glob headers

Document operator<< for monostate
Add operator<< for monostate
2022-07-05 10:48:50 +02:00 · 2022-01-31 19:19:49 +02:00 · 2022-01-31 17:55:26 +02:00 · 2021-12-09 21:28:35 +02:00 · 2021-12-09 19:59:01 +02:00 · 2021-12-09 18:39:03 +02:00
57 changed files with 3470 additions and 4016 deletions
--- a/.github/workflows/ci.yml
+++ b/.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
--- a/.travis.yml
+++ b/.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 $TRAVIS_BUILD_DIR/* libs/variant2
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@ -0,0 +1,49 @@
+# Generated by `boostdep --cmake variant2`
+# Copyright 2020, 2021 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+if(NOT DEFINED IDF_TARGET)
+
+cmake_minimum_required(VERSION 3.8...3.20)
+
+project(boost_variant2 VERSION "${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()
+
+else()
+
+FILE(GLOB_RECURSE headers include/*.h include/*.hpp)
+
+idf_component_register(
+    SRCS
+        ${headers}
+    INCLUDE_DIRS
+        include
+    REQUIRES
+        boost_assert
+        boost_config
+        boost_mp11
+)
+
+endif()
+
+
--- a/README.md
+++ b/README.md
@ -1,64 +1,18 @@
-# variant2
+# Boost.Variant2

-This repository contains a never-valueless C++11/14/17 implementation of
-[std::variant](http://en.cppreference.com/w/cpp/utility/variant) in
-[variant.hpp](include/boost/variant2/variant.hpp) and an implementation of
-`expected<T, E...>` in [expected.hpp](include/boost/variant2/expected.hpp)
-that is an extended version of `expected<T, E>` as proposed in
-[P0323R1](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0323r1.pdf)
-and the subsequent
-[D0323R2](https://github.com/viboes/std-make/blob/master/doc/proposal/expected/d0323r2.md).
+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/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/CTZztA).
+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
-* Visual Studio 2015, 2017
+* clang++ 3.9 or later with `-std=c++11` or above
+* Visual Studio 2015, 2017, 2019

-Tested on [Travis](https://travis-ci.org/pdimov/variant2/) and
-[Appveyor](https://ci.appveyor.com/project/pdimov/variant2/).
-
-## variant.hpp
-
-The class `boost::variant2::variant<T...>` is an almost conforming
-implementation of `std::variant` with the following differences:
-
-* A converting constructor from, e.g. `variant<int, float>` to
-  `variant<float, double, int>` is provided as an extension;
-* The reverse operation, going from `variant<float, double, int>` to
-  `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 trivial when all contained types are trivial.
-
-To avoid going into a valueless-by-exception state, this implementation falls
-back to using double storage unless
-
-* one of the alternatives is the type `monostate`,
-* one of the alternatives has a nonthrowing default constructor, or
-* all the contained types are nothrow move constructible.
-
-If the first two bullets don't hold, but the third does, the variant uses
-single storage, but `emplace` constructs a temporary and moves it into place
-if the construction of the object can throw. In case this is undesirable, one
-can force `emplace` into always constructing in-place by adding `monostate` as
-one of the alternatives.
-
-## expected.hpp
-
-The class `boost::variant2::expected<T, E...>` represents the return type of
-an operation that may potentially fail. It contains either the expected result
-of type `T`, or a reason for the failure, of one of the error types in `E...`.
-Internally, this is stored as `variant<T, E...>`.
-
-See [its documentation](doc/expected.md) for more information.
-
-Note that, while `variant` is production quality, `expected` is still a work
-in progress and has no test suite yet.
+Tested on [Github Actions](https://github.com/boostorg/variant2/actions) and
+[Appveyor](https://ci.appveyor.com/project/pdimov/variant2-fkab9).
--- a/appveyor.yml
+++ b/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
--- a/doc/.gitignore
+++ b/doc/.gitignore
@ -1 +1,2 @@
 /pdf/
+/html/
--- a/doc/expected.md
+++ b/doc/expected.md
@ -1,305 +0,0 @@
-# expected<T, E...>
-
-## Description
-
-The class `expected<T, E...>` presented here is an extended version of `expected<T, E>` as
-proposed in [P0323R1](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0323r1.pdf)
-and the subsequent [D0323R2](https://github.com/viboes/std-make/blob/master/doc/proposal/expected/d0323r2.md).
-
-The main difference is that this class takes more than one error type, which makes it more
-flexible. One example of a type of the `expected` family, [`outcome<T>`](https://ned14.github.io/boost.outcome/),
-on failure can store either an error of type `std::error_code`, or an exception in the form of `std::exception_ptr`.
-This can be represented naturally in this implementation via `expected<T, std::error_code, std::exception_ptr>`.
-
-In addition, libraries would generally differ in their choice of error types. It would be a
-common need in practice of having to combine the results of calling two different libraries,
-each with its own error type. Library 1 may use `lib1::error`:
-
-    namespace lib1
-    {
-
-    enum class error
-    {
-        division_by_zero,
-        other_error
-    };
-
-    expected<double, error> div( double x, double y );
-
-    } // namespace lib1
-
-while Library 2 might define its own `lib2::error`:
-
-    namespace lib2
-    {
-
-    enum class error
-    {
-        division_by_zero,
-        negative_logarithm
-    };
-
-    expected<double, error> log( double x );
-
-    } // namespace lib2
-
-In this proposal, combining the results of `lib1::div` and `lib2::log` can be achieved via
-simple composition:
-
-    expected<double, lib1::error, lib2::error> log_div_mul( double x, double y, double m )
-    {
-        auto r1 = lib1::div( x, y );
-        if( !r1 ) return r1.unexpected();
-
-        auto r2 = lib2::log( r1.value() );
-        if( !r2 ) return r2.unexpected();
-
-        return m * r2.value();
-    }
-
-An alternative approach that requires more effort is also supported:
-
-    enum class common_error
-    {
-        division_by_zero,
-        negative_logarithm,
-        other_error,
-        unknown_error
-    };
-
-    common_error make_common_error( lib1::error e );
-    common_error make_common_error( lib2::error e );
-
-    expected<double, common_error> log_div_mul2( double x, double y, double m )
-    {
-        static const auto rm = []( auto x ) { return make_common_error(x); };
-
-        auto r1 = lib1::div( x, y ).remap_errors( rm );
-        if( !r1 ) return r1.unexpected();
-
-        auto r2 = lib2::log( r1.value() ).remap_errors( rm );
-        if( !r2 ) return r2.unexpected();
-
-        return m * r2.value();
-    }
-
-`std::error_code` is a very good choice for a common error type, and it's supported
-natively by the overload of `.remap_errors()` that takes no arguments, which uses
-calls to `make_error_code` to translate the errors.
-
-When an attempt to access the value via `r.value()` is made and an error is present,
-an exception is thrown. By default, this exception is of type `bad_expected_access<E>`,
-as in D0323R2, but there are two differences. First, `bad_expected_access<E>` objects
-derive from a common base `bad_expected_access<void>` so that they can be caught at
-points where the set of possible `E` is unknown.
-
-Second, the thrown exception can be customized. The implementation calls
-`throw_on_unexpected(e)` unqualified, where `e` is the error object, and the user can
-define such a function in the namespace of the type of `e`. Two specialized overloads
-of `throw_on_unexpected` are provided, one for `std::error_code`, which throws the
-corresponding `std::system_error`, and one for `std::exception_ptr`, which rethrows
-the exception stored in it.
-
-For example, `lib1` from above may customize the exceptions associated with `lib1::error`
-via the following:
-
-    namespace lib1
-    {
-
-    enum class error
-    {
-        division_by_zero,
-        other_error
-    };
-
-    class exception: public std::exception
-    {
-    private:
-
-        error e_;
-
-    public:
-
-        explicit exception( error e ): e_( e ) {}
-        virtual const char * what() const noexcept;
-    };
-
-    void throw_on_unexpected( error e )
-    {
-        throw exception( e );
-    }
-
-    } // namespace lib1
-
-In this implementation, `unexpected_type<E...>` has been called `unexpected_<T...>` and is
-an alias for `variant<T...>`. It is unfortunately not possible to use the name `unexpected<T...>`,
-because a function `std::unexpected` already exists.
-
-The `make_...` helper functions have been omitted as unnecessary; class template argument deduction
-as in `expected{ 1.0 }` or `unexpected_{ lib1::division_by_zero }` suffices.
-
-Other functions have also been dropped as unnecessary, not providing sufficient value, dangerous, or
-a combination of the three, although the decision of what to include isn't final at this point. The aim
-is to produce a minimal interface that still covers the use cases.
-
-`expected<T, E1...>` can be converted to `expected<T, E2...>` if all error types in `E1...` are
-also in `E2...`. This allows composition as in the example above. Whether value convertibility ought
-to also be supported is an open question.
-
-A single monadic operation ("bind") is supported in the form of `operator>>`, allowing
-
-    auto log_div_mul3( double x, double y, double m )
-    {
-        return lib1::div( x, y ) >> [&]( auto && r1 ) {
-
-            return lib2::log( r1 ) >> [&]( auto && r2 ) -> expected<double, lib1::error, lib2::error> {
-
-                return m * r2;
-
-            };
-        };
-    }
-
-as well as the more concise in this example, although limited in utility for real world scenarios,
-
-    auto log_div_mul3( double x, double y, double m )
-    {
-        return lib1::div( x, y ) >> std::bind<expected<double, lib1::error, lib2::error>>( lib2::log, _1 ) >> m * _1;
-    }
-
-The more traditional name `then` was also a candidate for this operation, but `operator>>` has two advantages;
-it avoids the inevitable naming debates and does not require parentheses around the continuation lambda.
-
-## Synopsis
-
-    // unexpected_
-
-    template<class... E> using unexpected_ = variant<E...>;
-
-    // bad_expected_access
-
-    template<class E = void> class bad_expected_access;
-
-    template<> class bad_expected_access<void>: public std::exception
-    {
-    public:
-
-        bad_expected_access() noexcept;
-        char const * what() const noexcept;
-    };
-
-    template<class E> class bad_expected_access: public bad_expected_access<void>
-    {
-    public:
-
-        explicit bad_expected_access( E const& e );
-        E error() const;
-    };
-
-    // throw_on_unexpected
-
-    template<class E> void throw_on_unexpected( E const& e );
-    void throw_on_unexpected( std::error_code const& e );
-    void throw_on_unexpected( std::exception_ptr const& e );
-
-    // expected
-
-    template<class T, class... E> class expected
-    {
-    public:
-
-        // value constructors
-
-        constexpr expected() noexcept( /*see below*/ );
-
-        constexpr expected( T const& t ) noexcept( /*see below*/ );
-        constexpr expected( T&& t ) noexcept( /*see below*/ );
-
-        // unexpected constructor
-
-        template<class... E2>
-        constexpr expected( unexpected_<E2...> const& x );
-
-        template<class... E2>
-        constexpr expected( unexpected_<E2...>&& x );
-
-        // conversion constructor
-
-        template<class... E2>
-        constexpr expected( expected<T, E2...> const& x );
-
-        template<class... E2>
-        constexpr expected( expected<T, E2...>&& x );
-
-        // emplace
-
-        template<class... A> void emplace( A&&... a );
-        template<class V, class... A> void emplace( std::initializer_list<V> il, A&&... a );
-
-        // swap
-
-        void swap( expected& r ) noexcept( /*see below*/ );
-
-        // value queries
-
-        constexpr bool has_value() const noexcept;
-        constexpr explicit operator bool() const noexcept;
-
-        // checked value access
-
-        constexpr T& value() &;
-        constexpr T const& value() const&;
-        constexpr T&& value() &&;
-        constexpr T const&& value() const&&;
-
-        // unchecked value access
-
-        T* operator->() noexcept;
-        T const* operator->() const noexcept;
-
-        T& operator*() & noexcept;
-        T const& operator*() const & noexcept;
-        T&& operator*() && noexcept;
-        T const&& operator*() const && noexcept;
-
-        // error queries
-
-        template<class E2> constexpr bool has_error() const noexcept;
-        constexpr bool has_error() const noexcept;
-
-        // error access
-
-        unexpected_<E...> unexpected() const;
-
-        template<class E2> constexpr E2 error() const noexcept;
-        constexpr /*see below*/ error() const noexcept;
-
-        // error mapping
-
-        template<class F> /*see below*/ remap_errors( F&& f ) const;
-        expected<T, std::error_code> remap_errors() const;
-
-        // then
-
-        template<class F> /*see below*/ operator>>( F&& f ) const;
-    };
-
-    template<class T, class... E>
-    inline constexpr bool operator==( expected<T, E...> const& x1, expected<T, E...> const& x2 );
-    
-    template<class T, class... E>
-    inline constexpr bool operator!=( expected<T, E...> const& x1, expected<T, E...> const& x2 );
-
-    template<class T, class... E>
-    inline void swap( expected<T, E...>& x1, expected<T, E...>& x2 ) noexcept( /*see below*/ );
-
-    // is_expected
-
-    template<class T> struct is_expected;
-
-    } // namespace variant2
-    } // namespace boost
-
-## Reference
-
-...
--- a/doc/html/variant2.html
+++ b/doc/html/variant2.html
--- a/doc/variant2-docinfo-footer.html
+++ b/doc/variant2-docinfo-footer.html
@ -1,5 +1,6 @@
 <style>

 *:not(pre)>code { background: none; color: #600000; }
+:not(pre):not([class^=L])>code { background: none; color: #600000; }

 </style>
--- a/doc/variant2.adoc
+++ b/doc/variant2.adoc
@ -13,10 +13,15 @@ Peter Dimov
 :toclevels: 4
 :idprefix:
 :docinfo: private-footer
+:source-highlighter: rouge
+:source-language: c++

 :leveloffset: +1

 include::variant2/overview.adoc[]
+include::variant2/changelog.adoc[]
+include::variant2/design.adoc[]
+include::variant2/implementation.adoc[]
 include::variant2/reference.adoc[]
 include::variant2/copyright.adoc[]

--- a/doc/variant2/changelog.adoc
+++ b/doc/variant2/changelog.adoc
@ -0,0 +1,43 @@
+////
+Copyright 2019-2021 Peter Dimov
+Distributed under the Boost Software License, Version 1.0.
+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
+changed to provide the strong exception safety guarantee,
+instead of basic. `expected` has been removed.
--- a/doc/variant2/design.adoc
+++ b/doc/variant2/design.adoc
@ -0,0 +1,189 @@
+////
+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
+////
+
+[#design]
+# Design
+:idprefix: design_
+
+## Features
+
+This `variant` implementation has two distinguishing features:
+
+* It's never "valueless", that is, `variant<T1, T2, ..., Tn>` has an
+  invariant that it always contains a valid value of one of the types
+  `T1`, `T2`, ..., `Tn`.
+* It provides the strong exception safety guarantee on assignment and
+  `emplace`.
+
+This is achieved with the use of double storage, unless all of the
+contained types have a non-throwing move constructor.
+
+## Rationale
+
+### Never Valueless
+
+It makes intuitive sense that `variant<X, Y, Z>` can hold only values
+of type `X`, type `Y`, or type `Z`, and nothing else.
+
+If we think of `variant` as an extension of `union`, since a `union`
+has a state called "no active member", an argument can be made that a
+`variant<X, Y, Z>` should also have such an additional state, holding
+none of `X`, `Y`, `Z`.
+
+This however makes `variant` less convenient in practice and less useful
+as a building block. If we really need a variable that only holds `X`,
+`Y`, or `Z`, the additional empty state creates complications that need
+to be worked around. And in the case where we do need this additional
+empty state, we can just use `variant<empty, X, Y, Z>`, with a suitable
+`struct empty {};`.
+
+From a pure design perspective, the case for no additional empty state is
+solid. Implementation considerations, however, argue otherwise.
+
+When we replace the current value of the `variant` (of, say, type `X`) with
+another (of type `Y`), since the new value needs to occupy the same storage
+as the old one, we need to destroy the old `X` first, then construct a new
+`Y` in its place. But since this is {cpp}, the construction can fail with an
+exception. At this point the `variant` is in the "has no active member"
+state that we've agreed it cannot be in.
+
+This is a legitimate problem, and it is this problem that makes having
+an empty/valueless state so appealing. We just leave the `variant` empty on
+exception and we're done.
+
+As explained, though, this is undesirable from a design perspective as it
+makes the component less useful and less elegant.
+
+There are several ways around the issue. The most straightforward one is to
+just disallow types whose construction can throw. Since we can always create
+a temporary value first, then use the move constructor to initialize the one
+in the `variant`, it's enough to require a nonthrowing move constructor,
+rather than all constructors to be nonthrowing.
+
+Unfortunately, under at least one popular standard library implementation,
+node based containers such as `std::list` and `std::map` have a potentially
+throwing move constructor. Disallowing `variant<X, std::map<Y, Z>>` is hardly
+practical, so the exceptional case cannot be avoided.
+
+On exception, we could also construct some other value, leaving the `variant`
+valid; but in the general case, that construction can also throw. If one of
+the types has a nonthrowing default constructor, we can use it; but if not,
+we can't.
+
+The approach Boost.Variant takes here is to allocate a temporary copy of
+the value on the heap. On exception, a pointer to that temporary copy can be
+stored into the `variant`. Pointer operations don't throw.
+
+Another option is to use double buffering. If our `variant` occupies twice
+the storage, we can construct the new value in the unused half, then, once
+the construction succeeds, destroy the old value in the other half.
+
+When `std::variant` was standardized, none of those approaches was deemed
+palatable, as all of them either introduce overhead or are too restrictive
+with respect to the types a `variant` can contain. So as a compromise,
+`std::variant` took a way that can (noncharitably) be described as "having
+your cake and eating it too."
+
+Since the described exceptional situation is relatively rare, `std::variant`
+has a special case, called "valueless", into which it goes on exception,
+but the interface acknowledges its existence as little as possible, allowing
+users to pretend that it doesn't exist.
+
+This is, arguably, not that bad from a practical point of view, but it leaves
+many of us wanting. Rare states that "never" occur are undertested and when
+that "never" actually happens, it's usually in the most inconvenient of times.
+
+This implementation does not follow `std::variant`; it statically guarantees
+that `variant` is never in a valueless state. The function
+`valueless_by_exception` is provided for compatibility, but it always returns
+`false`.
+
+Instead, if the contained types are such that it's not possible to avoid an
+exceptional situation when changing the contained value, double storage is
+used.
+
+### Strong Exception Safety
+
+The initial submission only provided the basic exception safety guarantee.
+If an attempt to change the contained value (via assignment or `emplace`)
+failed with an exception, and a type with a nonthrowing default constructor
+existed among the alternatives, a value of that type was created into the
+`variant`. The upside of this decision was that double storage was needed
+less frequently.
+
+The reviewers were fairly united in hating it. Constructing a random type
+was deemed too unpredictable and not complying with the spirit of the
+basic guarantee. The default constructor of the chosen type, even if
+nonthrowing, may still have undesirable side effects. Or, if not that, a
+value of that type may have special significance for the surrounding code.
+Therefore, some argued, the `variant` should either remain with its
+old value, or transition into the new one, without synthesizing other
+states.
+
+At the other side of the spectrum, there were those who considered double
+storage unacceptable. But they considered it unacceptable in principle,
+regardless of the frequency with which it was used.
+
+As a result, providing the strong exception safety guarantee on assignment
+and `emplace` was declared an acceptance condition.
+
+In retrospect, this was the right decision. The reason the strong guarantee
+is generally not provided is because it doesn't compose. When `X` and `Y`
+provide the basic guarantee on assignment, so does `struct { X x; Y y; };`.
+Similarly, when `X` and `Y` have nonthrowing assignments, so does the
+`struct`. But this doesn't hold for the strong guarantee.
+
+The usual practice is to provide the basic guarantee on assignment and
+let the user synthesize a "strong" assignment out of either a nonthrowing
+`swap` or a nonthrowing move assignment. That is, given `x1` and `x2` of
+type `X`, instead of the "basic" `x1 = x2;`, use either `X(x2).swap(x1);`
+or `x1 = X(x2);`.
+
+Nearly all types provide a nonthrowing `swap` or a nonthrowing move
+assignment, so this works well. Nearly all, except `variant`, which in the
+general case has neither a nonthrowing `swap` nor a nonthrowing move
+assignment. If `variant` does not provide the strong guarantee itself, it's
+impossible for the user to synthesize it.
+
+So it should, and so it does.
+
+## Differences with std::variant
+
+The main differences between this implementation and `std::variant` are:
+
+* No valueless-by-exception state: `valueless_by_exception()` always
+  returns `false`.
+* Strong exception safety guarantee on assignment and `emplace`.
+* `emplace` first constructs the new value and then destroys the old one;
+  in the single storage case, this translates to constructing a temporary
+  and then moving it into place.
+* A converting constructor from, e.g. `variant<int, float>` to 
+  `variant<float, double, int>` is provided as an extension.
+* The reverse operation, going from `variant<float, double, int>` to
+  `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.)
+* `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.
+
+## Differences with Boost.Variant
+
+This library is API compatible with `std::variant`. As such, its interface
+is different from Boost.Variant's. For example, visitation is performed via
+`visit` instead of `apply_visitor`.
+
+Recursive variants are not supported.
+
+Double storage is used instead of temporary heap backup. This `variant` is
+always "stack-based", it never allocates, and never throws `bad_alloc` on
+its own.
--- a/doc/variant2/implementation.adoc
+++ b/doc/variant2/implementation.adoc
@ -0,0 +1,22 @@
+////
+Copyright 2019 Peter Dimov
+Distributed under the Boost Software License, Version 1.0.
+https://www.boost.org/LICENSE_1_0.txt
+////
+
+[#implementation]
+# Implementation
+:idprefix: implementation_
+
+## Dependencies
+
+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 3.9 or later with `-std=c++11` or above
+* Visual Studio 2015, 2017, 2019
+
+Tested on https://github.com/boostorg/variant2/actions[Github Actions] and
+https://ci.appveyor.com/project/pdimov/variant2-fkab9[Appveyor].
--- a/doc/variant2/overview.adoc
+++ b/doc/variant2/overview.adoc
@ -1,5 +1,5 @@
 ////
-Copyright 2018 Peter Dimov
+Copyright 2018, 2019 Peter Dimov

 Distributed under the Boost Software License, Version 1.0.

@ -9,32 +9,327 @@ http://www.boost.org/LICENSE_1_0.txt

 [#overview]
 # Overview
-:idprefix:
+:idprefix: overview_

-This library implements a type-safe discriminated union (variant) type,
-`variant<T...>`, that almost conforms to the {cpp}17 Standard's
-http://en.cppreference.com/w/cpp/utility/variant[`std::variant<T...>`]. The
-main differences between the two are:
+## Description

-* `variant<T...>` does not have a valueless-by-exception state;
-* A converting constructor from, e.g. `variant<int, float>` to 
-  `variant<float, double, int>` is provided as an extension;
-* The reverse operation, going from `variant<float, double, int>` to
-  `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 trivial when all contained types are trivial, as
-   mandated by {cpp}17.
+This library implements a type-safe discriminated/tagged union type,
+`variant<T...>`, that is API-compatible with the {cpp}17 Standard's
+http://en.cppreference.com/w/cpp/utility/variant[`std::variant<T...>`].

-To avoid the valueless-by-exception state, this implementation falls
-back to using double storage unless
+A `variant<T1, T2, ..., Tn>` variable can hold a value of any of the
+types `T1`, `T2`, ..., `Tn`. For example,
+`variant<int64_t, double, std::string>` can hold an `int64_t` value, a
+`double` value, or a `string` value.

-* one of the alternatives is the type `monostate`,
-* one of the alternatives has a nonthrowing default constructor, or
-* all the contained types are nothrow move constructible.
+Such a type is sometimes called a "tagged union", because it's roughly
+equivalent to

-If the first two bullets don't hold, but the third does, the variant uses
-single storage, but `emplace` constructs a temporary and moves it into place
-if the construction of the object can throw. In case this is undesirable, one
-can force `emplace` into always constructing in-place by adding `monostate` as
-one of the alternatives.
+```
+struct V
+{
+    enum tag { tag_int64_t, tag_double, tag_string };
+
+    tag tag_;
+
+    union
+    {
+        int64_t     i_;
+        double      d_;
+        std::string s_;
+    };
+};
+```
+
+## Usage Examples
+
+Variants can be used to represent dynamically-typed values. A configuration
+file of the form
+
+```
+server.host=test.example.com
+server.port=9174
+cache.max_load=0.7
+```
+
+can be represented as `std::map<std::string, variant<int64_t, double,
+std::string>>`.
+
+Variants can also represent polymorphism. To take a classic example, a
+polymorphic collection of shapes:
+
+```
+#define _USE_MATH_DEFINES
+#include <iostream>
+#include <vector>
+#include <memory>
+#include <cmath>
+
+class Shape
+{
+public:
+
+    virtual ~Shape() = default;
+    virtual double area() const = 0;
+};
+
+class Rectangle: public Shape
+{
+private:
+
+    double width_, height_;
+
+public:
+
+    Rectangle( double width, double height ):
+        width_( width ), height_( height ) {}
+
+    virtual double area() const { return width_ * height_; }
+};
+
+class Circle: public Shape
+{
+private:
+
+    double radius_;
+
+public:
+
+    explicit Circle( double radius ): radius_( radius ) {}
+    virtual double area() const { return M_PI * radius_ * radius_; }
+};
+
+double total_area( std::vector<std::unique_ptr<Shape>> const & v )
+{
+    double s = 0.0;
+
+    for( auto const& p: v )
+    {
+        s += p->area();
+    }
+
+    return s;
+}
+
+int main()
+{
+    std::vector<std::unique_ptr<Shape>> v;
+
+    v.push_back( std::unique_ptr<Shape>( new Circle( 1.0 ) ) );
+    v.push_back( std::unique_ptr<Shape>( new Rectangle( 2.0, 3.0 ) ) );
+
+    std::cout << "Total area: " << total_area( v ) << std::endl;
+}
+```
+
+can instead be represented as a collection of `variant<Rectangle, Circle>`
+values. This requires the possible `Shape` types be known in advance, as is
+often the case. In return, we no longer need virtual functions, or to allocate
+the values on the heap with `new Rectangle` and `new Circle`:
+
+```
+#define _USE_MATH_DEFINES
+#include <iostream>
+#include <vector>
+#include <cmath>
+
+#include <boost/variant2/variant.hpp>
+using namespace boost::variant2;
+
+struct Rectangle
+{
+    double width_, height_;
+    double area() const { return width_ * height_; }
+};
+
+struct Circle
+{
+    double radius_;
+    double area() const { return M_PI * radius_ * radius_; }
+};
+
+double total_area( std::vector<variant<Rectangle, Circle>> const & v )
+{
+    double s = 0.0;
+
+    for( auto const& x: v )
+    {
+        s += visit( []( auto const& y ){ return y.area(); }, x );
+    }
+
+    return s;
+}
+
+int main()
+{
+    std::vector<variant<Rectangle, Circle>> v;
+
+    v.push_back( Circle{ 1.0 } );
+    v.push_back( Rectangle{ 2.0, 3.0 } );
+
+    std::cout << "Total area: " << total_area( v ) << std::endl;
+}
+```
+
+## Construction and Assignment
+
+If we look at the
+
+```
+    v.push_back( Circle{ 1.0 } );
+```
+
+line, we can deduce that `variant<Rectangle, Circle>` can be (implicitly)
+constructed from `Circle` (and `Rectangle`), and indeed it can. It can also
+be assigned a `Circle` or a `Rectangle`:
+
+```
+variant<Rectangle, Circle> v = Circle{ 1.0 }; // v holds Circle
+v = Rectangle{ 2.0, 3.0 };                    // v now holds Rectangle
+```
+
+If we try to construct `variant<int, float>` from something that is neither
+`int` nor `float`, say, `(short)1`, the behavior is "as if" the `variant` has
+declared two constructors,
+
+```
+variant::variant(int x);
+variant::variant(float x);
+```
+
+and the standard overload resolution rules are used to pick the one that will
+be used. So `variant<int, float>((short)1)` will hold an `int`.
+
+## Inspecting the Value
+
+Putting values into a `variant` is easy, but taking them out is necessarily a
+bit more convoluted. It's not possible for `variant<int, float>` to define a
+member function `get() const`, because such a function will need its return
+type fixed at compile time, and whether the correct return type is `int` or
+`float` will only become known at run time.
+
+There are a few ways around that. First, there is the accessor member function
+
+```
+std::size_t variant::index() const noexcept;
+```
+
+that returns the zero-based index of the current type. For `variant<int,
+float>`, it will return `0` for `int` and `1` for `float`.
+
+Once we have the index, we can use the free function `get<N>` to obtain the
+value. Since we're passing the type index to `get`, it knows what to return.
+`get<0>(v)` will return `int`, and `get<1>(v)` will return `float`:
+
+```
+void f( variant<int, float> const& v )
+{
+    switch( v.index() )
+    {
+    case 0:
+
+        // use get<0>(v)
+        break;
+
+    case 1:
+
+        // use get<1>(v)
+        break;
+
+    default:
+
+        assert(false); // never happens
+    }
+}
+```
+
+If we call `get<0>(v)`, and `v.index()` is not currently `0`, an exception
+(of type `bad_variant_access`) will be thrown.
+
+An alternative approach is to use `get<int>(v)` or `get<float>(v)`. This
+works similarly.
+
+Another alternative that avoids the possibility of `bad_variant_access` is
+to use `get_if`. Instead of a reference to the contained value, it returns
+a pointer to it, returning `nullptr` to indicate type mismatch. `get_if`
+takes a pointer to the `variant`, so in our example we'll use something along
+the following lines:
+
+```
+void f( variant<int, float> const& v )
+{
+    if( int const * p = get_if<int>(&v) )
+    {
+        // use *p
+    }
+    else if( float const * p = get_if<float>(&v) )
+    {
+        // use *p
+    }
+    else
+    {
+        assert(false); // never happens
+    }
+}
+```
+
+## Visitation
+
+Last but not least, there's `visit`. `visit(f, v)` calls the a function object
+`f` with the value contained in the `variant` `v` and returns the result. When
+`v` is `variant<int, float>`, it will call `f` with either an `int` or a
+`float`. The function object must be prepared to accept both.
+
+In practice, this can be achieved by having the function take a type that can
+be passed either `int` or `float`, such as `double`:
+
+```
+double f( double x ) { return x; }
+
+double g( variant<int, float> const& v )
+{
+    return visit( f, v );
+}
+```
+
+By using a function object with an overloaded `operator()`:
+
+```
+struct F
+{
+    void operator()(int x) const { /* use x */ }
+    void operator()(float x) const { /* use x */ }
+};
+
+void g( variant<int, float> const& v )
+{
+    visit( F(), v );
+}
+```
+
+Or by using a polymorphic lambda, as we did in our `Circle`/`Rectangle`
+example:
+
+```
+void g( variant<int, float> const& v )
+{
+    visit( [&]( auto const& x ){ std::cout << x << std::endl; }, v );
+}
+```
+
+`visit` can also take more than one `variant`. `visit(f, v1, v2)` calls
+`f(x1, x2)`, where `x1` is the value contained in `v1` and `x2` is the value
+in `v2`.
+
+## Default Construction
+
+The default constructor of `variant` value-initializes the first type in
+the list. `variant<int, float>{}` holds `0` (of type `int`), and
+`variant<float, int>{}` holds `0.0f`.
+
+This is usually the desired behavior. However, in cases such as
+`variant<std::mutex, std::recursive_mutex>`, one might legitimately wish to
+avoid constructing a `std::mutex` by default. A provided type, `monostate`,
+can be used as the first type in those scenarios. `variant<monostate,
+std::mutex, std::recursive_mutex>` will default-construct a `monostate`,
+which is basically a no-op, as `monostate` is effectively an empty `struct`.
--- a/doc/variant2/reference.adoc
+++ b/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

@ -247,6 +274,9 @@ public:
 } // namespace boost
 ```

+In the descriptions that follow, let `i` be in the range `[0, sizeof...(T))`,
+and `Ti` be the `i`-th type in `T...`.
+
 #### Constructors

 ```
@ -383,24 +413,21 @@ Remarks: :: This function does not participate in overload resolution unless
 [none]
 * {blank}
 +
-Effects: :: Destroys the currently contained value.
+Effects: ::
+  Destroys the currently contained value.

 #### Assignment

 ```
 constexpr variant& operator=( const variant& r )
-  noexcept( mp_all<std::is_nothrow_copy_constructible<T>...,
-    std::is_nothrow_copy_assignable<T>...>::value );
+  noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );
 ```
 [none]
 * {blank}
 +
 Let `j` be `r.index()`.

-Effects: ::
- If `index() == j`, assigns the value contained in `r` to the value
-  contained in `*this`.
- Otherwise, equivalent to `emplace<j>(get<j>(r))`.
+Effects: :: `emplace<j>(get<j>(r))`.
 Returns: :: `*this`.
 Ensures: :: `index() == r.index()`.
 Remarks: :: This operator does not participate in overload resolution unless
@ -409,18 +436,14 @@ Remarks: :: This operator does not participate in overload resolution unless

 ```
 constexpr variant& operator=( variant&& r )
-  noexcept( mp_all<std::is_nothrow_move_constructible<T>...,
-    std::is_nothrow_move_assignable<T>...>::value );
+  noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );
 ```
 [none]
 * {blank}
 +
 Let `j` be `r.index()`.

-Effects: ::
- If `index() == j`, assigns the value contained in `std::move(r)` to the
-  value contained in `*this`.
- Otherwise, equivalent to `emplace<j>(get<j>(std::move(r)))`.
+Effects: :: `emplace<j>(get<j>(std::move(r)))`.
 Returns: :: `*this`.
 Ensures: :: `index() == r.index()`.
 Remarks: :: This operator does not participate in overload resolution unless
@ -439,18 +462,14 @@ Let `Tj` be a type that is determined as follows: build an imaginary function
 overload resolution for the expression `FUN(std::forward<U>(u))` defines the
 alternative `Tj` which is the type of the contained value after construction.

-Effects: ::
- If `index() == j`, assigns `std::forward<U>(u)` to the value contained in
-  `*this`.
- Otherwise, equivalent to `emplace<j>(std::forward<U>(u))`.
+Effects: :: `emplace<j>(std::forward<U>(u))`.
 Returns: :: `*this`.
 Ensures: :: `index() == j`.
 Remarks: ::
-  The expression inside `noexcept` is `std::is_nothrow_constructible_v<Tj, U>
-    && std::is_nothrow_assignable_v<Tj&, U>`.
+  The expression inside `noexcept` is `std::is_nothrow_constructible_v<Tj, U&&>`.
  This operator does not participate in overload resolution unless
  - `std::is_same_v<std::remove_cvref_t<T>, variant>` is `false`,
-  - `std::is_constructible_v<Tj, U> && std::is_assignable_v<Tj&, U>` is
+  - `std::is_constructible_v<Tj, U&&> && std::is_assignable_v<Tj&, U&&>` is
    `true`,  and
  - the expression `FUN(std::forward<U>(u))` (with `FUN` being the
    above-mentioned set of imaginary functions) is well-formed.
@ -469,7 +488,7 @@ Let `I` be the zero-based index of `U` in `T...`.
 Effects: :: Equivalent to: `return emplace<I>(std::forward<A>(a)...);`
 Remarks: ::
  This function shall not participate in overload resolution unless
-  `std::is_constructible_v<U, A...>` is `true` and `U` occurs exactly once
+  `std::is_constructible_v<U, A&&...>` is `true` and `U` occurs exactly once
  in `T...`.

 ```
@ -484,7 +503,7 @@ Let `I` be the zero-based index of `U` in `T...`.
 Effects: :: Equivalent to: `return emplace<I>(il, std::forward<A>(a)...);`
 Remarks: ::
  This function shall not participate in overload resolution unless
-  `std::is_constructible_v<U, std::initializer_list<V>&, A...>` is `true`
+  `std::is_constructible_v<U, std::initializer_list<V>&, A&&...>` is `true`
  and `U` occurs exactly once in `T...`.


@ -498,13 +517,16 @@ 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:   ::
+  Nothing unless the initialization of the new contained value throws.
+Exception Safety: :: Strong. On exception, the contained value is unchanged.
 Remarks:  ::
  This function shall not participate in overload resolution unless
-  `std::is_constructible_v<Ti, A...>` is `true`.
+  `std::is_constructible_v<Ti, A&&...>` is `true`.

 ```
 template<size_t I, class V, class... A>
@ -516,13 +538,16 @@ 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:   ::
+  Nothing unless the initialization of the new contained value throws.
+Exception Safety: :: Strong. On exception, the contained value is unchanged.
 Remarks:  ::
  This function shall not participate in overload resolution unless
-  `std::is_constructible_v<Ti, std::initializer_list<V>&, A...>` is `true`.
+  `std::is_constructible_v<Ti, std::initializer_list<V>&, A&&...>` is `true`.

 #### Value Status

@ -534,13 +559,16 @@ constexpr bool valueless_by_exception() const noexcept;
 +
 Returns:  :: `false`.

+NOTE: This function is provided purely for compatibility with `std::variant`.
+
 ```
 constexpr size_t index() const noexcept;
 ```
 [none]
 * {blank}
 +
-Returns:  :: The zero-based index of the active alternative.
+Returns:  ::
+  The zero-based index of the active alternative.

 #### Swap

@ -709,6 +737,8 @@ template<size_t I, class... T>
 +
 Effects: :: If `v.index()` is `I`, returns a reference to the object stored in
  the variant. Otherwise, throws `bad_variant_access`.
+Remarks: :: These functions do not participate in overload resolution
+  unless `I` < `sizeof...(T)`.

 ```
 template<class U, class... T>
@ -749,9 +779,10 @@ template<size_t I, class... T>
 [none]
 * {blank}
 +
-Requires: :: `I < sizeof...(U)`. Otherwise, the program is ill-formed.
 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)`.

 ```
 template<class U, class... T>
@ -771,6 +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

 ```
@ -780,7 +839,7 @@ template<class... T>
 [none]
 * {blank}
 +
-Returns: :: `v.index() == w.index && get<I>(v) == get<I>(w)`, where `I`
+Returns: :: `v.index() == w.index() && get<I>(v) == get<I>(w)`, where `I`
  is `v.index()`.

 ```
@ -799,7 +858,7 @@ template<class... T>
 [none]
 * {blank}
 +
-Returns: :: `v.index() < w.index || (v.index() == w.index && get<I>(v) < get<I>(w))`,
+Returns: :: `v.index() < w.index() || (v.index() == w.index() && get<I>(v) < get<I>(w))`,
  where `I` is `v.index()`.

 ```
@ -818,7 +877,7 @@ template<class... T>
 [none]
 * {blank}
 +
-Returns: :: `v.index() < w.index || (v.index() == w.index && get<I>(v) \<= get<I>(w))`,
+Returns: :: `v.index() < w.index() || (v.index() == w.index() && get<I>(v) \<= get<I>(w))`,
  where `I` is `v.index()`.

 ```
@ -828,19 +887,8 @@ template<class... T>
 [none]
 * {blank}
 +
-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()...`.
+Returns: ::
+  `w \<= v`.

 ### swap

@ -851,7 +899,69 @@ template<class... T>
 [none]
 * {blank}
 +
-Effects: :: Equivalent to `v.swap(w)`.
+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

@ -868,3 +978,7 @@ public:
    }
 };
 ```
+
+## <boost/variant2.hpp>
+
+This convenience header includes `<boost/variant2/variant.hpp>`.
--- a/include/boost/variant2.hpp
+++ b/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
--- a/include/boost/variant2/expected.hpp
+++ b/include/boost/variant2/expected.hpp
@ -1,446 +0,0 @@
-#ifndef BOOST_VARIANT2_EXPECTED_HPP_INCLUDED
-#define BOOST_VARIANT2_EXPECTED_HPP_INCLUDED
-
-//  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
-
-#ifndef BOOST_VARIANT2_VARIANT_HPP_INCLUDED
-#include <boost/variant2/variant.hpp>
-#endif
-#include <boost/core/demangle.hpp>
-#include <system_error>
-#include <type_traits>
-#include <typeinfo>
-#include <cassert>
-
-//
-
-namespace boost
-{
-namespace variant2
-{
-
-// unexpected_
-
-template<class... E> using unexpected_ = variant<E...>;
-
-// bad_expected_access
-
-template<class E = void> class bad_expected_access;
-
-template<> class bad_expected_access<void>: public std::exception
-{
-private:
-
-    std::string msg_;
-
-public:
-
-    bad_expected_access() noexcept
-    {
-    }
-
-    explicit bad_expected_access( std::string&& msg ) noexcept: msg_( std::move(msg) ) // extension
-    {
-    }
-
-    char const * what() const noexcept
-    {
-        return msg_.empty()? "bad_expected_access<>": msg_.c_str();
-    }
-};
-
-namespace detail
-{
-
-template<class E, class En = std::enable_if_t<!std::is_enum<E>::value>> std::string add_value( E const& /*e*/ )
-{
-    return std::string();
-}
-
-template<class E, class E1 = void, class E2 = std::enable_if_t<std::is_enum<E>::value>> std::string add_value( E const& e )
-{
-    return ": " + std::to_string( static_cast<int>(e) );
-}
-
-} // namespace detail
-
-template<class E> class bad_expected_access: public bad_expected_access<void>
-{
-private:
-
-    E e_;
-
-public:
-
-    explicit bad_expected_access( E const& e )
-        : bad_expected_access<void>( "bad_expected_access<" + boost::core::demangle( typeid(E).name() ) + ">" + variant2::detail::add_value( e ) ), e_( e )
-    {
-    }
-
-    E error() const
-    {
-        return e_;
-    }
-};
-
-// throw_on_unexpected
-
-template<class E> void throw_on_unexpected( E const& /*e*/ )
-{
-}
-
-void throw_on_unexpected( std::error_code const & e )
-{
-    throw std::system_error( e );
-}
-
-void throw_on_unexpected( std::exception_ptr const & e )
-{
-    if( e )
-    {
-        std::rethrow_exception( e );
-    }
-    else
-    {
-        throw bad_expected_access<>( "bad_expected_access<>: null exception_ptr" );
-    }
-}
-
-// expected
-
-template<class T, class... E> class expected;
-
-template<class T> struct is_expected: std::false_type {};
-template<class T, class... E> struct is_expected<expected<T, E...>>: std::true_type {};
-
-template<class T, class... E> class expected
-{
-private:
-
-    variant<T, E...> v_;
-
-private:
-
-    void _bad_access() const
-    {
-        mp_with_index<mp_size<expected>>( v_.index(), [&]( auto I )
-        {
-            if( I == 0 )
-            {
-                throw bad_expected_access<>( "bad_expected_access<>: value present on error request" );
-            }
-            else
-            {
-                auto const & e = get<I>(v_);
-
-                throw_on_unexpected( e );
-                throw bad_expected_access<std::decay_t<decltype(e)>>( e );
-            }
-        });
-    }
-
-public:
-
-    // value constructors
-
-    constexpr expected() noexcept( std::is_nothrow_default_constructible<T>::value )
-    {
-    }
-
-    constexpr expected( T const& t ) noexcept( std::is_nothrow_copy_constructible<T>::value ): v_( in_place_index<0>, t )
-    {
-    }
-
-    constexpr expected( T && t ) noexcept( std::is_nothrow_move_constructible<T>::value ): v_( in_place_index<0>, std::move(t) )
-    {
-    }
-
-    // template<class U> constexpr expected( U && u ); where U in E...?
-
-    // in-place constructor?
-
-    // unexpected constructor
-
-    template<class... E2,
-        class En = mp_if<mp_all<std::is_copy_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
-    constexpr expected( unexpected_<E2...> const & x ): v_( x )
-    {
-    }
-
-    template<class... E2,
-        class En = mp_if<mp_all<std::is_move_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
-    constexpr expected( unexpected_<E2...> && x ): v_( std::move(x) )
-    {
-    }
-
-    // conversion constructor
-
-    template<class... E2,
-        class En = mp_if<mp_all<std::is_copy_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
-    constexpr expected( expected<T, E2...> const & x ): v_( x.v_ )
-    {
-    }
-
-    template<class... E2,
-        class En = mp_if<mp_all<std::is_move_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
-    constexpr expected( expected<T, E2...> && x ): v_( std::move(x.v_) )
-    {
-    }
-
-    // emplace
-
-    template<class... A> void emplace( A&&... a )
-    {
-        v_.emplace( std::forward<A>(a)... );
-    }
-
-    template<class V, class... A> void emplace( std::initializer_list<V> il, A&&... a )
-    {
-        v_.emplace( il, std::forward<A>(a)... );
-    }
-
-    // swap
-
-    void swap( expected & r ) noexcept( noexcept( v_.swap( r.v_ ) ) )
-    {
-        v_.swap( r.v_ );
-    }
-
-    // value queries
-
-    constexpr bool has_value() const noexcept
-    {
-        return v_.index() == 0;
-    }
-
-    constexpr explicit operator bool() const noexcept
-    {
-        return v_.index() == 0;
-    }
-
-    // checked value access
-
-    constexpr T& value() &
-    {
-        if( !has_value() )
-        {
-            _bad_access();
-        }
-
-        return *get_if<0>(&v_);
-    }
-
-    constexpr T const& value() const&
-    {
-        if( !has_value() )
-        {
-            _bad_access();
-        }
-
-        return *get_if<0>(&v_);
-    }
-
-    constexpr T&& value() &&
-    {
-        return std::move( value() );
-    }
-
-    constexpr T const&& value() const&&
-    {
-        return std::move( value() );
-    }
-
-    // unchecked value access
-
-    T* operator->() noexcept
-    {
-        return get_if<0>(&v_);
-    }
-
-    T const* operator->() const noexcept
-    {
-        return get_if<0>(&v_);
-    }
-
-    T& operator*() & noexcept
-    {
-        T* p = get_if<0>(&v_);
-
-        assert( p != 0 );
-
-        return *p;
-    }
-
-    T const& operator*() const & noexcept
-    {
-        T const* p = get_if<0>(&v_);
-
-        assert( p != 0 );
-
-        return *p;
-    }
-
-    T&& operator*() && noexcept
-    {
-        return std::move(**this);
-    }
-
-    T const&& operator*() const && noexcept
-    {
-        return std::move(**this);
-    }
-
-    // error queries
-
-    template<class E2> constexpr bool has_error() const noexcept
-    {
-        using I = mp_find<expected, E2>;
-        return v_.index() == I::value;
-    }
-
-    constexpr bool has_error() const noexcept
-    {
-        static_assert( sizeof...(E) == 1, "has_error() is only valid when there is a single E" );
-        return has_error<mp_first<expected>>();
-    }
-
-    // error access
-
-    unexpected_<E...> unexpected() const
-    {
-        if( has_value() )
-        {
-            _bad_access();
-        }
-
-        return v_.template subset<E...>();
-    }
-
-    template<class E2> constexpr E2 error() const noexcept
-    {
-        using I = mp_find<expected, E2>;
-
-        if( v_.index() != I::value )
-        {
-            _bad_access();
-        }
-
-        return get<I>( v_ );
-    }
-
-    constexpr mp_first<expected> error() const noexcept
-    {
-        static_assert( sizeof...(E) == 1, "error() is only valid when there is a single E" );
-        return error<mp_first<expected>>();
-    }
-
-    // error mapping
-
-private:
-
-    template<class F> struct Qret
-    {
-        template<class... A> using fn = decltype( std::declval<F>()( std::declval<A>()... ) );
-    };
-
-    template<class F> using remapped = mp_append<expected<T>, mp_unique<mp_transform_q<Qret<F>, mp_list<E...>>>>;
-
-    template<class R, std::size_t I, class F, class V> static R _remap_error( mp_size_t<I>, F && f, V && v )
-    {
-        // return R( std::forward<F>(f)( std::forward<V>(v) ) );
-
-        auto e = std::forward<F>(f)( std::forward<V>(v) );
-
-        return unexpected_<decltype(e)>{ e };
-    }
-
-    template<class R, class F, class V> static R _remap_error( mp_size_t<0>, F && /*f*/, V && v )
-    {
-        return R( std::forward<V>(v) );
-    }
-
-public:
-
-    template<class F> remapped<F> remap_errors( F && f ) const
-    {
-        using R = remapped<F>;
-
-        return mp_with_index<mp_size<expected>>( v_.index(), [&]( auto I ) {
-
-            return this->_remap_error<R>( I, f, get<I>(v_) );
-
-        });
-    }
-
-    expected<T, std::error_code> remap_errors() const
-    {
-        using R = expected<T, std::error_code>;
-
-        auto f = []( auto const& e ){ return make_error_code(e); };
-
-        return mp_with_index<mp_size<expected>>( v_.index(), [&]( auto I ) {
-
-            return this->_remap_error<R>( I, f, get<I>(v_) );
-
-        });
-    }
-
-    // then
-
-private:
-
-    template<class F, class U> using then_result_ = decltype( std::declval<F>()( std::declval<U>() ) );
-
-    template<class F, class U, class R = then_result_<F, U>> using then_result = mp_if<is_expected<R>, R, expected<R, E...>>;
-
-public:
-
-    template<class F> then_result<F, T const&> then( F && f ) const
-    {
-        if( has_value() )
-        {
-            return std::forward<F>(f)( **this );
-        }
-        else
-        {
-            return unexpected();
-        }
-    }
-
-    template<class F> then_result<F, T const&> operator>>( F && f ) const
-    {
-        if( has_value() )
-        {
-            return std::forward<F>(f)( **this );
-        }
-        else
-        {
-            return unexpected();
-        }
-    }
-};
-
-template<class T, class... E> inline constexpr bool operator==( expected<T, E...> const & x1, expected<T, E...> const & x2 )
-{
-    return x1.v_ == x2.v_;
-}
-
-template<class T, class... E> inline constexpr bool operator!=( expected<T, E...> const & x1, expected<T, E...> const & x2 )
-{
-    return x1.v_ != x2.v_;
-}
-
-template<class T, class... E> inline void swap( expected<T, E...> & x1, expected<T, E...> & x2 ) noexcept( noexcept( x1.swap( x2 ) ) )
-{
-    x1.swap( x2 );
-}
-
-} // namespace variant2
-} // namespace boost
-
-#endif // #ifndef BOOST_VARIANT2_EXPECTED_HPP_INCLUDED
--- a/include/boost/variant2/outcome.hpp
+++ b/include/boost/variant2/outcome.hpp
@ -1,259 +0,0 @@
-#ifndef BOOST_VARIANT2_OUTCOME_HPP_INCLUDED
-#define BOOST_VARIANT2_OUTCOME_HPP_INCLUDED
-
-//  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
-
-#ifndef BOOST_VARIANT2_VARIANT_HPP_INCLUDED
-#include <boost/variant2/variant.hpp>
-#endif
-#include <system_error>
-#include <exception>
-#include <type_traits>
-#include <cassert>
-
-//
-
-namespace boost
-{
-namespace variant2
-{
-
-enum class outcome_errc
-{
-    not_initialized
-};
-
-class outcome_error_category: public std::error_category
-{
-public:
-
-    virtual const char * name() const noexcept
-    {
-        return "boost::variant2::outcome";
-    }
-
-    virtual std::string message( int e ) const
-    {
-        switch( e )
-        {
-        case (int)outcome_errc::not_initialized:
-
-            return "outcome<> not initialized";
-
-        default:
-
-            return "unknown outcome<> error";
-        }
-    }
-
-    static outcome_error_category const & instance()
-    {
-        static outcome_error_category cat;
-        return cat;
-    }
-};
-
-std::error_code make_error_code( outcome_errc e )
-{
-    return std::error_code( static_cast<int>( e ), outcome_error_category::instance() );
-}
-
-template<class T> class outcome
-{
-private:
-
-    variant<T, std::error_code, std::exception_ptr> v_;
-
-public:
-
-    // constructors
-
-    constexpr outcome() noexcept: v_( make_error_code( outcome_errc::not_initialized ) )
-    {
-    }
-
-    constexpr outcome( T const& t ): v_( t )
-    {
-    }
-
-    constexpr outcome( T&& t ): v_( std::move(t) )
-    {
-    }
-
-    constexpr outcome( std::error_code const & ec ) noexcept: v_( ec )
-    {
-    }
-
-    constexpr outcome( std::exception_ptr const & ep ) noexcept: v_( ep )
-    {
-    }
-
-    // queries
-
-    constexpr bool has_value() const noexcept
-    {
-        return v_.index() == 0;
-    }
-
-    constexpr bool has_error() const noexcept
-    {
-        return v_.index() == 1;
-    }
-
-    constexpr bool has_exception() const noexcept
-    {
-        return v_.index() == 2;
-    }
-
-    constexpr explicit operator bool() const noexcept
-    {
-        return v_.index() == 0;
-    }
-
-    // checked value access
-
-    constexpr T& value() &
-    {
-        if( has_value() )
-        {
-            return *get_if<0>(&v_);
-        }
-        else
-        {
-            throw std::system_error( *get_if<1>(&v_) );
-        }
-    }
-
-    constexpr T const& value() const&
-    {
-        if( has_value() )
-        {
-            return *get_if<0>(&v_);
-        }
-        else
-        {
-            throw std::system_error( *get_if<1>(&v_) );
-        }
-    }
-
-    constexpr T&& value() &&
-    {
-        return std::move( value() );
-    }
-
-    constexpr T const&& value() const&&
-    {
-        return std::move( value() );
-    }
-
-    // unchecked value access
-
-    T* operator->() noexcept
-    {
-        return get_if<0>(&v_);
-    }
-
-    T const* operator->() const noexcept
-    {
-        return get_if<0>(&v_);
-    }
-
-    T& operator*() & noexcept
-    {
-        T* p = get_if<0>(&v_);
-
-        assert( p != 0 );
-
-        return *p;
-    }
-
-    T const& operator*() const & noexcept
-    {
-        T const* p = get_if<0>(&v_);
-
-        assert( p != 0 );
-
-        return *p;
-    }
-
-    T&& operator*() && noexcept
-    {
-        return std::move(**this);
-    }
-
-    T const&& operator*() const && noexcept
-    {
-        return std::move(**this);
-    }
-
-    // error access
-
-    /*constexpr*/ std::error_code error() const noexcept
-    {
-        if( has_error() )
-        {
-            return *get_if<1>(&v_);
-        }
-        else
-        {
-            return std::error_code();
-        }
-    }
-
-    // exception access
-
-    std::exception_ptr exception() const noexcept
-    {
-        if( has_exception() )
-        {
-            return *get_if<2>(&v_);
-        }
-        else if( has_value() )
-        {
-            return std::exception_ptr();
-        }
-        else
-        {
-            return std::make_exception_ptr( std::system_error( *get_if<1>(&v_) ) );
-        }
-    }
-
-    // setters
-
-    void set_value( T const& t ) noexcept( std::is_nothrow_copy_constructible<T>::value )
-    {
-        v_.emplace<0>( t );
-    }
-
-    void set_value( T&& t ) noexcept( std::is_nothrow_move_constructible<T>::value )
-    {
-        v_.emplace<0>( std::move( t ) );
-    }
-
-    void set_error( std::error_code const & e ) noexcept
-    {
-        v_.emplace<1>( e );
-    }
-
-    void set_exception( std::exception_ptr const & x ) noexcept
-    {
-        v_.emplace<2>( x );
-    }
-
-    // swap
-
-    void swap( outcome& r ) noexcept( noexcept( v_.swap( r.v_ ) ) )
-    {
-        v_.swap( r.v_ );
-    }
-};
-
-} // namespace variant2
-} // namespace boost
-
-#endif // #ifndef BOOST_VARIANT2_OUTCOME_HPP_INCLUDED
--- a/include/boost/variant2/result.hpp
+++ b/include/boost/variant2/result.hpp
@ -1,226 +0,0 @@
-#ifndef BOOST_VARIANT2_RESULT_HPP_INCLUDED
-#define BOOST_VARIANT2_RESULT_HPP_INCLUDED
-
-//  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
-
-#ifndef BOOST_VARIANT2_VARIANT_HPP_INCLUDED
-#include <boost/variant2/variant.hpp>
-#endif
-#include <system_error>
-#include <type_traits>
-#include <cassert>
-
-//
-
-namespace boost
-{
-namespace variant2
-{
-
-enum class result_errc
-{
-    not_initialized
-};
-
-class result_error_category: public std::error_category
-{
-public:
-
-    virtual const char * name() const noexcept
-    {
-        return "boost::variant2::result";
-    }
-
-    virtual std::string message( int e ) const
-    {
-        switch( e )
-        {
-        case (int)result_errc::not_initialized:
-
-            return "result<> not initialized";
-
-        default:
-
-            return "unknown result<> error";
-        }
-    }
-
-    static result_error_category const & instance()
-    {
-        static result_error_category cat;
-        return cat;
-    }
-};
-
-std::error_code make_error_code( result_errc e )
-{
-    return std::error_code( static_cast<int>( e ), result_error_category::instance() );
-}
-
-template<class T> class result
-{
-private:
-
-    variant<T, std::error_code> v_;
-
-public:
-
-    // constructors
-
-    constexpr result() noexcept: v_( make_error_code( result_errc::not_initialized ) )
-    {
-    }
-
-    constexpr result( T const& t ): v_( t )
-    {
-    }
-
-    constexpr result( T&& t ): v_( std::move(t) )
-    {
-    }
-
-    constexpr result( std::error_code const & ec ) noexcept: v_( ec )
-    {
-    }
-
-    // queries
-
-    constexpr bool has_value() const noexcept
-    {
-        return v_.index() == 0;
-    }
-
-    constexpr bool has_error() const noexcept
-    {
-        return v_.index() == 1;
-    }
-
-    constexpr explicit operator bool() const noexcept
-    {
-        return v_.index() == 0;
-    }
-
-    // checked value access
-
-    constexpr T& value() &
-    {
-        if( has_value() )
-        {
-            return *get_if<0>(&v_);
-        }
-        else
-        {
-            throw std::system_error( *get_if<1>(&v_) );
-        }
-    }
-
-    constexpr T const& value() const&
-    {
-        if( has_value() )
-        {
-            return *get_if<0>(&v_);
-        }
-        else
-        {
-            throw std::system_error( *get_if<1>(&v_) );
-        }
-    }
-
-    constexpr T&& value() &&
-    {
-        return std::move( value() );
-    }
-
-    constexpr T const&& value() const&&
-    {
-        return std::move( value() );
-    }
-
-    // unchecked value access
-
-    T* operator->() noexcept
-    {
-        return get_if<0>(&v_);
-    }
-
-    T const* operator->() const noexcept
-    {
-        return get_if<0>(&v_);
-    }
-
-    T& operator*() & noexcept
-    {
-        T* p = get_if<0>(&v_);
-
-        assert( p != 0 );
-
-        return *p;
-    }
-
-    T const& operator*() const & noexcept
-    {
-        T const* p = get_if<0>(&v_);
-
-        assert( p != 0 );
-
-        return *p;
-    }
-
-    T&& operator*() && noexcept
-    {
-        return std::move(**this);
-    }
-
-    T const&& operator*() const && noexcept
-    {
-        return std::move(**this);
-    }
-
-    // error access
-
-    /*constexpr*/ std::error_code error() const noexcept
-    {
-        if( has_error() )
-        {
-            return *get_if<1>(&v_);
-        }
-        else
-        {
-            return std::error_code();
-        }
-    }
-
-    // setters
-
-    void set_value( T const& t ) noexcept( std::is_nothrow_copy_constructible<T>::value )
-    {
-        v_.emplace<0>( t );
-    }
-
-    void set_value( T&& t ) noexcept( std::is_nothrow_move_constructible<T>::value )
-    {
-        v_.emplace<0>( std::move( t ) );
-    }
-
-    void set_error( std::error_code const & e ) noexcept
-    {
-        v_.emplace<1>( e );
-    }
-
-    // swap
-
-    void swap( result& r ) noexcept( noexcept( v_.swap( r.v_ ) ) )
-    {
-        v_.swap( r.v_ );
-    }
-};
-
-} // namespace variant2
-} // namespace boost
-
-#endif // #ifndef BOOST_VARIANT2_RESULT_HPP_INCLUDED
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
--- a/index.html
+++ b/index.html
@ -4,7 +4,7 @@
 </head>
 <body>
 Automatic redirection failed, please go to
-<a href="doc/html/mp11.html">doc/html/variant2.html</a>.
+<a href="doc/html/variant2.html">doc/html/variant2.html</a>.
 </body>
 </html>
 <!--
--- a/meta/libraries.json
+++ b/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 implementation of std::variant.",
+    "description": "A never-valueless, strong guarantee implementation of std::variant.",
    "category": [
        "Containers", "Data"
-    ]
+    ],
+    "cxxstd": "11"
 }
--- a/test/CMakeLists.txt
+++ b/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()
--- a/test/Jamfile
+++ b/test/Jamfile
@ -1,6 +1,6 @@
 #  Boost.Variant2 Library Test Jamfile
 #
-#  Copyright 2015-2017 Peter Dimov
+#  Copyright 2015-2019 Peter Dimov
 #
 #  Distributed under the Boost Software License, Version 1.0.
 #  See accompanying file LICENSE_1_0.txt or copy at
@ -9,22 +9,36 @@
 import testing ;
 import ../../config/checks/config : requires ;

-project : requirements [ requires cxx11_variadic_templates cxx11_template_aliases cxx11_decltype cxx11_constexpr ] ;
+project
+  : default-build
+
+    <warnings>extra
+
+  : requirements
+
+    [ requires cxx11_variadic_templates cxx11_template_aliases cxx11_decltype cxx11_constexpr ]
+
+    <toolset>msvc:<warnings-as-errors>on
+    <toolset>gcc:<warnings-as-errors>on
+    <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 ;
@ -33,13 +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 ] ;
@ -77,3 +91,38 @@ run variant_convert_construct_throw.cpp ;

 run variant_copy_assign_throw.cpp ;
 run variant_move_assign_throw.cpp ;
+
+local NX =
+    <exception-handling>off
+    <toolset>msvc:<cxxflags>/wd4530
+    <toolset>msvc:<cxxflags>/wd4577
+    ;
+
+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 ;
+
+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 ;
+
+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 ;
--- a/test/cmake_install_test/CMakeLists.txt
+++ b/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>)
--- a/test/cmake_subdir_test/CMakeLists.txt
+++ b/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>)
--- a/test/is_output_streamable.cpp
+++ b/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();
+}
--- a/test/quick.cpp
+++ b/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;
+}
--- a/test/throw_exception.cpp
+++ b/test/throw_exception.cpp
@ -0,0 +1,20 @@
+
+// Copyright 2019 Peter Dimov.
+//
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/config.hpp>
+#include <exception>
+#include <cstdio>
+
+namespace boost
+{
+
+void throw_exception( std::exception const & e )
+{
+    std::fprintf( stderr, "Exception: %s\n", e.what() );
+    std::terminate();
+}
+
+} // namespace boost
--- a/test/variant_convert_construct_throw.cpp
+++ b/test/variant_convert_construct_throw.cpp
@ -6,6 +6,10 @@
 // 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: 4702 ) // unreachable code
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <stdexcept>
--- a/test/variant_copy_assign_throw.cpp
+++ b/test/variant_copy_assign_throw.cpp
@ -6,6 +6,10 @@
 // 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: 4702 ) // unreachable code
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <stdexcept>
--- a/test/variant_copy_construct.cpp
+++ b/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>>));
--- a/test/variant_copy_construct_throw.cpp
+++ b/test/variant_copy_construct_throw.cpp
@ -6,6 +6,10 @@
 // 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: 4702 ) // unreachable code
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <stdexcept>
--- a/test/variant_derived_construct.cpp
+++ b/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;
+}
--- a/test/variant_emplace_index.cpp
+++ 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();
 }
--- a/test/variant_emplace_type.cpp
+++ b/test/variant_emplace_type.cpp
@ -57,6 +57,21 @@ 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
+{
+};
+
+struct Y2
+{
+    ~Y2() {}
+};
+
+struct Guard
+{
+    explicit Guard(int) {}
+    Guard(Guard&&) = delete;
+};
+
 int main()
 {
    {
@ -165,5 +180,15 @@ int main()
        BOOST_TEST_EQ( get<0>(v).v, 4 );
    }

+    {
+        variant<Y1, Guard> v;
+        v.emplace<Guard>( 1 );
+    }
+
+    {
+        variant<Y2, Guard> v;
+        v.emplace<Guard>( 1 );
+    }
+
    return boost::report_errors();
 }
--- a/test/variant_eq_ne.cpp
+++ b/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();
 }
--- a/test/variant_eq_ne_cx.cpp
+++ b/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
--- a/test/variant_get_by_index.cpp
+++ b/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>
--- a/test/variant_get_by_type.cpp
+++ b/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>
--- a/test/variant_hash.cpp
+++ b/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();
+}
--- a/test/variant_in_place_index_construct_cx.cpp
+++ b/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
 }
--- a/test/variant_in_place_type_construct_cx.cpp
+++ b/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
 }
--- a/test/variant_lt_gt.cpp
+++ b/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();
 }
--- a/test/variant_lt_gt_cx.cpp
+++ b/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
--- a/test/variant_many_types.cpp
+++ b/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();
+}
--- a/test/variant_move_assign_throw.cpp
+++ b/test/variant_move_assign_throw.cpp
@ -6,6 +6,10 @@
 // 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: 4702 ) // unreachable code
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <stdexcept>
--- a/test/variant_move_construct_throw.cpp
+++ b/test/variant_move_construct_throw.cpp
@ -6,6 +6,10 @@
 // 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: 4702 ) // unreachable code
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <stdexcept>
--- a/test/variant_ostream_insert.cpp
+++ b/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();
+}
--- a/test/variant_special.cpp
+++ b/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();
+}
--- a/test/variant_subset.cpp
+++ b/test/variant_subset.cpp
@ -6,6 +6,10 @@
 // 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: 4702 ) // unreachable code
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
--- a/test/variant_trivial.cpp
+++ b/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();
+}
--- a/test/variant_valueless.cpp
+++ b/test/variant_valueless.cpp
@ -6,6 +6,10 @@
 // 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: 4702 ) // unreachable code
+#endif
+
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <type_traits>
@ -111,8 +115,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // basic guarantee; X1 is nothrow default-constructible
-            BOOST_TEST_EQ( v.index(), 1 );
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 0 );
        }
    }

@ -128,8 +132,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // basic guarantee; X1 is nothrow default-constructible
-            BOOST_TEST_EQ( v.index(), 0 );
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 1 );
        }
    }

@ -145,8 +149,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // basic guarantee; monostate
-            BOOST_TEST_EQ( v.index(), 2 );
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 0 );
        }
    }

@ -162,8 +166,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // basic guarantee; monostate
-            BOOST_TEST_EQ( v.index(), 2 );
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 1 );
        }
    }

@ -179,8 +183,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // basic guarantee; X1 is nothrow default-constructible
-            BOOST_TEST_EQ( v.index(), 2 );
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 0 );
        }
    }

@ -196,8 +200,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // basic guarantee; monostate
-            BOOST_TEST_EQ( v.index(), 3 );
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 0 );
        }
    }

@ -230,18 +234,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // X3 is not v2d::trivially_move_assignable on libstdc++ 4.x
-
-            if( v2d::is_trivially_move_assignable<X3>::value )
-            {
-                // all trivially destructible and move-assignable, no change
-                BOOST_TEST_EQ( v.index(), 0 );
-            }
-            else
-            {
-                // basic guarantee; X1 is nothrow default-constructible
-                BOOST_TEST_EQ( v.index(), 1 );
-            }
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 0 );
        }
    }

@ -257,18 +251,8 @@ int main()
        }
        catch( std::exception const& )
        {
-            // X3 is not v2d::trivially_move_assignable on libstdc++ 4.x
-
-            if( v2d::is_trivially_move_assignable<X3>::value )
-            {
-                // all trivially destructible and move-assignable, no change
-                BOOST_TEST_EQ( v.index(), 0 );
-            }
-            else
-            {
-                // basic guarantee; monostate
-                BOOST_TEST_EQ( v.index(), 2 );
-            }
+            // strong guarantee
+            BOOST_TEST_EQ( v.index(), 0 );
        }
    }

--- a/test/variant_visit.cpp
+++ b/test/variant_visit.cpp
@ -37,6 +37,10 @@ struct F

 int main()
 {
+    {
+        BOOST_TEST_EQ( (visit( []{ return 5; } )), 5 );
+    }
+
    {
        variant<int> v( 1 );

--- a/test/variant_visit_by_index.cpp
+++ b/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();
+}
--- a/test/variant_visit_derived.cpp
+++ b/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();
+}
--- a/test/variant_visit_r.cpp
+++ b/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();
+}
Author	SHA1	Message	Date
0xFEEDC0DE64	5c0274a76e	glob headers	2022-07-05 10:48:50 +02:00
Peter Dimov	d34658f4c8	Document operator<< for monostate	2022-01-31 19:19:49 +02:00
Peter Dimov	5e2bce1baa	Add operator<< for monostate	2022-01-31 17:55:26 +02:00
Peter Dimov	573ef77382	Move unsigned ix_ after the elements to avoid need to adjust rdi (closes #30 )	2021-12-09 21:28:35 +02:00
Peter Dimov	f6cad272d4	Add msvc-14.0 to GHA	2021-12-09 19:59:01 +02:00
Peter Dimov	3fc7540733	Only define operator<< when all contained types are streamable (refs #31 )	2021-12-09 18:39:03 +02:00
Peter Dimov	e4b4b3f02b	Extend msvc-14.2 workaround to 14.3	2021-10-29 02:18:23 +03:00
Peter Dimov	44d38c4111	Enable syntax highlighting	2021-10-28 23:23:13 +03:00
Peter Dimov	1ebfb3b689	Add msvc-14.3 to ci.yml	2021-10-28 23:22:20 +03:00
Peter Dimov	8af1fbde8e	Update cmake_subdir_test/CMakeLists.txt	2021-09-21 14:52:38 +03:00
Peter Dimov	26ce33597d	Regenerate CMakeLists.txt	2021-09-21 14:45:20 +03:00
Peter Dimov	c89713b978	Add CMake tests to ci.yml	2021-09-21 14:31:47 +03:00
Peter Dimov	10c441c8d7	Update appveyor.yml	2021-09-15 15:11:54 +03:00
Peter Dimov	09ec260780	Document operator<<	2021-09-15 03:53:13 +03:00
Peter Dimov	2da13befd7	Work around mysterious errors with gcc, msvc-14.1, and clang-cl	2021-09-15 03:28:29 +03:00
Peter Dimov	e668c099ce	Add operator<< for variant	2021-09-15 02:51:11 +03:00
Peter Dimov	aebcb9792d	Update documentation of visit_by_index	2021-09-15 02:00:33 +03:00
Peter Dimov	8fd9f830a9	Add proper type deduction to visit_by_index	2021-09-15 01:30:09 +03:00
Peter Dimov	ed4bebff3d	Document visit_by_index	2021-09-14 21:32:57 +03:00
Peter Dimov	ad06c9b923	Add visit_by_index	2021-09-14 21:06:01 +03:00
Peter Dimov	de4eb8b6d2	Update README.md	2021-09-11 19:47:34 +03:00
Peter Dimov	4d69db1441	Extend g++-10 workarounds to 11	2021-09-11 19:23:39 +03:00
Peter Dimov	095d9770c4	Update ci.yml	2021-09-11 18:36:43 +03:00
Peter Dimov	cca476fbbc	Update documentation	2021-09-11 18:29:17 +03:00
Peter Dimov	c76af4d3cf	Update README.md	2021-09-11 18:26:28 +03:00
Peter Dimov	1af7b165cf	BOOST_VARIANT2_CX14_ASSERT must be empty under C++11	2021-09-11 18:22:46 +03:00
Peter Dimov	49bff72be0	Define and use a helper macro BOOST_VARIANT2_CX14_ASSERT in constexpr functions	2021-09-11 18:11:17 +03:00
Peter Dimov	59780ba08e	Only assert when C++14	2021-09-11 18:00:29 +03:00
Peter Dimov	edd70cfd66	Update ci.yml	2021-09-11 17:43:05 +03:00
Peter Dimov	86c2782dff	Update documentation	2021-09-11 17:39:55 +03:00
Peter Dimov	dce8174550	Make unsafe_get public	2021-09-11 17:29:21 +03:00
Peter Dimov	9d40bc80f1	Use BOOST_ASSERT instead of assert	2021-09-11 17:23:29 +03:00
Peter Dimov	be6ddf9fdc	Add boost/variant2.hpp	2021-09-11 17:19:09 +03:00
Peter Dimov	4153a535a0	Update .github/workflows	2021-04-19 20:37:12 +03:00
Peter Dimov	f374ad68a3	Add -DBUILD_TESTING=ON to .yml files; it's not default anymore	2021-03-19 04:18:08 +02:00
Peter Dimov	3bc9a57c9b	Merge branch 'feature/issue-26' into develop	2021-03-13 18:23:39 +02:00
Peter Dimov	3ca95a65df	Use boost::hash in hash_value, not std::hash (closes #27 )	2021-03-13 05:08:03 +02:00
Peter Dimov	ae1f72671e	Add test for types supported by boost::hash, but not by std::hash	2021-03-13 04:50:01 +02:00
Peter Dimov	a2dab8c7d3	Refactor detail::hash_value_L	2021-03-13 04:10:41 +02:00
Peter Dimov	f39a71ed2f	Merge branch 'feature/issue-26-nofix' into feature/issue-26	2021-03-13 01:09:46 +02:00
Peter Dimov	06dde96400	Merge branch 'develop' into feature/issue-26-nofix	2021-03-13 01:09:17 +02:00
Peter Dimov	8be0a4a5fe	Update copyright	2021-03-12 23:42:32 +02:00
Peter Dimov	90cda5339f	Disable U&& constructor for derived types	2021-03-12 23:40:14 +02:00
Peter Dimov	afb0aafd64	Change clangs to bionic	2021-03-12 20:18:24 +02:00
Peter Dimov	f586dc8848	Add test for inherited constructors (refs #26 )	2021-03-12 20:15:09 +02:00
Peter Dimov	20b9175932	Add cxxstd=latest to Appveyor	2021-03-08 04:04:35 +02:00
Peter Dimov	fb3ce863ff	Add MSVC workaround for monospace constexpr (refs #25 )	2021-03-08 03:26:40 +02:00
Peter Dimov	8ec0bf448a	Add monospace relational tests (refs #25 )	2021-03-08 03:01:21 +02:00
Peter Dimov	6d19e6be68	Update revision history	2021-03-07 00:51:28 +02:00
Peter Dimov	d8552b23ae	UBSan on clang 7 is broken on Ubuntu	2021-03-01 06:35:38 +02:00
Peter Dimov	f44aba09a2	Update .travis.yml	2021-03-01 00:26:43 +02:00
Peter Dimov	99cc1db385	Fix documentation for emplace effects	2021-01-25 03:49:42 +02:00
Peter Dimov	6d848c5af6	Update GCC workarounds in constexpr in-place construct tests	2021-01-25 03:44:24 +02:00
Peter Dimov	3c9f4e56bf	Fix visit codegen regression when NDEBUG is not defined	2021-01-24 17:27:24 +02:00
Peter Dimov	3015e56bcb	GCC bug 63707 has been fixed for GCC 11	2021-01-23 20:36:12 +02:00
Peter Dimov	7c37053950	Merge branch 'develop' into feature/double-opt	2021-01-13 05:06:00 +02:00
Peter Dimov	60995edb41	Remove g++ 4.7 from GHA	2021-01-13 05:01:44 +02:00
Peter Dimov	84a2c175d0	Use variant=release for variant_visit_r under g++/windows	2021-01-13 04:44:56 +02:00
Peter Dimov	ea3268feb4	Add .github/workflows	2021-01-13 04:19:15 +02:00
Peter Dimov	6de876954a	Merge branch 'develop' into feature/double-opt	2021-01-13 02:32:32 +02:00
Peter Dimov	efc1d5acca	Add more tests to variant_emplace_index.cpp	2021-01-13 02:32:05 +02:00
Peter Dimov	453b00dec8	Use (i+1)*2+j for the internal index in the double-buffered case, to avoid branches	2021-01-13 02:10:12 +02:00
Peter Dimov	c6186e0a95	Merge pull request #22 from eldiener/develop Add "cxxstd" json field. The "cxxstd" json field is being added to ea…	2020-12-16 18:45:01 +02:00
Edward Diener	24ccee2104	Add "cxxstd" json field. The "cxxstd" json field is being added to each Boost library's meta json information for libraries whose minumum C++ standard compilation level is C++11 on up. The value of this field matches one of the values for 'cxxstd' in Boost.Build. The purpose of doing this is to provide information for the Boost website documentation for each library which will specify the minimum C++ standard compilation that an end-user must employ in order to use the particular library. This will aid end-users who want to know if they can successfully use a Boost library based on their C++ compiler's compilation level, without having to search the library's documentation to find this out.	2020-12-16 01:45:24 -05:00
Peter Dimov	93b8618e94	Update maintainer e-mail	2020-12-12 01:08:09 +02:00
Peter Dimov	1ebc29aa02	Remove boost_install call from CMakeLists.txt	2020-06-11 17:21:57 +03:00
Peter Dimov	d3db874762	Document visit<R>	2020-06-03 18:01:57 +03:00
Peter Dimov	5586ebaa64	Add support for visit<R>	2020-06-03 17:00:13 +03:00
Peter Dimov	bede3777a8	Add test for nullary visit	2020-06-03 14:29:24 +03:00
Peter Dimov	b302dd5912	Update changelog	2020-05-22 17:57:29 +03:00
Peter Dimov	2ad6fed07a	Update revision history	2020-05-10 19:34:12 +03:00
Peter Dimov	84ea994325	Add specialization for variant_storage_impl<mp_false, ...>	2020-05-09 19:56:27 +03:00
Peter Dimov	465e5bac3d	Add specialization for variant_storage_impl<mp_true, ...> to reduce instantiations	2020-05-09 18:14:12 +03:00
Peter Dimov	41829b0fb1	test/variant_many_types: cosmetic fixes	2020-05-09 17:55:42 +03:00
Peter Dimov	b57d75ff80	Merge branch 'develop' into feature/many-types	2020-05-09 14:41:29 +03:00
Peter Dimov	03019860df	Fix gcc-10 version check	2020-05-09 05:10:28 +03:00
Peter Dimov	8ec4720d2d	Merge branch 'feature/clang-10' into feature/gcc-10	2020-05-09 05:09:11 +03:00
Peter Dimov	75f574fc48	Move warning suppression before the includes	2020-05-09 04:42:27 +03:00
Peter Dimov	1d79adfca0	Add gcc-10 to Travis	2020-05-09 04:32:11 +03:00
Peter Dimov	fa92e40b35	Disable failing constexpr tests on g++ 10.1	2020-05-09 04:31:26 +03:00
Peter Dimov	a403f82691	Disable -Wdeprecated-volatile	2020-05-09 03:57:27 +03:00
Peter Dimov	a7d0da59ad	Add clang-10 to Travis	2020-05-09 03:50:20 +03:00
Peter Dimov	fa872cb835	test/variant_many_types: add a constructor to Y	2020-05-08 23:23:19 +03:00
Peter Dimov	93204676f5	Add test/variant_many_types	2020-05-08 22:22:49 +03:00
Peter Dimov	772ef0d312	Support derived types in visit	2020-05-06 20:11:03 +03:00
Peter Dimov	f3b3b533aa	Remove local mp_power_set implementation, as it's now in mp11	2020-03-20 02:26:01 +02:00
Peter Dimov	4b60dee4b6	Avoid gcc warning 'base class should be explicitly initialized in copy constructor'	2020-02-01 01:18:40 +02:00
Peter Dimov	76c67c9b21	Use <warnings>extra instead of `all`	2020-01-31 23:59:00 +02:00
Peter Dimov	308bd731a5	Update documentation	2020-01-23 21:17:49 +02:00
Peter Dimov	719c43316f	Disable is_copy_constructible tests on gcc 4.x	2020-01-23 19:13:11 +02:00
Peter Dimov	d6680df4e5	Add a test case to test/variant_copy_construct	2020-01-23 18:33:39 +02:00
Peter Dimov	8cf72527a6	Revert incorrect changes in test/variant_trivial	2020-01-23 18:09:01 +02:00
Peter Dimov	b649a39776	Check std::is_copy_constructible in test/variant_copy_construct	2020-01-23 17:38:03 +02:00
Peter Dimov	37f6efab5d	Fix typo	2020-01-23 17:34:36 +02:00
Peter Dimov	5259bdd5fc	Disable move triviality tests on libstdc++ 4.x	2020-01-23 17:22:45 +02:00
Peter Dimov	9964e77dd2	Add clang++/trusty to Travis (libstdc++ 4.8)	2020-01-23 15:50:45 +02:00
Peter Dimov	57ab36bd95	Add test/variant_special	2020-01-23 15:24:32 +02:00
Peter Dimov	fac5992e45	Add test/variant_trivial	2020-01-23 03:53:05 +02:00
Peter Dimov	972280e59f	Remove commented-out variant_copy_base	2020-01-23 01:08:32 +02:00
Peter Dimov	ef2ef292b9	The msvc-14.0 workarounds are no longer needed	2020-01-22 20:14:11 +02:00
Peter Dimov	3b2fb1a48a	Split variant_copy_base into separate cc/ca/mc/ma bases; avoids multiple inheritance, which is a penalty in the MS ABI	2020-01-22 19:59:59 +02:00
Peter Dimov	4f81882bfd	Explicitly disable move constructor when needed	2020-01-22 18:30:36 +02:00
Peter Dimov	17bc06b090	Apply msvc-14.0 workarounds	2020-01-22 17:38:45 +02:00
Peter Dimov	c4cad3e96a	Use an intermediate trivial base for trivial variants	2020-01-22 17:09:15 +02:00
Peter Dimov	83ff667813	Remove stray `compiler: g++`	2020-01-21 02:22:17 +02:00
Peter Dimov	5e76451843	Use BOOST_INCLUDE_LIBRARIES=variant2 in the CMake install test	2020-01-21 00:43:15 +02:00
Peter Dimov	c2b5d101eb	Update revision history	2020-01-11 15:22:13 +02:00
Peter Dimov	a92185a86d	Add Boost::container_hash to LINK_LIBRARIES	2020-01-11 06:54:23 +02:00
Peter Dimov	a795f9cf01	Add missing inline	2020-01-11 06:41:21 +02:00
Peter Dimov	f6e2e78d7d	Libc++ doesn't implement a primary std::hash template under C++11	2020-01-11 06:31:45 +02:00
Peter Dimov	1f05cdfb17	Include <functional>, for g++ trunk (10.0)	2020-01-11 06:06:54 +02:00
Peter Dimov	1203aa97c0	Add g++ 4.8 to Travis	2020-01-11 05:29:07 +02:00
Peter Dimov	b3229d48b7	gcc < 7 doesn't like specializing std::hash in a different namespace	2020-01-11 05:27:57 +02:00
Peter Dimov	be11d94adc	Update appveyor.yml	2020-01-11 05:10:28 +02:00
Peter Dimov	fece11142c	Add hashing support	2020-01-11 04:51:30 +02:00
Peter Dimov	58b4a21deb	Add CMake tests	2020-01-09 17:43:54 +02:00
Peter Dimov	5192a345ab	Fix .travis.yml	2019-12-28 22:13:37 +02:00
Peter Dimov	74e6a04a60	Update .travis.yml	2019-12-28 21:35:20 +02:00
Peter Dimov	2af66dd815	Add CMake install support	2019-12-28 18:52:32 +02:00
Peter Dimov	a15364bd81	Update README	2019-09-21 19:12:34 +03:00
Peter Dimov	12758c076d	Fix Travis and Appveyor links	2019-09-21 18:38:55 +03:00
Peter Dimov	a60f432b64	Add `dist: trusty` to .travis.yml	2019-06-17 15:07:45 +03:00
Peter Dimov	bc73c4b822	Fix return code of quick.cpp	2019-06-03 18:48:13 +03:00
Peter Dimov	566bde5fcc	Add CMakeLists.txt	2019-06-03 17:50:13 +03:00
Peter Dimov	2e831fbdb2	Add gcc9, clang8 to Travis	2019-06-03 16:07:29 +03:00
Peter Dimov	2738f081cd	Include boost/config.hpp for ::gets workaround on clang 3.8, 3.9	2019-06-03 15:43:43 +03:00
Peter Dimov	f5d9662946	Support -fno-exceptions	2019-06-03 02:46:59 +03:00
Peter Dimov	adeb970be6	Introduce detail::throw_bad_variant_access	2019-06-03 01:05:15 +03:00
Peter Dimov	6b617ae3a8	Update README	2019-05-17 17:39:42 +03:00
Peter Dimov	04f655e76a	Fix link	2019-05-17 16:12:59 +03:00
Peter Dimov	c4f7f2d63a	Delete doc/html	2019-05-17 16:11:43 +03:00
Peter Dimov	504abbe8eb	Update description	2019-05-17 16:10:06 +03:00
Peter Dimov	8cb1789167	Update README.md	2019-05-17 03:30:40 +03:00
Peter Dimov	69b25cb42a	Add design rationale	2019-05-13 19:15:53 +03:00
Peter Dimov	6b3a2b2b4d	Merge branch 'develop' into feature/documentation	2019-05-12 20:15:31 +03:00
Peter Dimov	2d990f774a	Remove _real_index	2019-05-12 19:34:21 +03:00
Peter Dimov	a083667a96	Apply corrections from #10 . Closes #10 .	2019-05-12 19:19:55 +03:00
Peter Dimov	e686dab3ea	Update README	2019-05-12 19:02:59 +03:00
Peter Dimov	3041fff85c	Remove expected	2019-05-12 18:56:54 +03:00
Peter Dimov	cacd99d69d	Update documentation	2019-05-12 18:43:56 +03:00
Peter Dimov	64ea067c79	Merge branch 'develop' into feature/documentation	2019-05-12 02:21:12 +03:00
Peter Dimov	6390b5ed20	Update html	2019-05-12 02:02:58 +03:00
Peter Dimov	8691721a9c	Update reference	2019-05-12 02:02:49 +03:00
Peter Dimov	7f7c74522b	Strong guarantee on assignment	2019-05-12 01:46:55 +03:00
Peter Dimov	c3f9beaadc	Strong guarantee on emplace	2019-05-12 00:38:21 +03:00
Peter Dimov	475ad691d6	Add changelog, design, implementation sections	2019-05-12 00:15:17 +03:00
Peter Dimov	5011dd8e1c	Update html	2019-05-12 00:13:38 +03:00
Peter Dimov	f2980f97fc	Add headings to overview	2019-05-12 00:08:27 +03:00
Peter Dimov	c8ebfd0481	Update html	2019-05-11 21:06:44 +03:00
Peter Dimov	6f190133be	Update overview	2019-05-11 21:06:31 +03:00
Peter Dimov	c138488551	Update footer for Asciidoctor 2	2019-05-11 21:05:24 +03:00
Peter Dimov	6b69a1deca	Asciidoctor 2 fixes	2019-05-11 21:04:45 +03:00
Peter Dimov	70fbd17be9	Change A... to A&&... in is_constructible invocations	2019-04-09 21:14:13 +03:00
Peter Dimov	e8e56139a8	Update msvc workarounds for msvc-14.2	2019-04-09 20:17:28 +03:00
Peter Dimov	2d6385031e	Fix emplace for types with a deleted move constructor (in the not trivially destructible case)	2019-04-09 19:22:04 +03:00
Peter Dimov	9f7e525984	Fix emplace for types with a deleted move constructor (in the trivially destructible case)	2019-04-09 18:28:18 +03:00
Peter Dimov	2e8ae0c796	Specify the exception safety of emplace in more detail	2019-03-30 20:26:13 +02:00
Peter Dimov	89986c7634	Merge branch 'feature/warnings-as-errors' into develop	2019-03-26 01:32:02 +02:00
Peter Dimov	1da3892239	Disable C4702 (unreachable code) for the throwing tests	2019-03-25 19:50:33 +02:00
Peter Dimov	3808c9b122	Fix variant_npos warning	2019-03-25 17:40:39 +02:00
Peter Dimov	f1b1e7cade	Enable warnings-as-errors in test/Jamfile	2019-03-25 17:40:20 +02:00
Peter Dimov	6d7be90ce0	Document the exception safety of emplace	2019-03-25 16:35:27 +02:00
Peter Dimov	9875cfdc98	Add variant_npos and valueless_by_exception()	2019-03-25 16:17:06 +02:00
Peter Dimov	688849f9e8	Remove outcome.hpp and result.hpp	2019-03-25 16:16:10 +02:00