Document operator<< for monostate

Add "cxxstd" json field. The "cxxstd" json field is being added to ea…

.github/workflows/ci.yml

@@ -0,0 +1,358 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+      - develop
+      - feature/**
+
+env:
+  UBSAN_OPTIONS: print_stacktrace=1
+
+jobs:
+  posix:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - toolset: gcc-4.8
+            cxxstd: "03,11"
+            os: ubuntu-18.04
+            install: g++-4.8
+          - toolset: gcc-5
+            cxxstd: "03,11,14,1z"
+            os: ubuntu-18.04
+            install: g++-5
+          - toolset: gcc-6
+            cxxstd: "03,11,14,1z"
+            os: ubuntu-18.04
+            install: g++-6
+          - toolset: gcc-7
+            cxxstd: "03,11,14,17"
+            os: ubuntu-18.04
+          - toolset: gcc-8
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-18.04
+            install: g++-8
+          - toolset: gcc-9
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+          - toolset: gcc-10
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+            install: g++-10
+          - toolset: gcc-11
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+            install: g++-11
+          - toolset: clang
+            compiler: clang++-3.9
+            cxxstd: "03,11,14"
+            os: ubuntu-18.04
+            install: clang-3.9
+          - toolset: clang
+            compiler: clang++-4.0
+            cxxstd: "03,11,14"
+            os: ubuntu-18.04
+            install: clang-4.0
+          - toolset: clang
+            compiler: clang++-5.0
+            cxxstd: "03,11,14,1z"
+            os: ubuntu-18.04
+            install: clang-5.0
+          - toolset: clang
+            compiler: clang++-6.0
+            cxxstd: "03,11,14,17"
+            os: ubuntu-18.04
+            install: clang-6.0
+          - toolset: clang
+            compiler: clang++-7
+            cxxstd: "03,11,14,17"
+            os: ubuntu-18.04
+            install: clang-7
+          - toolset: clang
+            compiler: clang++-8
+            cxxstd: "03,11,14,17"
+            os: ubuntu-20.04
+            install: clang-8
+          - toolset: clang
+            compiler: clang++-9
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+            install: clang-9
+          - toolset: clang
+            compiler: clang++-10
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+            install: clang-10
+          - toolset: clang
+            compiler: clang++-11
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+            install: clang-11
+          - toolset: clang
+            compiler: clang++-12
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+            install: clang-12
+          - toolset: clang
+            cxxstd: "03,11,14,17,2a"
+            os: macos-10.15
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Install packages
+        if: matrix.install
+        run: sudo apt install ${{matrix.install}}
+
+      - name: Setup Boost
+        run: |
+          echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
+          LIBRARY=${GITHUB_REPOSITORY#*/}
+          echo LIBRARY: $LIBRARY
+          echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
+          echo GITHUB_BASE_REF: $GITHUB_BASE_REF
+          echo GITHUB_REF: $GITHUB_REF
+          REF=${GITHUB_BASE_REF:-$GITHUB_REF}
+          REF=${REF#refs/heads/}
+          echo REF: $REF
+          BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
+          echo BOOST_BRANCH: $BOOST_BRANCH
+          cd ..
+          git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
+          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
+          ./bootstrap.sh
+          ./b2 -d0 headers
+
+      - name: Create user-config.jam
+        if: matrix.compiler
+        run: |
+          echo "using ${{matrix.toolset}} : : ${{matrix.compiler}} ;" > ~/user-config.jam
+
+      - name: Run tests
+        run: |
+          cd ../boost-root
+          ./b2 -j3 libs/$LIBRARY/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} variant=debug,release
+
+  windows:
+    strategy:
+      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,latest"
+            addrmd: 32,64
+            os: windows-2022
+          - toolset: gcc
+            cxxstd: "03,11,14,17,2a"
+            addrmd: 64
+            os: windows-2019
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup Boost
+        shell: cmd
+        run: |
+          echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
+          for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
+          echo LIBRARY: %LIBRARY%
+          echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
+          echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
+          echo GITHUB_REF: %GITHUB_REF%
+          if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
+          set BOOST_BRANCH=develop
+          for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
+          echo BOOST_BRANCH: %BOOST_BRANCH%
+          cd ..
+          git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
+          cd boost-root
+          xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
+          cmd /c bootstrap
+          b2 -d0 headers
+
+      - name: Run tests
+        shell: cmd
+        run: |
+          cd ../boost-root
+          b2 -j3 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release
+
+  posix-cmake-subdir:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-18.04
+          - os: ubuntu-20.04
+          - os: macos-10.15
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Install packages
+        if: matrix.install
+        run: sudo apt install ${{matrix.install}}
+
+      - name: Setup Boost
+        run: |
+          echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
+          LIBRARY=${GITHUB_REPOSITORY#*/}
+          echo LIBRARY: $LIBRARY
+          echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
+          echo GITHUB_BASE_REF: $GITHUB_BASE_REF
+          echo GITHUB_REF: $GITHUB_REF
+          REF=${GITHUB_BASE_REF:-$GITHUB_REF}
+          REF=${REF#refs/heads/}
+          echo REF: $REF
+          BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
+          echo BOOST_BRANCH: $BOOST_BRANCH
+          cd ..
+          git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
+          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" $LIBRARY
+
+      - name: Use library with add_subdirectory
+        run: |
+          cd ../boost-root/libs/$LIBRARY/test/cmake_subdir_test
+          mkdir __build__ && cd __build__
+          cmake ..
+          cmake --build .
+          ctest --output-on-failure --no-tests=error
+
+  posix-cmake-install:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-18.04
+          - os: ubuntu-20.04
+          - os: macos-10.15
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Install packages
+        if: matrix.install
+        run: sudo apt install ${{matrix.install}}
+
+      - name: Setup Boost
+        run: |
+          echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
+          LIBRARY=${GITHUB_REPOSITORY#*/}
+          echo LIBRARY: $LIBRARY
+          echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
+          echo GITHUB_BASE_REF: $GITHUB_BASE_REF
+          echo GITHUB_REF: $GITHUB_REF
+          REF=${GITHUB_BASE_REF:-$GITHUB_REF}
+          REF=${REF#refs/heads/}
+          echo REF: $REF
+          BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
+          echo BOOST_BRANCH: $BOOST_BRANCH
+          cd ..
+          git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
+          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" $LIBRARY
+
+      - name: Configure
+        run: |
+          cd ../boost-root
+          mkdir __build__ && cd __build__
+          cmake -DBOOST_INCLUDE_LIBRARIES=$LIBRARY -DCMAKE_INSTALL_PREFIX=~/.local ..
+
+      - name: Install
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target install
+
+      - name: Use the installed library
+        run: |
+          cd ../boost-root/libs/$LIBRARY/test/cmake_install_test && mkdir __build__ && cd __build__
+          cmake -DCMAKE_INSTALL_PREFIX=~/.local ..
+          cmake --build .
+          ctest --output-on-failure --no-tests=error
+
+  posix-cmake-test:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-18.04
+          - os: ubuntu-20.04
+          - os: macos-10.15
+
+    runs-on: ${{matrix.os}}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Install packages
+        if: matrix.install
+        run: sudo apt install ${{matrix.install}}
+
+      - name: Setup Boost
+        run: |
+          echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
+          LIBRARY=${GITHUB_REPOSITORY#*/}
+          echo LIBRARY: $LIBRARY
+          echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
+          echo GITHUB_BASE_REF: $GITHUB_BASE_REF
+          echo GITHUB_REF: $GITHUB_REF
+          REF=${GITHUB_BASE_REF:-$GITHUB_REF}
+          REF=${REF#refs/heads/}
+          echo REF: $REF
+          BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
+          echo BOOST_BRANCH: $BOOST_BRANCH
+          cd ..
+          git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
+          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" $LIBRARY
+
+      - name: Configure
+        run: |
+          cd ../boost-root
+          mkdir __build__ && cd __build__
+          cmake -DBOOST_INCLUDE_LIBRARIES=$LIBRARY -DBUILD_TESTING=ON ..
+
+      - name: Build tests
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target tests
+
+      - name: Run tests
+        run: |
+          cd ../boost-root/__build__
+          ctest --output-on-failure --no-tests=error

.travis.yml

@@ -1,12 +1,10 @@
-# Copyright 2016-2019 Peter Dimov
+# Copyright 2016-2021 Peter Dimov
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or copy at http://boost.org/LICENSE_1_0.txt)
 
 language: cpp
 
-sudo: false
-
-python: "2.7"
+dist: xenial
 
 branches:
   only:
@@ -25,8 +23,14 @@ matrix:
 
   include:
     - os: linux
-      compiler: g++
-      env: TOOLSET=gcc COMPILER=g++ CXXSTD=11
+      compiler: g++-4.8
+      env: TOOLSET=gcc COMPILER=g++-4.8 CXXSTD=11
+      addons:
+        apt:
+          packages:
+            - g++-4.8
+          sources:
+            - ubuntu-toolchain-r-test
 
     - os: linux
       compiler: g++-4.9
@@ -79,12 +83,23 @@ matrix:
             - ubuntu-toolchain-r-test
 
     - os: linux
-      compiler: g++-8
-      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-8 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
+      compiler: g++-9
+      env: TOOLSET=gcc COMPILER=g++-9 CXXSTD=11,14,17,2a
       addons:
         apt:
           packages:
-            - g++-8
+            - g++-9
+          sources:
+            - ubuntu-toolchain-r-test
+
+    - os: linux
+      dist: bionic
+      compiler: g++-10
+      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-10 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
+      addons:
+        apt:
+          packages:
+            - g++-10
           sources:
             - ubuntu-toolchain-r-test
 
@@ -98,7 +113,6 @@ matrix:
             - libstdc++-4.9-dev
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.5
 
     - os: linux
       compiler: clang++-3.6
@@ -109,7 +123,6 @@ matrix:
             - clang-3.6
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.6
 
     - os: linux
       compiler: clang++-3.7
@@ -120,7 +133,6 @@ matrix:
             - clang-3.7
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.7
 
     - os: linux
       compiler: clang++-3.8
@@ -131,7 +143,6 @@ matrix:
             - clang-3.8
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.8
 
     - os: linux
       compiler: clang++-3.9
@@ -142,7 +153,6 @@ matrix:
             - clang-3.9
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.9
 
     - os: linux
       compiler: clang++-4.0
@@ -153,9 +163,9 @@ matrix:
             - clang-4.0
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-4.0
 
     - os: linux
+      dist: bionic
       compiler: clang++-5.0
       env: TOOLSET=clang COMPILER=clang++-5.0 CXXSTD=11,14,1z
       addons:
@@ -164,9 +174,9 @@ matrix:
             - clang-5.0
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-5.0
 
     - os: linux
+      dist: bionic
       compiler: clang++-6.0
       env: TOOLSET=clang COMPILER=clang++-6.0 CXXSTD=11,14,17
       addons:
@@ -175,9 +185,9 @@ matrix:
             - clang-6.0
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-6.0
 
     - os: linux
+      dist: bionic
       compiler: clang++-7
       env: TOOLSET=clang COMPILER=clang++-7 CXXSTD=11,14,17,2a
       addons:
@@ -186,20 +196,74 @@ matrix:
             - clang-7
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-7
+            - llvm-toolchain-bionic-7
 
     - os: linux
-      compiler: clang++-6.0
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-6.0 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
+      dist: bionic
+      compiler: clang++-8
+      env: TOOLSET=clang COMPILER=clang++-8 CXXSTD=11,14,17,2a
       addons:
         apt:
           packages:
-            - clang-6.0
+            - clang-8
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-6.0
+            - llvm-toolchain-bionic-8
 
     - os: linux
+      dist: bionic
+      compiler: clang++-9
+      env: TOOLSET=clang COMPILER=clang++-9 CXXSTD=11,14,17,2a
+      addons:
+        apt:
+          packages:
+            - clang-9
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+
+    - os: linux
+      dist: bionic
+      compiler: clang++-10
+      env: TOOLSET=clang COMPILER=clang++-10 CXXSTD=11,14,17,2a
+      addons:
+        apt:
+          packages:
+            - clang-10
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic-10 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+
+    - os: linux
+      dist: bionic
+      compiler: clang++-11
+      env: TOOLSET=clang COMPILER=clang++-11 CXXSTD=11,14,17,2a
+      addons:
+        apt:
+          packages:
+            - clang-11
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic-11 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+
+    - os: linux
+      dist: bionic
+      compiler: clang++-12
+      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-12 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
+      addons:
+        apt:
+          packages:
+            - clang-12
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic-12 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+
+    - os: linux
+      dist: trusty
       compiler: clang++-libc++
       env: TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=11,14,1z
       addons:
@@ -208,8 +272,9 @@ matrix:
             - libc++-dev
 
     - os: linux
+      dist: bionic
       compiler: clang++-libc++
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=11,14,1z UBSAN_OPTIONS=print_stacktrace=1
+      env: TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=11,14,17,2a
       addons:
         apt:
           packages:
@@ -217,15 +282,46 @@ matrix:
 
     - os: osx
       compiler: clang++
-      env: TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z
+      env: UBSAN=1 TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z UBSAN_OPTIONS=print_stacktrace=1
+
+    - os: linux
+      env: CMAKE_TEST=1
+      script:
+        - mkdir __build__ && cd __build__
+        - cmake -DBOOST_ENABLE_CMAKE=1 -DBUILD_TESTING=ON -DBoost_VERBOSE=1 -DBOOST_INCLUDE_LIBRARIES=variant2 ..
+        - ctest --output-on-failure -R boost_variant2
+
+    - os: linux
+      env: CMAKE_SUBDIR_TEST=1
+      install:
+        - BOOST_BRANCH=develop && [ "$TRAVIS_BRANCH" == "master" ] && BOOST_BRANCH=master || true
+        - git clone -b $BOOST_BRANCH https://github.com/boostorg/assert.git ../assert
+        - git clone -b $BOOST_BRANCH https://github.com/boostorg/config.git ../config
+        - git clone -b $BOOST_BRANCH https://github.com/boostorg/core.git ../core
+        - git clone -b $BOOST_BRANCH https://github.com/boostorg/mp11.git ../mp11
+      script:
+      - cd test/cmake_subdir_test && mkdir __build__ && cd __build__
+      - cmake ..
+      - cmake --build .
+      - cmake --build . --target check
+
+    - os: linux
+      env: CMAKE_INSTALL_TEST=1
+      script:
+        - pip install --user cmake
+        - mkdir __build__ && cd __build__
+        - cmake -DBOOST_ENABLE_CMAKE=1 -DBoost_VERBOSE=1 -DBOOST_INCLUDE_LIBRARIES=variant2 -DCMAKE_INSTALL_PREFIX=~/.local ..
+        - cmake --build . --target install
+        - cd ../libs/variant2/test/cmake_install_test && mkdir __build__ && cd __build__
+        - cmake -DCMAKE_INSTALL_PREFIX=~/.local ..
+        - cmake --build .
+        - cmake --build . --target check
 
 install:
   - BOOST_BRANCH=develop && [ "$TRAVIS_BRANCH" == "master" ] && BOOST_BRANCH=master || true
   - cd ..
   - git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
   - cd boost-root
-  - git submodule update --init tools/build
-  - git submodule update --init libs/config
   - git submodule update --init tools/boostdep
   - mkdir -p libs/variant2
   - cp -r $TRAVIS_BUILD_DIR/* libs/variant2

CMakeLists.txt

@@ -0,0 +1,28 @@
+# Generated by `boostdep --cmake variant2`
+# Copyright 2020, 2021 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+cmake_minimum_required(VERSION 3.8...3.20)
+
+project(boost_variant2 VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
+
+add_library(boost_variant2 INTERFACE)
+add_library(Boost::variant2 ALIAS boost_variant2)
+
+target_include_directories(boost_variant2 INTERFACE include)
+
+target_link_libraries(boost_variant2
+  INTERFACE
+    Boost::assert
+    Boost::config
+    Boost::mp11
+)
+
+target_compile_features(boost_variant2 INTERFACE cxx_std_11)
+
+if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
+
+  add_subdirectory(test)
+
+endif()

README.md

@@ -1,22 +1,18 @@
-# variant2
+# Boost.Variant2
 
 This repository contains a never-valueless, strong guarantee, C++11/14/17
 implementation of [std::variant](http://en.cppreference.com/w/cpp/utility/variant).
-See [the documentation](https://www.boost.org/doc/libs/master/libs/variant2/)
+See [the documentation](https://www.boost.org/libs/variant2)
 for more information.
 
-The code requires [Boost.Mp11](https://github.com/boostorg/mp11) and
-Boost.Config.
-
-The repository is intended to be placed into the `libs/variant2` directory of
-a Boost clone or release, but the header `variant.hpp` will also work
-[standalone](https://godbolt.org/z/nVUNKX).
+The library is part of Boost, starting from release 1.71. It depends on
+Boost.Mp11, Boost.Config, and Boost.Assert.
 
 Supported compilers:
 
 * g++ 4.8 or later with `-std=c++11` or above
-* clang++ 3.5 or later with `-std=c++11` or above
+* clang++ 3.9 or later with `-std=c++11` or above
 * Visual Studio 2015, 2017, 2019
 
-Tested on [Travis](https://travis-ci.org/boostorg/variant2/) and
-[Appveyor](https://ci.appveyor.com/project/pdimov/variant2/).
+Tested on [Github Actions](https://github.com/boostorg/variant2/actions) and
+[Appveyor](https://ci.appveyor.com/project/pdimov/variant2-fkab9).

appveyor.yml

@@ -1,4 +1,4 @@
-# Copyright 2016-2018 Peter Dimov
+# Copyright 2016-2021 Peter Dimov
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or copy at http://boost.org/LICENSE_1_0.txt)
 
@@ -20,7 +20,19 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       TOOLSET: msvc-14.1
       ADDRMD: 32,64
-      CXXSTD: 14,17
+      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
+      ADDRMD: 32,64
+      CXXSTD: 14,17,latest
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      TOOLSET: clang-win
+      ADDRMD: 64
+      CXXSTD: 14,17,latest
 
 install:
   - set BOOST_BRANCH=develop
@@ -28,8 +40,6 @@ install:
   - cd ..
   - git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
   - cd boost-root
-  - git submodule update --init tools/build
-  - git submodule update --init libs/config
   - git submodule update --init tools/boostdep
   - xcopy /s /e /q %APPVEYOR_BUILD_FOLDER% libs\variant2\
   - python tools/boostdep/depinst/depinst.py variant2
@@ -41,4 +51,4 @@ build: off
 test_script:
   - if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
   - if not "%ADDRMD%" == "" set ADDRMD=address-model=%ADDRMD%
-  - b2 libs/variant2/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release
+  - b2 -j3 libs/variant2/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release embed-manifest-via=linker

doc/variant2.adoc

@@ -13,6 +13,8 @@ Peter Dimov
 :toclevels: 4
 :idprefix:
 :docinfo: private-footer
+:source-highlighter: rouge
+:source-language: c++
 
 :leveloffset: +1
 

doc/variant2/changelog.adoc

@@ -1,16 +1,41 @@
 ////
-Copyright 2019 Peter Dimov
-
+Copyright 2019-2021 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
 ////
 
 [#changelog]
 # Revision History
 :idprefix: changelog_
 
+## Changes in 1.79.0
+
+* Added `operator<<` for `monostate`.
+
+## Changes in 1.78.0
+
+* Added `<boost/variant2.hpp>`.
+* Added `unsafe_get<I>`.
+* Added `visit_by_index`.
+* Added `operator<<`.
+
+## Changes in 1.76.0
+
+* Improved generated code for the double buffered case.
+
+## Changes in 1.74.0
+
+* Added support for derived types in `visit`
+* Improved compilation performance for many (hundreds of) alternatives.
+* Added support for `visit<R>`
+
+## Changes in 1.73.0
+
+* Added support for `std::hash`, `boost::hash`.
+* `variant<T...>` is now trivial when all types in `T...` are trivial.
+  This improves performance by enabling it to be passed to, and returned
+  from, functions in registers.
+
 ## Changes in 1.71.0
 
 After the Boost formal review, the implementation has been

doc/variant2/design.adoc

@@ -169,8 +169,10 @@ The main differences between this implementation and `std::variant` are:
   `variant<int, float>` is provided as the member function `subset<U...>`.
   (This operation can throw if the current state of the variant cannot be
   represented.)
-* `variant<T...>` is not (yet) trivial when all contained types are trivial,
-   as mandated by {cpp}17.
+* `unsafe_get`, an unchecked alternative to `get` and `get_if`, is provided
+  as an extension.
+* `visit_by_index`, a visitation function that takes a single variant and a
+  number of function objects, one per alternative, is provided as an extension.
 * The {cpp}20 additions and changes to `std::variant` have not yet been
   implemented.
 

doc/variant2/implementation.adoc

@@ -1,10 +1,7 @@
 ////
 Copyright 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
 ////
 
 [#implementation]
@@ -13,13 +10,13 @@ http://www.boost.org/LICENSE_1_0.txt
 
 ## Dependencies
 
-This implementation only depends on Boost.Config and Boost.Mp11.
+This implementation only depends on Boost.Config, Boost.Assert, and Boost.Mp11.
 
 ## Supported Compilers
 
 * GCC 4.8 or later with `-std=c++11` or above
-* Clang 3.5 or later with `-std=c++11` or above
+* Clang 3.9 or later with `-std=c++11` or above
 * Visual Studio 2015, 2017, 2019
 
-Tested on https://travis-ci.org/pdimov/variant2/[Travis] and
-https://ci.appveyor.com/project/pdimov/variant2/[Appveyor].
+Tested on https://github.com/boostorg/variant2/actions[Github Actions] and
+https://ci.appveyor.com/project/pdimov/variant2-fkab9[Appveyor].

doc/variant2/reference.adoc

@@ -1,10 +1,7 @@
 ////
-Copyright 2018, 2019 Peter Dimov
-
+Copyright 2018-2021 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
 ////
 
 [#reference]
@@ -117,6 +114,21 @@ template<class U, class... T>
   constexpr add_pointer_t<const U>
     get_if(const variant<T...>* v) noexcept;
 
+// unsafe_get (extension)
+
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&
+    unsafe_get(variant<T...>& v);
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&&
+    unsafe_get(variant<T...>&& v);
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&
+    unsafe_get(const variant<T...>& v);
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&&
+    unsafe_get(const variant<T...>&& v);
+
 // relational operators
 
 template<class... T>
@@ -132,11 +144,21 @@ template<class... T>
 template<class... T>
   constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);
 
+// swap
+
+template<class... T>
+  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
+
 // visit
 
-template<class F, class... V>
+template<class R = /*unspecified*/, class F, class... V>
   constexpr /*see below*/ visit(F&& f, V&&... v);
 
+// visit_by_index (extension)
+
+template<class R = /*unspecified*/, class V, class... F>
+  constexpr /*see below*/ visit_by_index(V&& v, F&&.. f);
+
 // monostate
 
 struct monostate {};
@@ -148,10 +170,15 @@ constexpr bool operator>(monostate, monostate) noexcept { return false; }
 constexpr bool operator<=(monostate, monostate) noexcept { return true; }
 constexpr bool operator>=(monostate, monostate) noexcept { return true; }
 
-// swap
+// stream insertion (extension)
 
-template<class... T>
-  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
+template<class Ch, class Tr, class... T>
+  std::basic_ostream<Ch, Tr>&
+    operator<<( std::basic_ostream<Ch, Tr>& os, variant<T...> const& v );
+
+template<class Ch, class Tr>
+  std::basic_ostream<Ch, Tr>&
+    operator<<( std::basic_ostream<Ch, Tr>& os, monostate const& v );
 
 // bad_variant_access
 
@@ -490,8 +517,8 @@ template<size_t I, class... A>
 +
 Requires: :: `I < sizeof...(T)`.
 Effects:  ::
-  Destroys the currently contained value, then initializes a new contained
-  value as if using the expression `Ti(std::forward<A>(a)...)`.
+  Initializes a new contained value as if using the expression
+  `Ti(std::forward<A>(a)...)`, then destroys the currently contained value.
 Ensures:  :: `index() == I`.
 Returns:  :: A reference to the new contained value.
 Throws:   ::
@@ -511,8 +538,8 @@ template<size_t I, class V, class... A>
 +
 Requires: :: `I < sizeof...(T)`.
 Effects:  ::
-  Destroys the currently contained value, then initializes a new contained
-  value as if using the expression `Ti(il, std::forward<A>(a)...)`.
+  Initializes a new contained value as if using the expression
+  `Ti(il, std::forward<A>(a)...)`, then destroys the currently contained value.
 Ensures:  :: `index() == I`.
 Returns:  :: A reference to the new contained value.
 Throws:   ::
@@ -775,6 +802,34 @@ Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
 Effects:  :: Equivalent to: `return get_if<I>(v);` with `I` being
   the zero-based index of `U` in `T...`.
 
+### unsafe_get (extension)
+
+```
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&
+    unsafe_get(variant<T...>& v);
+```
+```
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&&
+    unsafe_get(variant<T...>&& v);
+```
+```
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&
+    unsafe_get(const variant<T...>& v);
+```
+```
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&&
+    unsafe_get(const variant<T...>&& v);
+```
+[none]
+* {blank}
++
+Requires: :: `v.index() == I`.
+Returns: :: a reference to the object stored in the variant.
+
 ### Relational Operators
 
 ```
@@ -835,18 +890,6 @@ template<class... T>
 Returns: ::
   `w \<= v`.
 
-### visit
-
-```
-template<class F, class... V>
-  constexpr /*see below*/ visit(F&& f, V&&... v);
-```
-[none]
-* {blank}
-+
-Returns: :: `std::forward<F>(f)(get<I>(std::forward<V>(v))...)`, where
-  `I...` is `v.index()...`.
-
 ### swap
 
 ```
@@ -859,6 +902,67 @@ template<class... T>
 Effects: ::
   Equivalent to `v.swap(w)`.
 
+### visit
+
+```
+template<class R = /*unspecified*/, class F, class... V>
+  constexpr /*see below*/ visit(F&& f, V&&... v);
+```
+[none]
+* {blank}
++
+Returns: :: `std::forward<F>(f)(get<I>(std::forward<V>(v))...)`, where
+  `I...` is `v.index()...`.
+Remarks: :: If `R` is given explicitly, as in `visit<int>`, the return
+  type is `R`. Otherwise, it's deduced from `F`. All possible applications
+  of `F` to the variant alternatives must have the same return type for
+  this deduction to succeed.
+
+### visit_by_index (extension)
+
+```
+template<class R = /*unspecified*/, class V, class... F>
+  constexpr /*see below*/ visit_by_index(V&& v, F&&.. f);
+```
+[none]
+* {blank}
++
+Requires: :: `variant_size<V>::value == sizeof...(F)`, or the program is ill-formed.
+Returns: :: `std::forward<Fi>(fi)(get<i>(std::forward<V>(v)))`, where
+  `i` is `v.index()` and `Fi` and `fi` are the `i`-th element of `F...` and `f...`
+  accordingly.
+Remarks: :: If `R` is given explicitly, as in `visit_by_index<int>`, the return
+  type is `R`. Otherwise, it's deduced from `F...` and `V`. All the applications
+  of `Fi` to the corresponding variant alternatives must have the same return type
+  for this deduction to succeed.
+
+### Stream Insertion (extension)
+```
+template<class Ch, class Tr, class... T>
+  std::basic_ostream<Ch, Tr>&
+    operator<<( std::basic_ostream<Ch, Tr>& os, variant<T...> const& v );
+```
+[none]
+* {blank}
++
+Requires: ::
+  `sizeof...(T) != 0`.
+Returns: ::
+  `os << get<I>(v)`, where `I` is `v.index()`.
+
+```
+template<class Ch, class Tr>
+  std::basic_ostream<Ch, Tr>&
+    operator<<( std::basic_ostream<Ch, Tr>& os, monostate const& v );
+```
+[none]
+* {blank}
++
+Effects: ::
+  `os << "monostate"`.
+Returns: ::
+  `os`.
+
 ### bad_variant_access
 
 ```
@@ -874,3 +978,7 @@ public:
     }
 };
 ```
+
+## <boost/variant2.hpp>
+
+This convenience header includes `<boost/variant2/variant.hpp>`.

include/boost/variant2.hpp

@@ -0,0 +1,10 @@
+#ifndef BOOST_VARIANT2_HPP_INCLUDED
+#define BOOST_VARIANT2_HPP_INCLUDED
+
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+
+#endif // #ifndef BOOST_VARIANT2_HPP_INCLUDED

meta/libraries.json

@@ -5,10 +5,11 @@
         "Peter Dimov"
     ],
     "maintainers": [
-        "Peter Dimov <pdimov -at- pdimov.com>"
+        "Peter Dimov <pdimov -at- gmail.com>"
     ],
     "description": "A never-valueless, strong guarantee implementation of std::variant.",
     "category": [
         "Containers", "Data"
-    ]
+    ],
+    "cxxstd": "11"
 }

test/CMakeLists.txt

@@ -0,0 +1,11 @@
+# Copyright 2018, 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
+
+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)
+
+endif()

test/Jamfile

@@ -12,7 +12,7 @@ import ../../config/checks/config : requires ;
 project
   : default-build
 
-    <warnings>all
+    <warnings>extra
 
   : requirements
 
@@ -23,20 +23,22 @@ project
     <toolset>clang:<warnings-as-errors>on
   ;
 
+run quick.cpp ;
+
 run variant_size.cpp ;
 run variant_alternative.cpp ;
 
 run variant_holds_alternative.cpp ;
-compile variant_holds_alternative_cx.cpp ;
+compile variant_holds_alternative_cx.cpp : <toolset>msvc-14.0:<build>no ;
 
 run variant_get_by_index.cpp ;
-compile variant_get_by_index_cx.cpp ;
+compile variant_get_by_index_cx.cpp : <toolset>msvc-14.0:<build>no ;
 
 run variant_get_by_type.cpp ;
-compile variant_get_by_type_cx.cpp ;
+compile variant_get_by_type_cx.cpp : <toolset>msvc-14.0:<build>no ;
 
 run variant_default_construct.cpp ;
-compile variant_default_construct_cx.cpp ;
+compile variant_default_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
 
 run variant_copy_construct.cpp ;
 compile variant_copy_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
@@ -45,13 +47,13 @@ run variant_move_construct.cpp ;
 compile variant_move_construct_cx.cpp : [ requires cxx14_constexpr ] ;
 
 run variant_value_construct.cpp ;
-compile variant_value_construct_cx.cpp ;
+compile variant_value_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
 
 run variant_in_place_index_construct.cpp ;
-compile variant_in_place_index_construct_cx.cpp ;
+compile variant_in_place_index_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
 
 run variant_in_place_type_construct.cpp ;
-compile variant_in_place_type_construct_cx.cpp ;
+compile variant_in_place_type_construct_cx.cpp : <toolset>msvc-14.0:<build>no ;
 
 run variant_copy_assign.cpp ;
 compile variant_copy_assign_cx.cpp : [ requires cxx14_constexpr ] ;
@@ -97,9 +99,30 @@ 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) : variant_get_by_index_cx_nx ;
+compile variant_get_by_index_cx.cpp : $(NX) <toolset>msvc-14.0:<build>no : 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) : variant_get_by_type_cx_nx ;
+compile variant_get_by_type_cx.cpp : $(NX) <toolset>msvc-14.0:<build>no : variant_get_by_type_cx_nx ;
 
 run variant_subset.cpp throw_exception.cpp : : : $(NX) : variant_subset_nx ;
+
+run variant_hash.cpp ;
+
+run variant_trivial.cpp ;
+run variant_special.cpp ;
+
+run variant_visit_derived.cpp ;
+
+run variant_many_types.cpp ;
+
+run variant_visit_r.cpp : : :
+    <toolset>gcc,<target-os>windows:<variant>release
+    <toolset>gcc,<target-os>cygwin:<variant>release
+    ;
+
+compile variant_derived_construct.cpp ;
+
+run variant_visit_by_index.cpp ;
+
+run variant_ostream_insert.cpp ;
+run is_output_streamable.cpp ;

test/cmake_install_test/CMakeLists.txt

@@ -0,0 +1,17 @@
+# Copyright 2018, 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
+
+cmake_minimum_required(VERSION 3.5...3.16)
+
+project(cmake_install_test LANGUAGES CXX)
+
+find_package(boost_variant2 REQUIRED)
+
+add_executable(quick ../quick.cpp)
+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>)

test/cmake_subdir_test/CMakeLists.txt

@@ -0,0 +1,21 @@
+# Copyright 2018, 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
+
+cmake_minimum_required(VERSION 3.5...3.16)
+
+project(cmake_subdir_test LANGUAGES CXX)
+
+add_subdirectory(../.. boostorg/variant2)
+
+add_subdirectory(../../../assert boostorg/assert)
+add_subdirectory(../../../config boostorg/config)
+add_subdirectory(../../../mp11 boostorg/mp11)
+
+add_executable(quick ../quick.cpp)
+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>)

test/is_output_streamable.cpp

@@ -0,0 +1,41 @@
+// Copyright 2021 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_trait.hpp>
+#include <ostream>
+#include <string>
+
+struct X
+{
+};
+
+struct Y
+{
+};
+
+std::ostream& operator<<( std::ostream& os, Y const& /*y*/ )
+{
+    os << "Y()";
+    return os;
+}
+
+int main()
+{
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, int>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, float>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, std::string>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, void>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, X>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, Y>));
+
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::monostate>));
+
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, std::string>>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, X>>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, Y>>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<boost::variant2::monostate, int, float>>));
+
+    return boost::report_errors();
+}

test/quick.cpp

@@ -0,0 +1,13 @@
+// Copyright 2019 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2.hpp>
+
+using namespace boost::variant2;
+
+int main()
+{
+    variant<float, int> v( 2 );
+    return get<1>( v ) == 2? 0: 1;
+}

test/variant_copy_construct.cpp

@@ -9,6 +9,8 @@
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
+#include <boost/config.hpp>
+#include <boost/config/workaround.hpp>
 #include <type_traits>
 #include <utility>
 #include <string>
@@ -48,12 +50,21 @@ struct Y
     Y( Y const& ) = delete;
 };
 
+struct D
+{
+    ~D() {}
+};
+
+inline bool operator==( D, D ) { return true; }
+
 template<class V> static void test( V const & v )
 {
     V v2( v );
 
     BOOST_TEST_EQ( v.index(), v2.index() );
     BOOST_TEST( v == v2 );
+
+    BOOST_TEST_TRAIT_TRUE((std::is_copy_constructible<V>));
 }
 
 int main()
@@ -115,6 +126,12 @@ int main()
         test( v );
     }
 
+#if !BOOST_WORKAROUND( __GNUC__, < 5 )
+
+    test( variant<D>() );
+
+#endif
+
     {
         BOOST_TEST_TRAIT_TRUE((std::is_nothrow_copy_constructible<variant<int>>));
         BOOST_TEST_TRAIT_TRUE((std::is_nothrow_copy_constructible<variant<int const>>));

test/variant_derived_construct.cpp

@@ -0,0 +1,33 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+
+using namespace boost::variant2;
+
+template<class... T> class X: variant<T...>
+{
+    using base = variant<T...>;
+    using base::base;
+};
+
+struct Y
+{
+    Y( Y const& rhs ) = default;
+
+    template<class T> Y( T const& t )
+    {
+        t.bar();
+    }
+};
+
+int main()
+{
+    using W = X<int, double, Y>;
+
+    W a( 1 );
+    W b( a );
+
+    (void)b;
+}

test/variant_emplace_index.cpp

@@ -57,6 +57,60 @@ STATIC_ASSERT( !std::is_nothrow_move_constructible<X2>::value );
 STATIC_ASSERT( !std::is_nothrow_copy_assignable<X2>::value );
 STATIC_ASSERT( !std::is_nothrow_move_assignable<X2>::value );
 
+struct Y1
+{
+    int v = 1;
+
+    Y1() = default;
+    Y1(Y1 const&) = delete;
+    Y1(Y1&&) = delete;
+};
+
+STATIC_ASSERT( !std::is_copy_constructible<Y1>::value );
+STATIC_ASSERT( !std::is_move_constructible<Y1>::value );
+
+struct Y2
+{
+    int v = 2;
+
+    Y2() = default;
+    Y2(Y2 const&) = delete;
+    Y2(Y2&&) = delete;
+};
+
+STATIC_ASSERT( !std::is_copy_constructible<Y2>::value );
+STATIC_ASSERT( !std::is_move_constructible<Y2>::value );
+
+struct Z1
+{
+    static int instances;
+
+    int v = 1;
+
+    Z1() { ++instances; }
+    ~Z1() { --instances; }
+
+    Z1(Z1 const&) = delete;
+    Z1(Z1&&) = delete;
+};
+
+int Z1::instances = 0;
+
+struct Z2
+{
+    static int instances;
+
+    int v = 2;
+
+    Z2() { ++instances; }
+    ~Z2() { --instances; }
+
+    Z2(Z2 const&) = delete;
+    Z2(Z2&&) = delete;
+};
+
+int Z2::instances = 0;
+
 int main()
 {
     {
@@ -177,5 +231,62 @@ int main()
         BOOST_TEST_EQ( get<0>(v).v, 4 );
     }
 
+    {
+        variant<Y1, Y2> v;
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v).v, 1 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v).v, 1 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+        BOOST_TEST_EQ( get<1>(v).v, 2 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+        BOOST_TEST_EQ( get<1>(v).v, 2 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v).v, 1 );
+    }
+
+    {
+        variant<Z1, Z2> v;
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v).v, 1 );
+        BOOST_TEST_EQ( Z1::instances, 1 );
+        BOOST_TEST_EQ( Z2::instances, 0 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v).v, 1 );
+        BOOST_TEST_EQ( Z1::instances, 1 );
+        BOOST_TEST_EQ( Z2::instances, 0 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+        BOOST_TEST_EQ( get<1>(v).v, 2 );
+        BOOST_TEST_EQ( Z1::instances, 0 );
+        BOOST_TEST_EQ( Z2::instances, 1 );
+
+        v.emplace<1>();
+        BOOST_TEST_EQ( v.index(), 1 );
+        BOOST_TEST_EQ( get<1>(v).v, 2 );
+        BOOST_TEST_EQ( Z1::instances, 0 );
+        BOOST_TEST_EQ( Z2::instances, 1 );
+
+        v.emplace<0>();
+        BOOST_TEST_EQ( v.index(), 0 );
+        BOOST_TEST_EQ( get<0>(v).v, 1 );
+        BOOST_TEST_EQ( Z1::instances, 1 );
+        BOOST_TEST_EQ( Z2::instances, 0 );
+    }
+
+    BOOST_TEST_EQ( Z1::instances, 0 );
+    BOOST_TEST_EQ( Z2::instances, 0 );
+
     return boost::report_errors();
 }

test/variant_eq_ne.cpp

@@ -88,5 +88,12 @@ int main()
         BOOST_TEST_NOT( v1 != v2 );
     }
 
+    {
+        variant<monostate> v1, v2;
+
+        BOOST_TEST( v1 == v2 );
+        BOOST_TEST_NOT( v1 != v2 );
+    }
+
     return boost::report_errors();
 }

test/variant_eq_ne_cx.cpp

@@ -93,6 +93,13 @@ int main()
         STATIC_ASSERT( !(v1 == v2) );
         STATIC_ASSERT( !(v1 != v2) );
     }
+
+    {
+        constexpr variant<monostate> v1, v2;
+
+        STATIC_ASSERT( v1 == v2 );
+        STATIC_ASSERT( !(v1 != v2) );
+    }
 }
 
 #endif

test/variant_get_by_index.cpp

@@ -6,6 +6,12 @@
 // See accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt
 
+#if defined( __clang__ ) && defined( __has_warning )
+# if __has_warning( "-Wdeprecated-volatile" )
+#  pragma clang diagnostic ignored "-Wdeprecated-volatile"
+# endif
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <boost/core/lightweight_test_trait.hpp>

test/variant_get_by_type.cpp

@@ -6,6 +6,12 @@
 // See accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt
 
+#if defined( __clang__ ) && defined( __has_warning )
+# if __has_warning( "-Wdeprecated-volatile" )
+#  pragma clang diagnostic ignored "-Wdeprecated-volatile"
+# endif
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <boost/core/lightweight_test_trait.hpp>

test/variant_hash.cpp

@@ -0,0 +1,86 @@
+
+// Copyright 2020 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#if defined(_MSC_VER)
+# pragma warning( disable: 4244 ) // conversion from int to float, possible loss of data
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/container_hash/hash.hpp>
+#include <boost/config/workaround.hpp>
+#include <vector>
+
+using namespace boost::variant2;
+
+template<template<class...> class Hash, class T1, class T2, class T3> void test()
+{
+    variant<T1, T2, T3> v1( in_place_index_t<0>{} );
+    std::size_t h1 = Hash<decltype(v1)>()( v1 );
+
+    variant<T1, T2, T3> v2( in_place_index_t<1>{} );
+    std::size_t h2 = Hash<decltype(v2)>()( v2 );
+
+    variant<T1, T2, T3> v3( in_place_index_t<2>{} );
+    std::size_t h3 = Hash<decltype(v3)>()( v3 );
+
+    BOOST_TEST_NE( h1, h2 );
+    BOOST_TEST_NE( h1, h3 );
+    BOOST_TEST_NE( h2, h3 );
+}
+
+template<template<class...> class Hash, class T> void test2()
+{
+    variant<T> v1( 0 );
+    std::size_t h1 = Hash<decltype(v1)>()( v1 );
+
+    variant<T> v2( 1 );
+    std::size_t h2 = Hash<decltype(v2)>()( v2 );
+
+    variant<T> v3( 2 );
+    std::size_t h3 = Hash<decltype(v3)>()( v3 );
+
+    BOOST_TEST_NE( h1, h2 );
+    BOOST_TEST_NE( h1, h3 );
+    BOOST_TEST_NE( h2, h3 );
+}
+
+struct X
+{
+    int m = 0;
+};
+
+std::size_t hash_value( X const& x )
+{
+    return boost::hash<int>()( x.m );
+}
+
+struct Y {}; // no hash support
+
+int main()
+{
+    test<std::hash, monostate, monostate, monostate>();
+    test<std::hash, int, int, float>();
+
+    test<boost::hash, monostate, monostate, monostate>();
+    test<boost::hash, int, int, float>();
+
+    test<boost::hash, monostate, X, std::vector<X>>();
+
+    test2<std::hash, int>();
+    test2<std::hash, float>();
+
+    test2<boost::hash, int>();
+    test2<boost::hash, float>();
+
+#if !BOOST_WORKAROUND(BOOST_MSVC, < 1910) && ( !defined(_LIBCPP_STD_VER) || _LIBCPP_STD_VER > 11 )
+
+    BOOST_TEST_TRAIT_FALSE(( detail::is_hash_enabled<Y> ));
+
+#endif
+
+    return boost::report_errors();
+}

test/variant_in_place_index_construct_cx.cpp

@@ -7,11 +7,8 @@
 // http://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
-#include <boost/core/lightweight_test.hpp>
-#include <boost/core/lightweight_test_trait.hpp>
-#include <type_traits>
-#include <utility>
-#include <string>
+#include <boost/config.hpp>
+#include <boost/config/workaround.hpp>
 
 using namespace boost::variant2;
 
@@ -108,9 +105,17 @@ int main()
         STATIC_ASSERT( v.index() == 4 );
     }
 
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 120000)
+
+    // no idea why this fails on g++ 10/11
+
+#else
+
     {
         constexpr variant<int, int, float, float, X, X> v( in_place_index_t<5>{}, 0, 0 );
 
         STATIC_ASSERT( v.index() == 5 );
     }
+
+#endif
 }

test/variant_in_place_type_construct_cx.cpp

@@ -7,6 +7,8 @@
 // http://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/variant2/variant.hpp>
+#include <boost/config.hpp>
+#include <boost/config/workaround.hpp>
 
 using namespace boost::variant2;
 
@@ -100,6 +102,12 @@ int main()
         STATIC_ASSERT( holds_alternative<X>(v) );
     }
 
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 120000)
+
+    // no idea why this fails on g++ 10/11
+
+#else
+
     {
         constexpr variant<int, int, float, float, X> v( in_place_type_t<X>{}, 0, 0 );
 
@@ -107,4 +115,6 @@ int main()
 
         STATIC_ASSERT( holds_alternative<X>(v) );
     }
+
+#endif
 }

test/variant_lt_gt.cpp

@@ -81,5 +81,14 @@ int main()
         BOOST_TEST_NOT( v1 >= v2 );
     }
 
+    {
+        variant<monostate> v1, v2;
+
+        BOOST_TEST_NOT( v1 < v2 );
+        BOOST_TEST_NOT( v1 > v2 );
+        BOOST_TEST( v1 <= v2 );
+        BOOST_TEST( v1 >= v2 );
+    }
+
     return boost::report_errors();
 }

test/variant_lt_gt_cx.cpp

@@ -86,6 +86,15 @@ int main()
         STATIC_ASSERT( !(v1 <= v2) );
         STATIC_ASSERT( !(v1 >= v2) );
     }
+
+    {
+        constexpr variant<monostate> v1, v2;
+
+        STATIC_ASSERT( !(v1 < v2) );
+        STATIC_ASSERT( !(v1 > v2) );
+        STATIC_ASSERT( v1 <= v2 );
+        STATIC_ASSERT( v1 >= v2 );
+    }
 }
 
 #endif

test/variant_many_types.cpp

@@ -0,0 +1,109 @@
+
+// Copyright 2020 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<class I> struct X
+{
+    static int const value = I::value;
+    int v_;
+};
+
+template<class I> int const X<I>::value;
+
+template<class I> struct Y
+{
+    static int const value = I::value;
+    int v_;
+
+    Y() = default;
+    Y( Y const& ) = default;
+
+    explicit Y( int v ): v_( v ) {}
+
+    Y& operator=( Y const& ) noexcept = default;
+
+    Y& operator=( Y&& r ) noexcept
+    {
+        v_ = r.v_;
+        return *this;
+    }
+};
+
+template<class I> int const Y<I>::value;
+
+template<class I> struct Z
+{
+    static int const value = I::value;
+    int v_;
+
+    ~Z() {}
+};
+
+template<class I> int const Z<I>::value;
+
+template<class V> struct F1
+{
+    template<class T> void operator()( T ) const
+    {
+        int const i = T::value;
+
+        T t{ i * 2 };
+
+        using boost::variant2::get;
+
+        {
+            V v( t );
+
+            BOOST_TEST_EQ( v.index(), i );
+            BOOST_TEST_EQ( get<i>( v ).v_, t.v_ );
+            BOOST_TEST_EQ( get<T>( v ).v_, t.v_ );
+        }
+
+        {
+            V const v( t );
+
+            BOOST_TEST_EQ( v.index(), i );
+            BOOST_TEST_EQ( get<i>( v ).v_, t.v_ );
+            BOOST_TEST_EQ( get<T>( v ).v_, t.v_ );
+        }
+
+        {
+            V v;
+
+            v = t;
+
+            BOOST_TEST_EQ( v.index(), i );
+            BOOST_TEST_EQ( get<i>( v ).v_, t.v_ );
+            BOOST_TEST_EQ( get<T>( v ).v_, t.v_ );
+        }
+    }
+};
+
+template<class V> void test()
+{
+    mp_for_each<V>( F1<V>() );
+}
+
+int main()
+{
+    int const N = 32;
+
+    using V = mp_rename<mp_iota_c<N>, boost::variant2::variant>;
+
+    test< mp_transform<X, V> >();
+    test< mp_transform<Y, V> >();
+    test< mp_transform<Z, V> >();
+
+    return boost::report_errors();
+}

test/variant_ostream_insert.cpp

@@ -0,0 +1,43 @@
+// Copyright 2021 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>
+#include <sstream>
+#include <string>
+
+using namespace boost::variant2;
+
+template<class T> std::string to_string( T const& t )
+{
+    std::ostringstream os;
+
+    os << t;
+
+    return os.str();
+}
+
+int main()
+{
+    {
+        BOOST_TEST_EQ( to_string( monostate() ), "monostate" );
+    }
+
+    {
+        variant<monostate, int, float, std::string> v;
+
+        BOOST_TEST_EQ( to_string( v ), to_string( monostate() ) );
+
+        v = 1;
+        BOOST_TEST_EQ( to_string( v ), to_string( 1 ) );
+
+        v = 3.14f;
+        BOOST_TEST_EQ( to_string( v ), to_string( 3.14f ) );
+
+        v = "test";
+        BOOST_TEST_EQ( to_string( v ), to_string( "test" ) );
+    }
+
+    return boost::report_errors();
+}

test/variant_special.cpp

@@ -0,0 +1,103 @@
+// Copyright 2020 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#if defined(_MSC_VER) && _MSC_VER < 1910
+# pragma warning(disable: 4503) // decorated name length exceeded
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/config.hpp>
+#include <boost/config/workaround.hpp>
+
+#include <boost/mp11.hpp>
+using namespace boost::mp11;
+
+//
+
+using namespace boost::variant2;
+
+struct D
+{
+    ~D() noexcept {}
+};
+
+struct CC1
+{
+    CC1( CC1 const& ) {}
+};
+
+struct CC2
+{
+    CC2( CC2 const& ) = delete;
+};
+
+struct MC1
+{
+    MC1( MC1 && ) {}
+};
+
+struct MC2
+{
+    MC2( MC2 && ) = delete;
+};
+
+struct CA1
+{
+    CA1& operator=( CA1 const& ) { return *this; }
+};
+
+struct CA2
+{
+    CA2& operator=( CA2 const& ) = delete;
+};
+
+struct MA1
+{
+    MA1& operator=( MA1 && ) { return *this; }
+};
+
+struct MA2
+{
+    MA2& operator=( MA2 && ) = delete;
+};
+
+struct test
+{
+    template<class... T> void operator()( mp_list<T...> ) const noexcept
+    {
+        using U = mp_inherit<T...>;
+
+#if !BOOST_WORKAROUND( __GNUC__, < 5 )
+
+        BOOST_TEST_EQ( std::is_copy_constructible<variant<U>>::value, std::is_copy_constructible<U>::value );
+        BOOST_TEST_EQ( std::is_nothrow_copy_constructible<variant<U>>::value, std::is_nothrow_copy_constructible<U>::value );
+
+#endif
+
+#if !BOOST_WORKAROUND(BOOST_MSVC, < 1910)
+
+        BOOST_TEST_EQ( std::is_move_constructible<variant<U>>::value, std::is_move_constructible<U>::value );
+
+#else
+
+        BOOST_TEST_GE( std::is_move_constructible<variant<U>>::value, std::is_move_constructible<U>::value );
+
+#endif
+
+        BOOST_TEST_EQ( std::is_nothrow_move_constructible<variant<U>>::value, std::is_nothrow_move_constructible<U>::value );
+
+        BOOST_TEST_EQ( std::is_copy_assignable<variant<U>>::value, std::is_copy_constructible<U>::value && std::is_copy_assignable<U>::value );
+        BOOST_TEST_EQ( std::is_nothrow_copy_assignable<variant<U>>::value, std::is_nothrow_copy_constructible<U>::value && std::is_copy_assignable<U>::value );
+
+        BOOST_TEST_EQ( std::is_move_assignable<variant<U>>::value, std::is_move_constructible<U>::value && std::is_move_assignable<U>::value );
+        BOOST_TEST_EQ( std::is_nothrow_move_assignable<variant<U>>::value, std::is_nothrow_move_constructible<U>::value && std::is_move_assignable<U>::value );
+    }
+};
+
+int main()
+{
+    mp_for_each< mp_power_set< mp_list<D, CC1, CC2, MC1, MC2, CA1, CA2, MA1, MA2> > >( test() );
+    return boost::report_errors();
+}

test/variant_trivial.cpp

@@ -0,0 +1,95 @@
+// Copyright 2020 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#if defined(_MSC_VER) && _MSC_VER < 1910
+# pragma warning(disable: 4503) // decorated name length exceeded
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/config.hpp>
+#include <boost/config/workaround.hpp>
+
+#include <boost/mp11.hpp>
+using namespace boost::mp11;
+
+//
+
+struct D
+{
+    ~D() noexcept {}
+};
+
+struct CC1
+{
+    CC1( CC1 const& ) noexcept {}
+};
+
+struct CC2
+{
+    CC2( CC2 const& ) = delete;
+};
+
+struct MC1
+{
+    MC1( MC1 && ) noexcept {}
+};
+
+struct MC2
+{
+    MC2( MC2 && ) = delete;
+};
+
+struct CA1
+{
+    CA1& operator=( CA1 const& ) noexcept { return *this; }
+};
+
+struct CA2
+{
+    CA2& operator=( CA2 const& ) = delete;
+};
+
+struct MA1
+{
+    MA1& operator=( MA1 && ) noexcept { return *this; }
+};
+
+struct MA2
+{
+    MA2& operator=( MA2 && ) = delete;
+};
+
+using namespace boost::variant2;
+namespace v2d = boost::variant2::detail;
+
+struct test
+{
+    template<class... T> void operator()( mp_list<T...> ) const noexcept
+    {
+        using U = mp_inherit<T...>;
+
+#if !BOOST_WORKAROUND( __GNUC__, < 5 )
+
+        BOOST_TEST_EQ( v2d::is_trivially_copy_constructible<variant<U>>::value, v2d::is_trivially_copy_constructible<U>::value );
+        BOOST_TEST_EQ( v2d::is_trivially_copy_assignable<variant<U>>::value, std::is_trivially_destructible<U>::value && v2d::is_trivially_copy_constructible<U>::value && v2d::is_trivially_copy_assignable<U>::value );
+
+#endif
+
+        BOOST_TEST_EQ( std::is_trivially_destructible<variant<U>>::value, std::is_trivially_destructible<U>::value );
+
+#if !BOOST_WORKAROUND(BOOST_LIBSTDCXX_VERSION, < 50000)
+
+        BOOST_TEST_EQ( v2d::is_trivially_move_constructible<variant<U>>::value, v2d::is_trivially_move_constructible<U>::value );
+        BOOST_TEST_EQ( v2d::is_trivially_move_assignable<variant<U>>::value, std::is_trivially_destructible<U>::value && v2d::is_trivially_move_constructible<U>::value && v2d::is_trivially_move_assignable<U>::value );
+
+#endif
+    }
+};
+
+int main()
+{
+    mp_for_each< mp_power_set< mp_list<D, CC1, CC2, MC1, MC2, CA1, CA2, MA1, MA2> > >( test() );
+    return boost::report_errors();
+}

test/variant_visit.cpp

@@ -37,6 +37,10 @@ struct F
 
 int main()
 {
+    {
+        BOOST_TEST_EQ( (visit( []{ return 5; } )), 5 );
+    }
+
     {
         variant<int> v( 1 );
 

test/variant_visit_by_index.cpp

@@ -0,0 +1,126 @@
+// Copyright 2017, 2021 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>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/mp11.hpp>
+#include <boost/config.hpp>
+
+using namespace boost::variant2;
+using boost::mp11::mp_int;
+
+struct X
+{
+};
+
+struct F1
+{
+    int operator()( X& ) const { return 1; }
+    int operator()( X const& ) const { return 2; }
+    int operator()( X&& ) const { return 3; }
+    int operator()( X const&& ) const { return 4; }
+};
+
+struct F2
+{
+    mp_int<1> operator()( X& ) const { return {}; }
+    mp_int<2> operator()( X const& ) const { return {}; }
+    mp_int<3> operator()( X&& ) const { return {}; }
+    mp_int<4> operator()( X const&& ) const { return {}; }
+};
+
+int main()
+{
+    {
+        variant<int, int, float> v;
+
+        visit_by_index( v,
+            []( int& x ){ BOOST_TEST_EQ( x, 0 ); },
+            []( int&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> v( in_place_index_t<0>(), 1 );
+
+        visit_by_index( v,
+            []( int const& x ){ BOOST_TEST_EQ( x, 1 ); },
+            []( int&         ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int, int, float> const v( in_place_index_t<1>(), 2 );
+
+        visit_by_index( v,
+            []( int const&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const& x ){ BOOST_TEST_EQ( x, 2 ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> const v( 3.14f );
+
+        visit_by_index( v,
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& x ){ BOOST_TEST_EQ( x, 3.14f ); } );
+    }
+
+    {
+        variant<int, float> const v( 7 );
+
+        auto r = visit_by_index( v,
+            []( int const& x   ) -> double { return x; },
+            []( float const& x ) -> double { return x; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 7.0 );
+    }
+
+    {
+        variant<int, float> const v( 2.0f );
+
+        auto r = visit_by_index( v,
+            []( int const& x   ) { return x + 0.0; },
+            []( float const& x ) { return x + 0.0; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 2.0 );
+    }
+
+    {
+        variant<int, float, double> const v( 3.0 );
+
+        auto r = visit_by_index<double>( v,
+            []( int const& x    ) { return x; },
+            []( float const& x  ) { return x; },
+            []( double const& x ) { return x; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 3.0 );
+    }
+
+    {
+        variant<X> v;
+        variant<X> const cv;
+
+        F1 f1;
+
+        BOOST_TEST_EQ( visit_by_index( v, f1 ), 1 );
+        BOOST_TEST_EQ( visit_by_index( cv, f1 ), 2 );
+        BOOST_TEST_EQ( visit_by_index( std::move( v ), f1 ), 3 );
+        BOOST_TEST_EQ( visit_by_index( std::move( cv ), f1 ), 4 );
+
+        F2 f2;
+
+        BOOST_TEST_EQ( visit_by_index<int>( v, f2 ), 1 );
+        BOOST_TEST_EQ( visit_by_index<int>( cv, f2 ), 2 );
+        BOOST_TEST_EQ( visit_by_index<int>( std::move( v ), f2 ), 3 );
+        BOOST_TEST_EQ( visit_by_index<int>( std::move( cv ), f2 ), 4 );
+    }
+
+    return boost::report_errors();
+}

test/variant_visit_derived.cpp

@@ -0,0 +1,56 @@
+// Copyright 2017, 2020 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: 4244 ) // conversion from float to int, possible loss of data
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config.hpp>
+#include <boost/config/workaround.hpp>
+#include <utility>
+
+struct X: boost::variant2::variant<int, float>
+{
+#if BOOST_WORKAROUND( BOOST_MSVC, < 1940 )
+
+    template<class T> explicit X( T&& t ): variant( std::forward<T>( t ) ) {};
+
+#else
+
+    using variant::variant;
+
+#endif
+};
+
+template<class... T> struct Y: boost::variant2::variant<T...>
+{
+    using boost::variant2::variant<T...>::variant;
+};
+
+int main()
+{
+    {
+        X v1( 1 );
+        X const v2( 3.14f );
+
+        BOOST_TEST_EQ( (visit( []( int x1, float x2 ){ return (int)(x1 * 1000) + (int)(x2 * 100); }, v1, v2 )), 1314 );
+
+        visit( []( int x1, float x2 ){ BOOST_TEST_EQ( x1, 1 ); BOOST_TEST_EQ( x2, 3.14f ); }, v1, v2 );
+        visit( []( int x1, float x2 ){ BOOST_TEST_EQ( x1, 1 ); BOOST_TEST_EQ( x2, 3.14f ); }, std::move(v1), std::move(v2) );
+    }
+
+    {
+        Y<int, float> v1( 1 );
+        Y<int, float> const v2( 3.14f );
+
+        BOOST_TEST_EQ( (visit( []( int x1, float x2 ){ return (int)(x1 * 1000) + (int)(x2 * 100); }, v1, v2 )), 1314 );
+
+        visit( []( int x1, float x2 ){ BOOST_TEST_EQ( x1, 1 ); BOOST_TEST_EQ( x2, 3.14f ); }, v1, v2 );
+        visit( []( int x1, float x2 ){ BOOST_TEST_EQ( x1, 1 ); BOOST_TEST_EQ( x2, 3.14f ); }, std::move(v1), std::move(v2) );
+    }
+
+    return boost::report_errors();
+}

test/variant_visit_r.cpp

@@ -0,0 +1,98 @@
+
+// Copyright 2017 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
+
+#if defined(_MSC_VER)
+# pragma warning( disable: 4244 ) // conversion from float to int, possible loss of data
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::variant2;
+
+struct F1
+{
+    template<class T1> T1 operator()( T1 t1 ) const
+    {
+        return t1;
+    }
+};
+
+struct F2
+{
+    template<class T1, class T2> auto operator()( T1 t1, T2 t2 ) const -> decltype( t1 + t2 )
+    {
+        return t1 + t2;
+    }
+};
+
+struct F3
+{
+    template<class T1, class T2, class T3> auto operator()( T1 t1, T2 t2, T3 t3 ) const -> decltype( t1 + t2 + t3 )
+    {
+        return t1 + t2 + t3;
+    }
+};
+
+struct F4
+{
+    template<class T1, class T2, class T3, class T4> auto operator()( T1 t1, T2 t2, T3 t3, T4 t4 ) const -> decltype( t1 + t2 + t3 + t4 )
+    {
+        return t1 + t2 + t3 + t4;
+    }
+};
+
+int main()
+{
+    {
+        BOOST_TEST_EQ( (visit<int>( []{ return 3.14f; } )), 3 );
+    }
+
+    {
+        variant<int, float> v( 1 );
+
+        BOOST_TEST_EQ( visit<int>( F1(), v ), 1 );
+        BOOST_TEST_EQ( visit<float>( F1(), v ), 1.0f );
+    }
+
+    {
+        variant<int, float> const v( 3.14f );
+
+        BOOST_TEST_EQ( visit<int>( F1(), v ), 3 );
+        BOOST_TEST_EQ( visit<float>( F1(), v ), 3.14f );
+    }
+
+    {
+        variant<int, float> v1( 1 );
+        variant<int, float> const v2( 3.14f );
+
+        BOOST_TEST_EQ( visit<int>( F2(), v1, v2 ), 4 );
+        BOOST_TEST_EQ( visit<float>( F2(), v1, v2 ), 1 + 3.14f );
+    }
+
+    {
+        variant<int, float, double> v1( 1 );
+        variant<int, float, double> const v2( 3.14f );
+        variant<int, float, double> v3( 6.28 );
+
+        BOOST_TEST_EQ( visit<int>( F3(), v1, v2, v3 ), 10 );
+        BOOST_TEST_EQ( visit<float>( F3(), v1, v2, v3 ), static_cast<float>( 1 + 3.14f + 6.28 ) );
+    }
+
+    {
+        variant<int, float, double, char> v1( 1 );
+        variant<int, float, double, char> const v2( 3.14f );
+        variant<int, float, double, char> v3( 6.28 );
+        variant<int, float, double, char> const v4( 'A' );
+
+        BOOST_TEST_EQ( visit<int>( F4(), v1, v2, v3, v4 ), 10 + 'A' );
+        BOOST_TEST_EQ( visit<float>( F4(), v1, v2, v3, v4 ), static_cast<float>( 1 + 3.14f + 6.28 + 'A' ) );
+    }
+
+    return boost::report_errors();
+}