> test( V const v )
 {
-    return get<T>( v );
+    return get<I>( v );
 }
 
 int main()
 {
     {
         constexpr variant<int> v( 1 );
-        constexpr auto w = test<int>( v );
+        constexpr auto w = test<0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<X> v( 1 );
-        constexpr auto w = test<X>( v );
+        constexpr auto w = test<0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -57,7 +62,7 @@ int main()
 
     {
         constexpr variant<Y> v( 1 );
-        constexpr auto w = test<Y>( v );
+        constexpr auto w = test<0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -65,31 +70,31 @@ int main()
 
     {
         constexpr variant<int, float> v( 1 );
-        constexpr auto w = test<int>( v );
+        constexpr auto w = test<0>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<int, float> v( 3.0f );
-        constexpr auto w = test<float>( v );
+        constexpr auto w = test<1>( v );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
         constexpr variant<int, int, float> v( 3.0f );
-        constexpr auto w = test<float>( v );
+        constexpr auto w = test<2>( v );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
         constexpr variant<E, E, X> v( 1 );
-        constexpr auto w = test<X>( v );
+        constexpr auto w = test<2>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<int, int, float, float, X> v( X(1) );
-        constexpr auto w = test<X>( v );
+        constexpr auto w = test<4>( v );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -98,15 +103,17 @@ int main()
 
     {
         constexpr variant<E, E, Y> v( 1 );
-        constexpr auto w = test<Y>( v );
+        constexpr auto w = test<2>( v );
         STATIC_ASSERT( w == 1 );
     }
 
     {
         constexpr variant<int, int, float, float, Y> v( Y(1) );
-        constexpr auto w = test<Y>( v );
+        constexpr auto w = test<4>( v );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 }
+
+#endif

test/variant_default_construct_cx.cpp

@@ -1,12 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1910
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_MSVC < 1910" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -49,3 +54,5 @@ int main()
         STATIC_ASSERT( get<0>(v) == 0 );
     }
 }
+
+#endif

test/variant_default_construct_cx_2.cpp

@@ -0,0 +1,32 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(__cpp_constexpr_dynamic_alloc) || __cpp_constexpr_dynamic_alloc < 201907L
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because __cpp_constexpr_dynamic_alloc < 201907L")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X
+{
+    constexpr ~X() {}
+};
+
+int main()
+{
+    constexpr variant<int, X> v;
+
+    STATIC_ASSERT( v.index() == 0 );
+    STATIC_ASSERT( get<0>(v) == 0 );
+}
+
+#endif

test/variant_default_construct_cx_3.cpp

@@ -0,0 +1,37 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(__cpp_constexpr_dynamic_alloc) || __cpp_constexpr_dynamic_alloc < 201907L
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because __cpp_constexpr_dynamic_alloc < 201907L")
+int main() {}
+
+#elif defined(BOOST_CLANG) && BOOST_CLANG_VERSION < 180000
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because BOOST_CLANG_VERSION < 180000")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X
+{
+	~X() {}
+};
+
+int main()
+{
+    constexpr variant<int, X> v;
+
+    STATIC_ASSERT( v.index() == 0 );
+    STATIC_ASSERT( get<0>(v) == 0 );
+}
+
+#endif

test/variant_default_construct_cx_4.cpp

@@ -0,0 +1,33 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <string>
+
+#if !defined(__cpp_constexpr_dynamic_alloc) || __cpp_constexpr_dynamic_alloc < 201907L
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because __cpp_constexpr_dynamic_alloc < 201907L")
+int main() {}
+
+#elif defined(BOOST_CLANG) && BOOST_CLANG_VERSION < 180000
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because BOOST_CLANG_VERSION < 180000")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+int main()
+{
+    constexpr variant<int, std::string> v;
+
+    STATIC_ASSERT( v.index() == 0 );
+    STATIC_ASSERT( get<0>(v) == 0 );
+}
+
+#endif

test/variant_default_construct_cx_5.cpp

@@ -0,0 +1,49 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(__cpp_constexpr_dynamic_alloc) || __cpp_constexpr_dynamic_alloc < 201907L
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because __cpp_constexpr_dynamic_alloc < 201907L")
+int main() {}
+
+#elif defined(BOOST_MSVC) && BOOST_MSVC < 1960
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because BOOST_MSVC < 1960")
+int main() {}
+
+#elif defined(BOOST_CLANG) && BOOST_CLANG_VERSION < 180000
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because BOOST_CLANG_VERSION < 180000")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X
+{
+};
+
+struct Y
+{
+    constexpr ~Y() {}
+};
+
+struct Z
+{
+    ~Z() {}
+};
+
+int main()
+{
+    constexpr variant<X, Y, Z> v;
+    STATIC_ASSERT( v.index() == 0 );
+}
+
+#endif

test/variant_derived_construct2.cpp

@@ -0,0 +1,60 @@
+// Copyright 2024 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct expr: boost::variant2::variant<bool, int, double>
+{
+    using variant::variant;
+};
+
+int main()
+{
+    using boost::variant2::get;
+
+    {
+        expr v{ true };
+
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v), true );
+    }
+
+    {
+        expr v{ 2 };
+
+        BOOST_TEST_EQ( v.index(), 1 );
+        BOOST_TEST_EQ( get<1>(v), 2 );
+    }
+
+    {
+        expr v{ 3.0 };
+
+        BOOST_TEST_EQ( v.index(), 2 );
+        BOOST_TEST_EQ( get<2>(v), 3.0 );
+    }
+
+    {
+        expr v( true );
+
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v), true );
+    }
+
+    {
+        expr v( 2 );
+
+        BOOST_TEST_EQ( v.index(), 1 );
+        BOOST_TEST_EQ( get<1>(v), 2 );
+    }
+
+    {
+        expr v( 3.0 );
+
+        BOOST_TEST_EQ( v.index(), 2 );
+        BOOST_TEST_EQ( get<2>(v), 3.0 );
+    }
+
+    return boost::report_errors();
+}

test/variant_emplace_index.cpp

@@ -288,5 +288,39 @@ int main()
     BOOST_TEST_EQ( Z1::instances, 0 );
     BOOST_TEST_EQ( Z2::instances, 0 );
 
+    {
+        variant<monostate, monostate> v;
+        BOOST_TEST_EQ( v.index(), 0 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+    }
+
+    {
+        variant<monostate, int> v;
+        BOOST_TEST_EQ( v.index(), 0 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+    }
+
     return boost::report_errors();
 }

test/variant_emplace_index_cx.cpp

@@ -1,12 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -134,3 +139,5 @@ int main()
 
 #endif
 }
+
+#endif

test/variant_emplace_type_cx.cpp

@@ -1,12 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -28,24 +33,24 @@ struct Y
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
-template<class V, class T, class A> constexpr A test( A const& a )
+template<class V, std::size_t I, class A, class T = variant_alternative_t<I, V>> constexpr T test( A const& a )
 {
     V v;
 
     v.template emplace<T>( a );
 
-    return get<T>(v);
+    return get<I>(v);
 }
 
 int main()
 {
     {
-        constexpr auto w = test<variant<int>, int>( 1 );
+        constexpr auto w = test<variant<int>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<X>, X>( 1 );
+        constexpr auto w = test<variant<X>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -53,29 +58,29 @@ int main()
 #else
 
     {
-        constexpr auto w = test<variant<Y>, Y>( 1 );
+        constexpr auto w = test<variant<Y>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 
     {
-        constexpr auto w = test<variant<int, float>, int>( 1 );
+        constexpr auto w = test<variant<int, float>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<int, float>, float>( 3.0f );
+        constexpr auto w = test<variant<int, float>, 1>( 3.0f );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float>, float>( 3.0f );
+        constexpr auto w = test<variant<int, int, float>, 2>( 3.0f );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float, float, X>, X>( 1 );
+        constexpr auto w = test<variant<int, int, float, float, X>, 4>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -83,9 +88,11 @@ int main()
 #else
 
     {
-        constexpr auto w = test<variant<int, int, float, float, Y>, Y>( 1 );
+        constexpr auto w = test<variant<int, int, float, float, Y>, 4>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 }
+
+#endif

test/variant_eq_ne_cx.cpp

@@ -1,24 +1,26 @@
-
 // Copyright 2017, 2019 Peter Dimov
-//
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
-using namespace boost::variant2;
-
-#if !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
-
+#include <boost/mp11.hpp>
+#include <boost/config.hpp>
 #include <boost/config/pragma_message.hpp>
 
-BOOST_PRAGMA_MESSAGE("Skipping constexpr op==, op!= test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
 
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#elif !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr op==, op!= test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
 int main() {}
 
 #else
 
+using namespace boost::variant2;
+
 struct X
 {
 };

test/variant_get_by_index_cx.cpp

@@ -1,13 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
 #include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1910
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_MSVC < 1910" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -58,7 +62,7 @@ int main()
     {
         constexpr variant<int, float> v( 3.14f );
 
-        STATIC_ASSERT( get<1>(v) == 3.14f );
+        STATIC_ASSERT( get<1>(v) == (float)3.14f ); // see FLT_EVAL_METHOD
 
         STATIC_ASSERT_IF( get_if<0>(&v) == nullptr );
         STATIC_ASSERT_IF( get_if<1>(&v) == &get<1>(v) );
@@ -87,10 +91,12 @@ int main()
     {
         constexpr variant<int, int, float> v( 3.14f );
 
-        STATIC_ASSERT( get<2>(v) == 3.14f );
+        STATIC_ASSERT( get<2>(v) == (float)3.14f );
 
         STATIC_ASSERT_IF( get_if<0>(&v) == nullptr );
         STATIC_ASSERT_IF( get_if<1>(&v) == nullptr );
         STATIC_ASSERT_IF( get_if<2>(&v) == &get<2>(v) );
     }
 }
+
+#endif

test/variant_get_by_type.cpp

@@ -1,10 +1,6 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #if defined( __clang__ ) && defined( __has_warning )
 # if __has_warning( "-Wdeprecated-volatile" )
@@ -217,6 +213,9 @@ int main()
         BOOST_TEST_EQ( get<int>(v), 0 );
         BOOST_TEST_EQ( get_if<int>(&v), &get<int>(v) );
 
+        BOOST_TEST_THROWS( get<float>(v), bad_variant_access );
+        BOOST_TEST_EQ( get_if<float>(&v), nullptr );
+
         BOOST_TEST_EQ( get<int>(std::move(v)), 0 );
     }
 
@@ -226,6 +225,9 @@ int main()
         BOOST_TEST_EQ( get<int>(v), 0 );
         BOOST_TEST_EQ( get_if<int>(&v), &get<int>(v) );
 
+        BOOST_TEST_THROWS( get<float>(v), bad_variant_access );
+        BOOST_TEST_EQ( get_if<float>(&v), nullptr );
+
         BOOST_TEST_EQ( get<int>(std::move(v)), 0 );
     }
 
@@ -235,6 +237,9 @@ int main()
         BOOST_TEST_EQ( get<int>(v), 1 );
         BOOST_TEST_EQ( get_if<int>(&v), &get<int>(v) );
 
+        BOOST_TEST_THROWS( get<float>(v), bad_variant_access );
+        BOOST_TEST_EQ( get_if<float>(&v), nullptr );
+
         BOOST_TEST_EQ( get<int>(std::move(v)), 1 );
     }
 
@@ -244,12 +249,18 @@ int main()
         BOOST_TEST_EQ( get<int>(v), 1 );
         BOOST_TEST_EQ( get_if<int>(&v), &get<int>(v) );
 
+        BOOST_TEST_THROWS( get<float>(v), bad_variant_access );
+        BOOST_TEST_EQ( get_if<float>(&v), nullptr );
+
         BOOST_TEST_EQ( get<int>(std::move(v)), 1 );
     }
 
     {
         variant<int, int, float> v( 3.14f );
 
+        BOOST_TEST_THROWS( get<int>(v), bad_variant_access );
+        BOOST_TEST_EQ( get_if<int>(&v), nullptr );
+
         BOOST_TEST_EQ( get<float>(v), 3.14f );
         BOOST_TEST_EQ( get_if<float>(&v), &get<float>(v) );
 
@@ -259,6 +270,9 @@ int main()
     {
         variant<int, int, float> const v( 3.14f );
 
+        BOOST_TEST_THROWS( get<int>(v), bad_variant_access );
+        BOOST_TEST_EQ( get_if<int>(&v), nullptr );
+
         BOOST_TEST_EQ( get<float>(v), 3.14f );
         BOOST_TEST_EQ( get_if<float>(&v), &get<float>(v) );
 
@@ -301,11 +315,13 @@ int main()
 
     {
         variant<int, int, float> * p = 0;
+        BOOST_TEST_EQ( get_if<int>(p), nullptr );
         BOOST_TEST_EQ( get_if<float>(p), nullptr );
     }
 
     {
         variant<int, int, float> const * p = 0;
+        BOOST_TEST_EQ( get_if<int>(p), nullptr );
         BOOST_TEST_EQ( get_if<float>(p), nullptr );
     }
 

test/variant_get_by_type_cx.cpp

@@ -1,13 +1,18 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/mp11.hpp>
 #include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Test skipped because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -58,7 +63,7 @@ int main()
     {
         constexpr variant<int, float> v( 3.14f );
 
-        STATIC_ASSERT( get<float>(v) == 3.14f );
+        STATIC_ASSERT( get<float>(v) == (float)3.14f ); // see FLT_EVAL_METHOD
 
         STATIC_ASSERT_IF( get_if<int>(&v) == nullptr );
         STATIC_ASSERT_IF( get_if<float>(&v) == &get<float>(v) );
@@ -70,6 +75,7 @@ int main()
         STATIC_ASSERT( get<int>(v) == 0 );
 
         STATIC_ASSERT_IF( get_if<int>(&v) == &get<int>(v) );
+        STATIC_ASSERT_IF( get_if<float>(&v) == nullptr );
     }
 
     {
@@ -78,13 +84,17 @@ int main()
         STATIC_ASSERT( get<int>(v) == 1 );
 
         STATIC_ASSERT_IF( get_if<int>(&v) == &get<int>(v) );
+        STATIC_ASSERT_IF( get_if<float>(&v) == nullptr );
     }
 
     {
         constexpr variant<int, int, float> v( 3.14f );
 
-        STATIC_ASSERT( get<float>(v) == 3.14f );
+        STATIC_ASSERT( get<float>(v) == (float)3.14f );
 
+        STATIC_ASSERT_IF( get_if<int>(&v) == nullptr );
         STATIC_ASSERT_IF( get_if<float>(&v) == &get<float>(v) );
     }
 }
+
+#endif

test/variant_holds_alternative.cpp

@@ -1,10 +1,6 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
@@ -34,13 +30,27 @@ int main()
     {
         variant<int, float, float> v;
         BOOST_TEST( holds_alternative<int>( v ) );
+        BOOST_TEST( !holds_alternative<float>( v ) );
     }
 
     {
         variant<int, int, float> v( 3.14f );
+        BOOST_TEST( !holds_alternative<int>( v ) );
         BOOST_TEST( holds_alternative<float>( v ) );
     }
 
+    {
+        variant<int, int, float> v( in_place_index_t<0>{} );
+        BOOST_TEST( holds_alternative<int>( v ) );
+        BOOST_TEST( !holds_alternative<float>( v ) );
+    }
+
+    {
+        variant<int, int, float> v( in_place_index_t<1>{} );
+        BOOST_TEST( holds_alternative<int>( v ) );
+        BOOST_TEST( !holds_alternative<float>( v ) );
+    }
+
     {
         variant<int, float, std::string> v;
         BOOST_TEST( holds_alternative<int>( v ) );
@@ -64,15 +74,31 @@ int main()
 
     {
         variant<int, int, float, std::string> v( 3.14f );
+        BOOST_TEST( !holds_alternative<int>( v ) );
         BOOST_TEST( holds_alternative<float>( v ) );
         BOOST_TEST( !holds_alternative<std::string>( v ) );
     }
 
     {
         variant<int, int, float, std::string> v( "text" );
+        BOOST_TEST( !holds_alternative<int>( v ) );
         BOOST_TEST( !holds_alternative<float>( v ) );
         BOOST_TEST( holds_alternative<std::string>( v ) );
     }
 
+    {
+        variant<int, int, float, std::string> v( in_place_index_t<0>{} );
+        BOOST_TEST( holds_alternative<int>( v ) );
+        BOOST_TEST( !holds_alternative<float>( v ) );
+        BOOST_TEST( !holds_alternative<std::string>( v ) );
+    }
+
+    {
+        variant<int, int, float, std::string> v( in_place_index_t<1>{} );
+        BOOST_TEST( holds_alternative<int>( v ) );
+        BOOST_TEST( !holds_alternative<float>( v ) );
+        BOOST_TEST( !holds_alternative<std::string>( v ) );
+    }
+
     return boost::report_errors();
 }

test/variant_holds_alternative_cx.cpp

@@ -1,12 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1910
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_MSVC < 1910" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -34,10 +39,20 @@ int main()
     {
         constexpr variant<int, float, float> v;
         STATIC_ASSERT( holds_alternative<int>( v ) );
+        STATIC_ASSERT( !holds_alternative<float>( v ) );
     }
 
     {
         constexpr variant<int, int, float> v( 3.14f );
+        STATIC_ASSERT( !holds_alternative<int>( v ) );
         STATIC_ASSERT( holds_alternative<float>( v ) );
     }
+
+    {
+        constexpr variant<int, int, float> v;
+        STATIC_ASSERT( holds_alternative<int>( v ) );
+        STATIC_ASSERT( !holds_alternative<float>( v ) );
+    }
 }
+
+#endif

test/variant_in_place_index_construct_cx.cpp

@@ -1,14 +1,18 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
 #include <boost/config.hpp>
 #include <boost/config/workaround.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1910
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_MSVC < 1910" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -68,7 +72,7 @@ int main()
         constexpr variant<int, float> v( in_place_index_t<1>{}, 3.14f );
 
         STATIC_ASSERT( v.index() == 1 );
-        STATIC_ASSERT( get<1>(v) == 3.14f );
+        STATIC_ASSERT( get<1>(v) == (float)3.14f ); // see FLT_EVAL_METHOD
     }
 
     {
@@ -89,14 +93,14 @@ int main()
         constexpr variant<int, int, float, float, X, X> v( in_place_index_t<2>{}, 3.14f );
 
         STATIC_ASSERT( v.index() == 2 );
-        STATIC_ASSERT( get<2>(v) == 3.14f );
+        STATIC_ASSERT( get<2>(v) == (float)3.14f );
     }
 
     {
         constexpr variant<int, int, float, float, X, X> v( in_place_index_t<3>{}, 3.14f );
 
         STATIC_ASSERT( v.index() == 3 );
-        STATIC_ASSERT( get<3>(v) == 3.14f );
+        STATIC_ASSERT( get<3>(v) == (float)3.14f );
     }
 
     {
@@ -119,3 +123,5 @@ int main()
 
 #endif
 }
+
+#endif

test/variant_in_place_type_construct_cx.cpp

@@ -1,14 +1,18 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
 #include <boost/config.hpp>
 #include <boost/config/workaround.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1910
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_MSVC < 1910" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -80,7 +84,7 @@ int main()
         constexpr variant<int, float> v( in_place_type_t<float>{}, 3.14f );
 
         STATIC_ASSERT( v.index() == 1 );
-        STATIC_ASSERT( get<1>(v) == 3.14f );
+        STATIC_ASSERT( get<1>(v) == (float)3.14f ); // see FLT_EVAL_METHOD
 
         STATIC_ASSERT( holds_alternative<float>(v) );
     }
@@ -89,7 +93,7 @@ int main()
         constexpr variant<int, int, float, X> v( in_place_type_t<float>{}, 3.14f );
 
         STATIC_ASSERT( v.index() == 2 );
-        STATIC_ASSERT( get<2>(v) == 3.14f );
+        STATIC_ASSERT( get<2>(v) == (float)3.14f );
 
         STATIC_ASSERT( holds_alternative<float>(v) );
     }
@@ -118,3 +122,5 @@ int main()
 
 #endif
 }
+
+#endif

test/variant_index_type.cpp

@@ -0,0 +1,86 @@
+// Copyright 2024 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#if defined(_MSC_VER)
+# pragma warning( disable: 4503 ) // decorated name length exceeded
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/mp11.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::mp11;
+
+template<std::size_t I> struct X
+{
+};
+
+template<class N> using Xf = X< N::value >;
+
+template<std::size_t I> struct Y
+{
+    Y() = default;
+    Y( Y&& ) {}
+};
+
+template<class N> using Yf = Y< N::value >;
+
+template<std::size_t I> struct Z
+{
+    ~Z() {}
+};
+
+template<class N> using Zf = Z< N::value >;
+
+template<std::size_t I> struct W
+{
+    ~W() {}
+
+    W() = default;
+    W( W&& ) {}
+};
+
+template<class N> using Wf = W< N::value >;
+
+template<class L, template<class...> class F> void test()
+{
+    using V = mp_rename< mp_transform<F, L>, boost::variant2::variant >;
+
+    constexpr std::size_t N = mp_size<L>::value;
+
+    using last_type = F<mp_size_t<N-1>>;
+
+    {
+        V v( last_type{} );
+        BOOST_TEST_EQ( v.index(), N-1 );
+    }
+
+    {
+        V v;
+        BOOST_TEST_EQ( v.index(), 0 );
+
+        v.template emplace<last_type>();
+        BOOST_TEST_EQ( v.index(), N-1 );
+    }
+}
+
+int main()
+{
+    constexpr std::size_t N = LIST_SIZE;
+    using L = mp_iota_c<N>;
+
+    static_assert( !boost::variant2::variant< X<0>, X<1> >::uses_double_storage(), "" );
+    test<L, Xf>();
+
+    static_assert( boost::variant2::variant< Y<0>, Y<1> >::uses_double_storage(), "" );
+    test<L, Yf>();
+
+    static_assert( !boost::variant2::variant< Z<0>, Z<1> >::uses_double_storage(), "" );
+    test<L, Zf>();
+
+    static_assert( boost::variant2::variant< W<0>, W<1> >::uses_double_storage(), "" );
+    test<L, Wf>();
+
+    return boost::report_errors();
+}

test/variant_issue_55.cpp

@@ -0,0 +1,194 @@
+// Copyright 2025 Peter Dimov
+// Copyright 2025 Vinnie Falco
+//
+// Distributed under the Boost Software License, Version 1.0.
+//
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+
+// GCC 12+ -Wmaybe-uninitialized false positive tests
+// https://github.com/boostorg/variant2/issues/55
+//
+// GCC 12+'s improved dataflow analysis sees code paths for all alternatives
+// in mp_with_index and warns that members may be uninitialized, even though
+// the variant's discriminator guarantees only initialized alternatives are
+// accessed.
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config.hpp>
+#include <exception>
+#include <string>
+
+// Check for C++17 std::optional support
+#if BOOST_CXX_VERSION >= 201703L
+# include <optional>
+# define BOOST_VARIANT2_TEST_HAS_OPTIONAL 1
+#endif
+
+// Check for C++20 coroutine support
+#if defined(__cpp_impl_coroutine) && __cpp_impl_coroutine >= 201902L
+# include <coroutine>
+# define BOOST_VARIANT2_TEST_HAS_CORO 1
+#endif
+
+using result_void = boost::system::result<void, std::exception_ptr>;
+using result_string = boost::system::result<std::string, std::exception_ptr>;
+
+void testGccUninitialized()
+{
+    // Test 1: Simple copy construction
+    {
+        result_void r1;
+        result_void r2(r1);
+        (void)r2;
+    }
+
+    // Test 2: Copy assignment
+    {
+        result_void r1;
+        result_void r2;
+        r2 = r1;
+        (void)r2;
+    }
+
+#ifdef BOOST_VARIANT2_TEST_HAS_OPTIONAL
+    // Test 3: std::optional assignment (matches spawn pattern)
+    {
+        std::optional<result_void> opt;
+        opt = result_void{};
+        (void)opt;
+    }
+#endif
+
+    // Test 4: Pass to function via copy
+    {
+        auto fn = [](result_void r) { (void)r; };
+        fn(result_void{});
+    }
+
+#ifdef BOOST_VARIANT2_TEST_HAS_OPTIONAL
+    // Test 5: Lambda capture + optional (closest to spawn)
+    {
+        auto fn = [](result_void r) {
+            std::optional<result_void> opt;
+            opt = r;
+            return opt.has_value();
+        };
+        (void)fn(result_void{});
+    }
+#endif
+
+    // Test 6: Non-void result with string (triggers string warning)
+    {
+        result_string r1;
+        result_string r2(r1);
+        (void)r2;
+    }
+
+    // Test 7: Assign exception to result holding value
+    {
+        result_string r1{"hello"};
+        r1 = std::make_exception_ptr(std::runtime_error("test"));
+        (void)r1;
+    }
+
+#ifdef BOOST_VARIANT2_TEST_HAS_OPTIONAL
+    // Test 8: Optional with string result
+    {
+        std::optional<result_string> opt;
+        opt = result_string{};
+        (void)opt;
+    }
+#endif
+
+#ifdef BOOST_VARIANT2_TEST_HAS_CORO
+    // Minimal fire-and-forget coroutine for testing
+    struct fire_and_forget
+    {
+        struct promise_type
+        {
+            fire_and_forget get_return_object() { return {}; }
+            std::suspend_never initial_suspend() noexcept { return {}; }
+            std::suspend_never final_suspend() noexcept { return {}; }
+            void return_void() {}
+            void unhandled_exception() { std::terminate(); }
+        };
+    };
+
+    // Test 9: Coroutine returning result (mimics spawn)
+    {
+        auto coro = []() -> fire_and_forget {
+            result_void r{};
+            (void)r;
+            co_return;
+        };
+        coro();
+    }
+
+    // Test 10: Coroutine with handler call (closest to actual spawn)
+    {
+        std::optional<result_void> received;
+        auto handler = [&](result_void r) {
+            received = r;
+        };
+        auto coro = [&]() -> fire_and_forget {
+            handler(result_void{});
+            co_return;
+        };
+        coro();
+        (void)received;
+    }
+
+    // Test 11: Coroutine with try/catch like spawn
+    {
+        std::optional<result_void> received;
+        auto handler = [&](result_void r) {
+            received = r;
+        };
+        auto coro = [&]() -> fire_and_forget {
+            try
+            {
+                handler(result_void{});
+            }
+            catch (...)
+            {
+                handler(result_void{std::current_exception()});
+            }
+            co_return;
+        };
+        coro();
+        (void)received;
+    }
+
+    // Test 12: Coroutine with string result
+    {
+        std::optional<result_string> received;
+        auto handler = [&](result_string r) {
+            received = r;
+        };
+        auto coro = [&]() -> fire_and_forget {
+            try
+            {
+                handler(result_string{"test"});
+            }
+            catch (...)
+            {
+                handler(result_string{std::current_exception()});
+            }
+            co_return;
+        };
+        coro();
+        (void)received;
+    }
+#endif
+}
+
+int main()
+{
+    boost::core::lwt_init();
+
+    testGccUninitialized();
+
+    return boost::report_errors();
+}

test/variant_json_value_from.cpp

@@ -0,0 +1,47 @@
+// Copyright 2022 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/config.hpp>
+
+#if defined(BOOST_GCC) && BOOST_GCC < 50000
+# define BOOST_ALLOW_DEPRECATED
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/json/value_from.hpp>
+#include <boost/json/serialize.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <string>
+
+using namespace boost::variant2;
+namespace json = boost::json;
+
+int main()
+{
+    {
+        monostate m;
+        json::value w = json::value_from( m );
+        BOOST_TEST_EQ( w, json::value( nullptr ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v;
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( nullptr ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v( 17 );
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( 17 ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v( "test" );
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( "test" ) );
+    }
+
+    return boost::report_errors();
+}

test/variant_json_value_to.cpp

@@ -0,0 +1,48 @@
+// Copyright 2022 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/config.hpp>
+
+#if defined(BOOST_GCC) && BOOST_GCC < 50000
+# define BOOST_ALLOW_DEPRECATED
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/json/value_to.hpp>
+#include <boost/json/serialize.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::variant2;
+namespace json = boost::json;
+
+int main()
+{
+    {
+        json::value v;
+        auto r = json::try_value_to<monostate>( v );
+        BOOST_TEST( r.has_value() );
+    }
+
+    using V = variant<monostate, int, std::string>;
+
+    {
+        json::value v;
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V() );
+    }
+
+    {
+        json::value v( 12 );
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V(12) );
+    }
+
+    {
+        json::value v( "test" );
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V("test") );
+    }
+
+    return boost::report_errors();
+}

test/variant_lt_gt_cx.cpp

@@ -1,24 +1,26 @@
-
 // Copyright 2017, 2019 Peter Dimov
-//
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
-using namespace boost::variant2;
-
-#if !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
-
+#include <boost/mp11.hpp>
+#include <boost/config.hpp>
 #include <boost/config/pragma_message.hpp>
 
-BOOST_PRAGMA_MESSAGE("Skipping constexpr op<, op<= test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
 
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#elif !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr op<, op<= test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
 int main() {}
 
 #else
 
+using namespace boost::variant2;
+
 struct X
 {
 };

test/variant_move_assign_cx.cpp

@@ -1,14 +1,24 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
 #include <utility>
 
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#elif defined(BOOST_GCC) && BOOST_GCC < 70000
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_GCC < 70000" )
+int main() {}
+
+#else
+
 using namespace boost::variant2;
 
 struct X
@@ -34,24 +44,24 @@ enum E
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
-template<class V, class T, class A> constexpr T test( A&& a )
+template<class V, std::size_t I, class A> constexpr variant_alternative_t<I, V> test( A&& a )
 {
     V v;
 
     v = std::forward<A>(a);
 
-    return get<T>(v);
+    return get<I>(v);
 }
 
 int main()
 {
     {
-        constexpr auto w = test<variant<int>, int>( variant<int>( 1 ) );
+        constexpr auto w = test<variant<int>, 0>( variant<int>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<X>, X>( variant<X>( 1 ) );
+        constexpr auto w = test<variant<X>, 0>( variant<X>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -59,34 +69,34 @@ int main()
 #else
 
     {
-        constexpr auto w = test<variant<Y>, Y>( variant<Y>( 1 ) );
+        constexpr auto w = test<variant<Y>, 0>( variant<Y>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 
     {
-        constexpr auto w = test<variant<int, float>, int>( variant<int, float>( 1 ) );
+        constexpr auto w = test<variant<int, float>, 0>( variant<int, float>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<int, float>, float>( variant<int, float>( 3.0f ) );
+        constexpr auto w = test<variant<int, float>, 1>( variant<int, float>( 3.0f ) );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float>, float>( variant<int, int, float>( 3.0f ) );
+        constexpr auto w = test<variant<int, int, float>, 2>( variant<int, int, float>( 3.0f ) );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<variant<E, E, X>, X>( variant<E, E, X>( 1 ) );
+        constexpr auto w = test<variant<E, E, X>, 2>( variant<E, E, X>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float, float, X>, X>( variant<int, int, float, float, X>( X(1) ) );
+        constexpr auto w = test<variant<int, int, float, float, X>, 4>( variant<int, int, float, float, X>( X(1) ) );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -94,14 +104,16 @@ int main()
 #else
 
     {
-        constexpr auto w = test<variant<E, E, Y>, Y>( variant<E, E, Y>( 1 ) );
+        constexpr auto w = test<variant<E, E, Y>, 2>( variant<E, E, Y>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float, float, Y>, Y>( variant<int, int, float, float, Y>( Y(1) ) );
+        constexpr auto w = test<variant<int, int, float, float, Y>, 4>( variant<int, int, float, float, Y>( Y(1) ) );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 }
+
+#endif

test/variant_move_assign_throw.cpp

@@ -45,7 +45,7 @@ void test()
     variant<Y1, Y2> v1( in_place_type_t<Y1>{} );
     variant<Y1, Y2> v2( in_place_type_t<Y2>{} );
 
-    BOOST_TEST_THROWS( v1 = std::move( v2 ), std::runtime_error )
+    BOOST_TEST_THROWS( v1 = std::move( v2 ), std::runtime_error );
 }
 
 int main()

test/variant_move_construct_cx.cpp

@@ -1,14 +1,19 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
 #include <utility>
 
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#else
+
 using namespace boost::variant2;
 
 struct X
@@ -34,21 +39,21 @@ enum E
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
-template<class T, class V> constexpr T test( V&& v )
+template<std::size_t I, class V> constexpr variant_alternative_t<I, V> test( V&& v )
 {
     V v2( std::forward<V>(v) );
-    return get<T>( v2 );
+    return get<I>( v2 );
 }
 
 int main()
 {
     {
-        constexpr auto w = test<int>( variant<int>( 1 ) );
+        constexpr auto w = test<0>( variant<int>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<X>( variant<X>( 1 ) );
+        constexpr auto w = test<0>( variant<X>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -56,34 +61,34 @@ int main()
 #else
 
     {
-        constexpr auto w = test<Y>( variant<Y>( 1 ) );
+        constexpr auto w = test<0>( variant<Y>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 
     {
-        constexpr auto w = test<int>( variant<int, float>( 1 ) );
+        constexpr auto w = test<0>( variant<int, float>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<float>( variant<int, float>( 3.0f ) );
+        constexpr auto w = test<1>( variant<int, float>( 3.0f ) );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<float>( variant<int, int, float>( 3.0f ) );
+        constexpr auto w = test<2>( variant<int, int, float>( 3.0f ) );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<X>( variant<E, E, X>( 1 ) );
+        constexpr auto w = test<2>( variant<E, E, X>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<X>( variant<int, int, float, float, X>( X(1) ) );
+        constexpr auto w = test<4>( variant<int, int, float, float, X>( X(1) ) );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -91,14 +96,16 @@ int main()
 #else
 
     {
-        constexpr auto w = test<Y>( variant<E, E, Y>( 1 ) );
+        constexpr auto w = test<2>( variant<E, E, Y>( 1 ) );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<Y>( variant<int, int, float, float, Y>( Y(1) ) );
+        constexpr auto w = test<4>( variant<int, int, float, float, Y>( Y(1) ) );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 }
+
+#endif

test/variant_uses_double_storage.cpp

@@ -0,0 +1,50 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <type_traits>
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X1
+{
+};
+
+STATIC_ASSERT( std::is_nothrow_move_constructible<X1>::value );
+STATIC_ASSERT( std::is_trivially_destructible<X1>::value );
+
+struct X2
+{
+    ~X2() {}
+};
+
+STATIC_ASSERT( std::is_nothrow_move_constructible<X2>::value );
+STATIC_ASSERT( !std::is_trivially_destructible<X2>::value );
+
+struct X3
+{
+    X3( X3&& ) {}
+};
+
+STATIC_ASSERT( !std::is_nothrow_move_constructible<X3>::value );
+STATIC_ASSERT( std::is_trivially_destructible<X3>::value );
+
+struct X4
+{
+    ~X4() {}
+    X4( X4&& ) {}
+};
+
+STATIC_ASSERT( !std::is_nothrow_move_constructible<X4>::value );
+STATIC_ASSERT( !std::is_trivially_destructible<X4>::value );
+
+//
+
+STATIC_ASSERT( !variant<int, float>::uses_double_storage() );
+STATIC_ASSERT( !variant<int, float, X1>::uses_double_storage() );
+STATIC_ASSERT( !variant<int, float, X2>::uses_double_storage() );
+STATIC_ASSERT( variant<int, float, X3>::uses_double_storage() );
+STATIC_ASSERT( variant<int, float, X4>::uses_double_storage() );

test/variant_value_assign_cx.cpp

@@ -1,12 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -33,24 +38,24 @@ enum E
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
-template<class V, class T, class A> constexpr A test( A const& a )
+template<class V, std::size_t I, class A> constexpr variant_alternative_t<I, V> test( A const& a )
 {
     V v;
 
     v = a;
 
-    return get<T>(v);
+    return get<I>(v);
 }
 
 int main()
 {
     {
-        constexpr auto w = test<variant<int>, int>( 1 );
+        constexpr auto w = test<variant<int>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<X>, X>( 1 );
+        constexpr auto w = test<variant<X>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -58,34 +63,34 @@ int main()
 #else
 
     {
-        constexpr auto w = test<variant<Y>, Y>( 1 );
+        constexpr auto w = test<variant<Y>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 
     {
-        constexpr auto w = test<variant<int, float>, int>( 1 );
+        constexpr auto w = test<variant<int, float>, 0>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<int, float>, float>( 3.0f );
+        constexpr auto w = test<variant<int, float>, 1>( 3.0f );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float>, float>( 3.0f );
+        constexpr auto w = test<variant<int, int, float>, 2>( 3.0f );
         STATIC_ASSERT( w == 3.0f );
     }
 
     {
-        constexpr auto w = test<variant<E, E, X>, X>( 1 );
+        constexpr auto w = test<variant<E, E, X>, 2>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float, float, X>, X>( X(1) );
+        constexpr auto w = test<variant<int, int, float, float, X>, 4>( X(1) );
         STATIC_ASSERT( w == 1 );
     }
 
@@ -93,14 +98,16 @@ int main()
 #else
 
     {
-        constexpr auto w = test<variant<E, E, Y>, Y>( 1 );
+        constexpr auto w = test<variant<E, E, Y>, 2>( 1 );
         STATIC_ASSERT( w == 1 );
     }
 
     {
-        constexpr auto w = test<variant<int, int, float, float, Y>, Y>( Y(1) );
+        constexpr auto w = test<variant<int, int, float, float, Y>, 4>( Y(1) );
         STATIC_ASSERT( w == 1 );
     }
 
 #endif
 }
+
+#endif

test/variant_value_construct_cx.cpp

@@ -1,12 +1,17 @@
-
-// Copyright 2017 Peter Dimov.
-//
+// Copyright 2017, 2026 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
-//
-// See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1910
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_MSVC < 1910" )
+int main() {}
+
+#else
 
 using namespace boost::variant2;
 
@@ -82,7 +87,7 @@ int main()
 
         STATIC_ASSERT( v.index() == 1 );
         STATIC_ASSERT( holds_alternative<float>(v) );
-        STATIC_ASSERT( get<1>(v) == 3.14f );
+        STATIC_ASSERT( get<1>(v) == (float)3.14f ); // see FLT_EVAL_METHOD
     }
 
     {
@@ -97,7 +102,7 @@ int main()
 
         STATIC_ASSERT( v.index() == 2 );
         STATIC_ASSERT( holds_alternative<float>(v) );
-        STATIC_ASSERT( get<2>(v) == 3.14f );
+        STATIC_ASSERT( get<2>(v) == (float)3.14f );
     }
 
     {
@@ -107,3 +112,5 @@ int main()
         STATIC_ASSERT( holds_alternative<X>(v) );
     }
 }
+
+#endif

test/variant_value_construct_cx_2.cpp

@@ -0,0 +1,32 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(__cpp_constexpr_dynamic_alloc) || __cpp_constexpr_dynamic_alloc < 201907L
+
+BOOST_PRAGMA_MESSAGE("Skipping test because __cpp_constexpr_dynamic_alloc < 201907L")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X
+{
+    constexpr ~X() {}
+};
+
+int main()
+{
+    constexpr variant<X, int> v( 5 );
+
+    STATIC_ASSERT( v.index() == 1 );
+    STATIC_ASSERT( get<1>(v) == 5 );
+}
+
+#endif

test/variant_value_construct_cx_3.cpp

@@ -0,0 +1,37 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(__cpp_constexpr_dynamic_alloc) || __cpp_constexpr_dynamic_alloc < 201907L
+
+BOOST_PRAGMA_MESSAGE("Skipping test because __cpp_constexpr_dynamic_alloc < 201907L")
+int main() {}
+
+#elif defined(BOOST_CLANG) && BOOST_CLANG_VERSION < 180000
+
+BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_CLANG_VERSION < 180000")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X
+{
+	~X() {}
+};
+
+int main()
+{
+    constexpr variant<X, int> v( 5 );
+
+    STATIC_ASSERT( v.index() == 1 );
+    STATIC_ASSERT( get<1>(v) == 5 );
+}
+
+#endif

test/variant_value_construct_cx_4.cpp

@@ -0,0 +1,56 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(__cpp_constexpr_dynamic_alloc) || __cpp_constexpr_dynamic_alloc < 201907L
+
+BOOST_PRAGMA_MESSAGE("Skipping test because __cpp_constexpr_dynamic_alloc < 201907L")
+int main() {}
+
+#elif defined(BOOST_MSVC) && BOOST_MSVC < 1960
+
+BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_MSVC < 1960")
+int main() {}
+
+#elif defined(BOOST_CLANG) && BOOST_CLANG_VERSION < 180000
+
+BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_CLANG_VERSION < 180000")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X
+{
+};
+
+struct Y
+{
+    constexpr ~Y() {}
+};
+
+struct Z
+{
+    ~Z() {}
+};
+
+int main()
+{
+    {
+        constexpr variant<X, Y, Z> v( X{} );
+        STATIC_ASSERT( v.index() == 0 );
+    }
+
+    {
+        constexpr variant<X, Y, Z> v( Y{} );
+        STATIC_ASSERT( v.index() == 1 );
+    }
+}
+
+#endif

test/variant_visit_cx.cpp

@@ -0,0 +1,57 @@
+// Copyright 2017, 2024, 2026 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/mp11.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#elif !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr visit test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+struct X
+{
+    constexpr operator int() const { return 2; }
+};
+
+struct F
+{
+    constexpr int operator()( int x ) const
+    {
+        return x;
+    }
+};
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+int main()
+{
+    {
+        constexpr variant<int> v( 1 );
+        STATIC_ASSERT( visit( F(), v ) == 1 );
+    }
+
+    {
+        constexpr variant<char, int> v( 'a' );
+        STATIC_ASSERT( visit( F(), v ) == 'a' );
+    }
+
+    {
+        constexpr variant<char, int, X> v( X{} );
+        STATIC_ASSERT( visit( F(), v ) == 2 );
+    }
+}
+
+#endif

test/variant_visit_cx_2.cpp

@@ -0,0 +1,53 @@
+// Copyright 2017, 2024 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !( defined(__cpp_constexpr) && __cpp_constexpr >= 201603L )
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr visit test because __cpp_constexpr < 201603L")
+int main() {}
+
+#elif !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr visit test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+struct F
+{
+    constexpr int operator()( int x1, int x2 ) const
+    {
+        return x1 + x2;
+    }
+
+    constexpr int operator()( int x1, int x2, int x3 ) const
+    {
+        return x1 + x2 + x3;
+    }
+};
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+int main()
+{
+    {
+        constexpr variant<char, int> v1( 1 );
+        constexpr variant<char, int> v2( '2' );
+        STATIC_ASSERT( visit( F(), v1, v2 ) == 1 + '2' );
+    }
+
+    {
+        constexpr variant<char, int> v1( 1 );
+        constexpr variant<char, int> v2( 2 );
+        constexpr variant<char, int> v3( '3' );
+        STATIC_ASSERT( visit( F(), v1, v2, v3 ) == 1 + 2 + '3' );
+    }
+}
+
+#endif

test/variant_visit_r.cpp

@@ -61,18 +61,18 @@ int main()
     }
 
     {
-        variant<int, float> const v( 3.14f );
+        variant<int, float> const v( 3.125f );
 
         BOOST_TEST_EQ( visit<int>( F1(), v ), 3 );
-        BOOST_TEST_EQ( visit<float>( F1(), v ), 3.14f );
+        BOOST_TEST_EQ( visit<float>( F1(), v ), 3.125f );
     }
 
     {
         variant<int, float> v1( 1 );
-        variant<int, float> const v2( 3.14f );
+        variant<int, float> const v2( 3.125f );
 
         BOOST_TEST_EQ( visit<int>( F2(), v1, v2 ), 4 );
-        BOOST_TEST_EQ( visit<float>( F2(), v1, v2 ), 1 + 3.14f );
+        BOOST_TEST_EQ( visit<float>( F2(), v1, v2 ), 1 + 3.125f );
     }
 
     {

test/variant_visit_r_cx.cpp

@@ -0,0 +1,58 @@
+// Copyright 2017, 2024, 2026 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/mp11.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX14_CONSTEXPR is defined" )
+int main() {}
+
+#elif !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr visit test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
+int main() {}
+
+#else
+
+using namespace boost::variant2;
+
+struct X
+{
+    int v;
+    constexpr operator int() const { return v; }
+};
+
+struct F
+{
+    template<class T> constexpr T operator()( T x ) const
+    {
+        return x;
+    }
+};
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+int main()
+{
+    {
+        constexpr variant<int> v( 1 );
+        STATIC_ASSERT( visit<int>( F(), v ) == 1 );
+    }
+
+    {
+        constexpr variant<char, int> v( 'a' );
+        STATIC_ASSERT( visit<int>( F(), v ) == 'a' );
+    }
+
+    {
+        constexpr variant<char, int, X> v( X{2} );
+        STATIC_ASSERT( visit<int>( F(), v ) == 2 );
+    }
+}
+
+#endif