Merge branch 'feature/issue-26-nofix' into feature/issue-26

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

.github/workflows/ci.yml

@@ -0,0 +1,197 @@
+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-16.04
+            install: g++-4.8
+          - toolset: gcc-4.9
+            cxxstd: "03,11"
+            os: ubuntu-16.04
+            install: g++-4.9
+          - toolset: gcc-5
+            cxxstd: "03,11,14,1z"
+            os: ubuntu-16.04
+          - toolset: gcc-6
+            cxxstd: "03,11,14,1z"
+            os: ubuntu-16.04
+            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
+          - toolset: gcc-9
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-18.04
+          - toolset: gcc-10
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-18.04
+          - toolset: clang
+            compiler: clang++-3.5
+            cxxstd: "03,11,14"
+            os: ubuntu-16.04
+            install: clang-3.5
+          - toolset: clang
+            compiler: clang++-3.6
+            cxxstd: "03,11,14"
+            os: ubuntu-16.04
+            install: clang-3.6
+          - toolset: clang
+            compiler: clang++-3.7
+            cxxstd: "03,11,14"
+            os: ubuntu-16.04
+            install: clang-3.7
+          - toolset: clang
+            compiler: clang++-3.8
+            cxxstd: "03,11,14"
+            os: ubuntu-16.04
+            install: clang-3.8
+          - toolset: clang
+            compiler: clang++-3.9
+            cxxstd: "03,11,14"
+            os: ubuntu-16.04
+            install: clang-3.9
+          - toolset: clang
+            compiler: clang++-4.0
+            cxxstd: "03,11,14"
+            os: ubuntu-16.04
+            install: clang-4.0
+          - toolset: clang
+            compiler: clang++-5.0
+            cxxstd: "03,11,14,1z"
+            os: ubuntu-16.04
+            install: clang-5.0
+          - toolset: clang
+            compiler: clang++-6.0
+            cxxstd: "03,11,14,17"
+            os: ubuntu-18.04
+          - toolset: clang
+            compiler: clang++-7
+            cxxstd: "03,11,14,17"
+            os: ubuntu-18.04
+            install: clang-7
+          - toolset: clang
+            compiler: clang++-8
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+          - toolset: clang
+            compiler: clang++-9
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+          - toolset: clang
+            compiler: clang++-10
+            cxxstd: "03,11,14,17,2a"
+            os: ubuntu-20.04
+          - 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-14.1
+            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: 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

.travis.yml

@@ -1,4 +1,4 @@
-# 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)
 
@@ -95,7 +95,7 @@ matrix:
     - os: linux
       dist: bionic
       compiler: g++-10
-      env: TOOLSET=gcc COMPILER=g++-10 CXXSTD=11,14,17,2a
+      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:
@@ -103,21 +103,6 @@ matrix:
           sources:
             - 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
-      addons:
-        apt:
-          packages:
-            - g++-8
-          sources:
-            - ubuntu-toolchain-r-test
-
-    - os: linux
-      dist: trusty
-      compiler: clang++
-      env: DIST=trusty TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z
-
     - os: linux
       compiler: clang++-3.5
       env: TOOLSET=clang COMPILER=clang++-3.5 CXXSTD=11,14,1z
@@ -180,6 +165,7 @@ matrix:
             - ubuntu-toolchain-r-test
 
     - os: linux
+      dist: bionic
       compiler: clang++-5.0
       env: TOOLSET=clang COMPILER=clang++-5.0 CXXSTD=11,14,1z
       addons:
@@ -190,6 +176,7 @@ matrix:
             - ubuntu-toolchain-r-test
 
     - os: linux
+      dist: bionic
       compiler: clang++-6.0
       env: TOOLSET=clang COMPILER=clang++-6.0 CXXSTD=11,14,17
       addons:
@@ -200,6 +187,7 @@ matrix:
             - ubuntu-toolchain-r-test
 
     - os: linux
+      dist: bionic
       compiler: clang++-7
       env: TOOLSET=clang COMPILER=clang++-7 CXXSTD=11,14,17,2a
       addons:
@@ -208,9 +196,10 @@ matrix:
             - clang-7
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-xenial-7
+            - llvm-toolchain-bionic-7
 
     - os: linux
+      dist: bionic
       compiler: clang++-8
       env: TOOLSET=clang COMPILER=clang++-8 CXXSTD=11,14,17,2a
       addons:
@@ -219,10 +208,10 @@ matrix:
             - clang-8
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-xenial-8
+            - llvm-toolchain-bionic-8
 
     - os: linux
-      dist: xenial
+      dist: bionic
       compiler: clang++-9
       env: TOOLSET=clang COMPILER=clang++-9 CXXSTD=11,14,17,2a
       addons:
@@ -231,11 +220,11 @@ matrix:
             - clang-9
           sources:
             - ubuntu-toolchain-r-test
-            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-9 main'
+            - 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: xenial
+      dist: bionic
       compiler: clang++-10
       env: TOOLSET=clang COMPILER=clang++-10 CXXSTD=11,14,17,2a
       addons:
@@ -244,19 +233,34 @@ matrix:
             - clang-10
           sources:
             - ubuntu-toolchain-r-test
-            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-10 main'
+            - sourceline: 'deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic-10 main'
               key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - os: linux
-      compiler: clang++-8
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-8 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
+      dist: bionic
+      compiler: clang++-11
+      env: TOOLSET=clang COMPILER=clang++-11 CXXSTD=11,14,17,2a
       addons:
         apt:
           packages:
-            - clang-8
+            - clang-11
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-xenial-8
+            - 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
@@ -268,18 +272,14 @@ matrix:
             - libc++-dev
 
     - os: linux
-      dist: trusty
+      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:
             - libc++-dev
 
-    - os: osx
-      compiler: clang++
-      env: TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z
-
     - os: osx
       compiler: clang++
       env: UBSAN=1 TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z UBSAN_OPTIONS=print_stacktrace=1
@@ -308,6 +308,7 @@ matrix:
     - 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

CMakeLists.txt

@@ -17,13 +17,6 @@ target_link_libraries(boost_variant2
     Boost::mp11
 )
 
-if(BOOST_SUPERPROJECT_VERSION)
-
-  include(BoostInstall)
-  boost_install(TARGETS boost_variant2 HEADER_DIRECTORY include/)
-
-endif()
-
 if(BUILD_TESTING)
 
   add_subdirectory(test)

appveyor.yml

@@ -20,15 +20,15 @@ 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
+      CXXSTD: 14,17,latest
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: msvc-14.2
       ADDRMD: 32,64
-      CXXSTD: 14,17
+      CXXSTD: 14,17,latest
 
 install:
   - set BOOST_BRANCH=develop

doc/variant2/changelog.adoc

@@ -1,5 +1,5 @@
 ////
-Copyright 2019, 2020 Peter Dimov
+Copyright 2019-2021 Peter Dimov
 
 Distributed under the Boost Software License, Version 1.0.
 
@@ -11,9 +11,15 @@ http://www.boost.org/LICENSE_1_0.txt
 # Revision History
 :idprefix: changelog_
 
+## Changes in 1.76.0
+
+* Improved generated code for the double buffered case.
+
 ## Changes in 1.74.0
 
-* Improve compilation performance for many (hundreds of) alternatives.
+* 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
 

doc/variant2/reference.adoc

@@ -134,7 +134,7 @@ template<class... T>
 
 // visit
 
-template<class F, class... V>
+template<class R = /*unspecified*/, class F, class... V>
   constexpr /*see below*/ visit(F&& f, V&&... v);
 
 // monostate
@@ -490,8 +490,8 @@ template<size_t I, class... A>
 +
 Requires: :: `I < sizeof...(T)`.
 Effects:  ::
-  Destroys the currently contained value, then initializes a new contained
-  value as if using the expression `Ti(std::forward<A>(a)...)`.
+  Initializes a new contained value as if using the expression
+  `Ti(std::forward<A>(a)...)`, then destroys the currently contained value.
 Ensures:  :: `index() == I`.
 Returns:  :: A reference to the new contained value.
 Throws:   ::
@@ -511,8 +511,8 @@ template<size_t I, class V, class... A>
 +
 Requires: :: `I < sizeof...(T)`.
 Effects:  ::
-  Destroys the currently contained value, then initializes a new contained
-  value as if using the expression `Ti(il, std::forward<A>(a)...)`.
+  Initializes a new contained value as if using the expression
+  `Ti(il, std::forward<A>(a)...)`, then destroys the currently contained value.
 Ensures:  :: `index() == I`.
 Returns:  :: A reference to the new contained value.
 Throws:   ::
@@ -838,7 +838,7 @@ Returns: ::
 ### visit
 
 ```
-template<class F, class... V>
+template<class R = /*unspecified*/, class F, class... V>
   constexpr /*see below*/ visit(F&& f, V&&... v);
 ```
 [none]
@@ -846,6 +846,10 @@ template<class F, class... V>
 +
 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.
 
 ### swap
 

include/boost/variant2/variant.hpp

@@ -81,6 +81,8 @@ struct monostate
 {
 };
 
+#if !BOOST_WORKAROUND(BOOST_MSVC, < 1940)
+
 constexpr bool operator<(monostate, monostate) noexcept { return false; }
 constexpr bool operator>(monostate, monostate) noexcept { return false; }
 constexpr bool operator<=(monostate, monostate) noexcept { return true; }
@@ -88,6 +90,17 @@ constexpr bool operator>=(monostate, monostate) noexcept { return true; }
 constexpr bool operator==(monostate, monostate) noexcept { return true; }
 constexpr bool operator!=(monostate, monostate) noexcept { return false; }
 
+#else
+
+constexpr bool operator<(monostate const&, monostate const&) noexcept { return false; }
+constexpr bool operator>(monostate const&, monostate const&) noexcept { return false; }
+constexpr bool operator<=(monostate const&, monostate const&) noexcept { return true; }
+constexpr bool operator>=(monostate const&, monostate const&) noexcept { return true; }
+constexpr bool operator==(monostate const&, monostate const&) noexcept { return true; }
+constexpr bool operator!=(monostate const&, monostate const&) noexcept { return false; }
+
+#endif
+
 // variant forward declaration
 
 template<class... T> class variant;
@@ -520,11 +533,11 @@ template<class T1, class... T> union variant_storage_impl<mp11::mp_false, T1, T.
     T1 first_;
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
     {
     }
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-1>(), std::forward<A>(a)... )
+    template<std::size_t I, class... A> constexpr variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-1>(), std::forward<A>(a)... )
     {
     }
 
@@ -564,18 +577,18 @@ template<class T0, class T1, class T2, class T3, class T4, class T5, class T6, c
 
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
+    template<std::size_t I, class... A> constexpr variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
 
     ~variant_storage_impl()
     {
@@ -637,11 +650,11 @@ template<class T1, class... T> union variant_storage_impl<mp11::mp_true, T1, T..
     T1 first_;
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
     {
     }
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-1>(), std::forward<A>(a)... )
+    template<std::size_t I, class... A> constexpr variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-1>(), std::forward<A>(a)... )
     {
     }
 
@@ -687,18 +700,18 @@ template<class T0, class T1, class T2, class T3, class T4, class T5, class T6, c
 
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
+    template<std::size_t I, class... A> constexpr variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
 
     template<class... A> void emplace_impl( mp11::mp_false, mp11::mp_size_t<0>, A&&... a ) { ::new( &t0_ ) T0( std::forward<A>(a)... ); }
     template<class... A> void emplace_impl( mp11::mp_false, mp11::mp_size_t<1>, A&&... a ) { ::new( &t1_ ) T1( std::forward<A>(a)... ); }
@@ -814,21 +827,21 @@ struct none {};
 // trivially destructible, single buffered
 template<class... T> struct variant_base_impl<true, true, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
+    unsigned ix_;
+    variant_storage<none, T...> st_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): ix_( 0 ), st_( mp11::mp_size_t<0>() )
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
     {
     }
 
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ::new( &st_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
         ix_ = I::value + 1;
     }
 
@@ -843,7 +856,7 @@ template<class... T> struct variant_base_impl<true, true, T...>
 
         assert( ix_ == J );
 
-        return st1_.get( mp11::mp_size_t<J>() );
+        return st_.get( mp11::mp_size_t<J>() );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
@@ -851,14 +864,14 @@ template<class... T> struct variant_base_impl<true, true, T...>
         // size_t const J = I+1;
         // assert( ix_ == I+1 );
 
-        return st1_.get( mp11::mp_size_t<I+1>() );
+        return st_.get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t J, class U, class... A> BOOST_CXX14_CONSTEXPR void emplace_impl( mp11::mp_true, A&&... a )
     {
         static_assert( std::is_nothrow_constructible<U, A&&...>::value, "Logic error: U must be nothrow constructible from A&&..." );
 
-        st1_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
+        st_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
         ix_ = J;
     }
 
@@ -868,7 +881,7 @@ template<class... T> struct variant_base_impl<true, true, T...>
 
         U tmp( std::forward<A>(a)... );
 
-        st1_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
+        st_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
         ix_ = J;
     }
 
@@ -884,84 +897,78 @@ template<class... T> struct variant_base_impl<true, true, T...>
 // trivially destructible, double buffered
 template<class... T> struct variant_base_impl<true, false, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
-    variant_storage<none, T...> st2_;
+    unsigned ix_;
+    variant_storage<none, T...> st_[ 2 ];
 
-    constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() ), st2_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): ix_( 0 ), st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), st2_( mp11::mp_size_t<0>() )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( ( I::value + 1 ) * 2 ), st_{ { mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... }, { mp11::mp_size_t<0>() } }
     {
     }
 
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
-        ix_ = I::value + 1;
+        ::new( &st_[ 0 ] ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ix_ = ( I::value + 1 ) * 2;
     }
 
     constexpr std::size_t index() const noexcept
     {
-        return ix_ >= 0? ix_ - 1: -ix_ - 1;
+        return ix_ / 2 - 1;
     }
 
     template<std::size_t I> BOOST_CXX14_CONSTEXPR mp11::mp_at_c<variant<T...>, I>& _get_impl( mp11::mp_size_t<I> ) noexcept
     {
+        assert( index() == I );
+
         size_t const J = I+1;
 
-        assert( ix_ == J || -ix_ == J );
-
         constexpr mp11::mp_size_t<J> j{};
-        return ix_ >= 0? st1_.get( j ): st2_.get( j );
+        return st_[ ix_ & 1 ].get( j );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
     {
+        // assert( index() == I );
         // size_t const J = I+1;
-        // assert( ix_ == J || -ix_ == J );
         // constexpr mp_size_t<J> j{};
 
-        return ix_ >= 0? st1_.get( mp11::mp_size_t<I+1>() ): st2_.get( mp11::mp_size_t<I+1>() );
+        return st_[ ix_ & 1 ].get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t I, class... A> BOOST_CXX14_CONSTEXPR void emplace( A&&... a )
     {
         size_t const J = I+1;
 
-        if( ix_ >= 0 )
-        {
-            st2_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            ix_ = -static_cast<int>( J );
-        }
-        else
-        {
-            st1_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            ix_ = J;
-        }
+        unsigned i2 = 1 - ( ix_ & 1 );
+
+        st_[ i2 ].emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
+
+        ix_ = J * 2 + i2;
     }
 };
 
 // not trivially destructible, single buffered
 template<class... T> struct variant_base_impl<false, true, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
+    unsigned ix_;
+    variant_storage<none, T...> st_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): ix_( 0 ), st_( mp11::mp_size_t<0>() )
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
     {
     }
 
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ::new( &st_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
         ix_ = I::value + 1;
     }
 
@@ -977,7 +984,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
         template<class I> void operator()( I ) const noexcept
         {
             using U = mp11::mp_at<mp11::mp_list<none, T...>, I>;
-            this_->st1_.get( I() ).~U();
+            this_->st_.get( I() ).~U();
         }
     };
 
@@ -1005,7 +1012,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
 
         assert( ix_ == J );
 
-        return st1_.get( mp11::mp_size_t<J>() );
+        return st_.get( mp11::mp_size_t<J>() );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
@@ -1013,7 +1020,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
         // size_t const J = I+1;
         // assert( ix_ == J );
 
-        return st1_.get( mp11::mp_size_t<I+1>() );
+        return st_.get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t I, class... A> void emplace( A&&... a )
@@ -1028,7 +1035,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
 
         _destroy();
 
-        st1_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
+        st_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
         ix_ = J;
     }
 };
@@ -1036,23 +1043,61 @@ template<class... T> struct variant_base_impl<false, true, T...>
 // not trivially destructible, double buffered
 template<class... T> struct variant_base_impl<false, false, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
-    variant_storage<none, T...> st2_;
+    unsigned ix_;
+
+#if defined(__GNUC__) && __GNUC__ < 11 && !defined(__clang__) && !defined(__INTEL_COMPILER)
+
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63707 :-(
+
+    variant_storage<none, T...> st1_, st2_;
 
     constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() ), st2_( mp11::mp_size_t<0>() )
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), st2_( mp11::mp_size_t<0>() )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( ( I::value + 1 ) * 2 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), st2_( mp11::mp_size_t<0>() )
     {
     }
 
+    BOOST_CXX14_CONSTEXPR variant_storage<none, T...>& storage( unsigned i2 ) noexcept
+    {
+        return i2 == 0? st1_: st2_;
+    }
+
+    constexpr variant_storage<none, T...> const& storage( unsigned i2 ) const noexcept
+    {
+        return i2 == 0? st1_: st2_;
+    }
+
+#else
+
+    variant_storage<none, T...> st_[ 2 ];
+
+    constexpr variant_base_impl(): ix_( 0 ), st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }
+    {
+    }
+
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( ( I::value + 1 ) * 2 ), st_{ { mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... }, { mp11::mp_size_t<0>() } }
+    {
+    }
+
+    BOOST_CXX14_CONSTEXPR variant_storage<none, T...>& storage( unsigned i2 ) noexcept
+    {
+        return st_[ i2 ];
+    }
+
+    constexpr variant_storage<none, T...> const& storage( unsigned i2 ) const noexcept
+    {
+        return st_[ i2 ];
+    }
+
+#endif
+
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
-        ix_ = I::value + 1;
+        ::new( &storage( 0 ) ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ix_ = ( I::value + 1 ) * 2;
     }
 
     //[&]( auto I ){
@@ -1063,35 +1108,18 @@ template<class... T> struct variant_base_impl<false, false, T...>
     struct _destroy_L1
     {
         variant_base_impl * this_;
+        unsigned i2_;
 
         template<class I> void operator()( I ) const noexcept
         {
             using U = mp11::mp_at<mp11::mp_list<none, T...>, I>;
-            this_->st1_.get( I() ).~U();
-        }
-    };
-
-    struct _destroy_L2
-    {
-        variant_base_impl * this_;
-
-        template<class I> void operator()( I ) const noexcept
-        {
-            using U = mp11::mp_at<mp11::mp_list<none, T...>, I>;
-            this_->st2_.get( I() ).~U();
+            this_->storage( i2_ ).get( I() ).~U();
         }
     };
 
     void _destroy() noexcept
     {
-        if( ix_ > 0 )
-        {
-            mp11::mp_with_index<1 + sizeof...(T)>( ix_, _destroy_L1{ this } );
-        }
-        else if( ix_ < 0 )
-        {
-            mp11::mp_with_index<1 + sizeof...(T)>( -ix_, _destroy_L2{ this } );
-        }
+        mp11::mp_with_index<1 + sizeof...(T)>( ix_ / 2, _destroy_L1{ this, ix_ & 1 } );
     }
 
     ~variant_base_impl() noexcept
@@ -1101,46 +1129,38 @@ template<class... T> struct variant_base_impl<false, false, T...>
 
     constexpr std::size_t index() const noexcept
     {
-        return ix_ >= 0? ix_ - 1: -ix_ - 1;
+        return ix_ / 2 - 1;
     }
 
     template<std::size_t I> BOOST_CXX14_CONSTEXPR mp11::mp_at_c<variant<T...>, I>& _get_impl( mp11::mp_size_t<I> ) noexcept
     {
+        assert( index() == I );
+
         size_t const J = I+1;
 
-        assert( ix_ == J || -ix_ == J );
-
         constexpr mp11::mp_size_t<J> j{};
-        return ix_ >= 0? st1_.get( j ): st2_.get( j );
+        return storage( ix_ & 1 ).get( j );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
     {
+        // assert( index() == I );
         // size_t const J = I+1;
-        // assert( ix_ == J || -ix_ == J );
         // constexpr mp_size_t<J> j{};
 
-        return ix_ >= 0? st1_.get( mp11::mp_size_t<I+1>() ): st2_.get( mp11::mp_size_t<I+1>() );
+        return storage( ix_ & 1 ).get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t I, class... A> void emplace( A&&... a )
     {
         size_t const J = I+1;
 
-        if( ix_ >= 0 )
-        {
-            st2_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            _destroy();
+        unsigned i2 = 1 - ( ix_ & 1 );
 
-            ix_ = -static_cast<int>( J );
-        }
-        else
-        {
-            st1_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            _destroy();
+        storage( i2 ).emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
+        _destroy();
 
-            ix_ = J;
-        }
+        ix_ = J * 2 + i2;
     }
 };
 
@@ -1561,7 +1581,7 @@ public:
 
     template<class U,
         class Ud = typename std::decay<U>::type,
-        class E1 = typename std::enable_if< !std::is_same<Ud, variant>::value && !detail::is_in_place_index<Ud>::value && !detail::is_in_place_type<Ud>::value >::type,
+        class E1 = typename std::enable_if< !std::is_same<Ud, variant>::value && !std::is_base_of<variant, Ud>::value && !detail::is_in_place_index<Ud>::value && !detail::is_in_place_type<Ud>::value >::type,
         class V = detail::resolve_overload_type<U&&, T...>,
         class E2 = typename std::enable_if<std::is_constructible<V, U&&>::value>::type
         >
@@ -2009,11 +2029,26 @@ template<class L> using front_if_same = mp11::mp_if<mp11::mp_apply<mp11::mp_same
 
 template<class V> using apply_cv_ref = mp11::mp_product<copy_cv_ref_t, extract_variant_base<V>, mp11::mp_list<V>>;
 
-template<class F, class... V> using Vret = front_if_same<mp11::mp_product_q<Qret<F>, apply_cv_ref<V>...>>;
+struct deduced {};
+
+#if !BOOST_WORKAROUND( BOOST_MSVC, < 1920 )
+
+template<class R, class F, class... V> using Vret = mp11::mp_eval_if_not< std::is_same<R, deduced>, R, front_if_same, mp11::mp_product_q<Qret<F>, apply_cv_ref<V>...> >;
+
+#else
+
+template<class R, class F, class... V> struct Vret_impl
+{
+    using type = mp11::mp_eval_if_not< std::is_same<R, deduced>, R, front_if_same, mp11::mp_product_q<Qret<F>, apply_cv_ref<V>...> >;
+};
+
+template<class R, class F, class... V> using Vret = typename Vret_impl<R, F, V...>::type;
+
+#endif
 
 } // namespace detail
 
-template<class F> constexpr auto visit( F&& f ) -> decltype(std::forward<F>(f)())
+template<class R = detail::deduced, class F> constexpr auto visit( F&& f ) -> detail::Vret<R, F>
 {
     return std::forward<F>(f)();
 }
@@ -2021,12 +2056,12 @@ template<class F> constexpr auto visit( F&& f ) -> decltype(std::forward<F>(f)()
 namespace detail
 {
 
-template<class F, class V1> struct visit_L1
+template<class R, class F, class V1> struct visit_L1
 {
     F&& f;
     V1&& v1;
 
-    template<class I> auto operator()( I ) const -> Vret<F, V1>
+    template<class I> auto operator()( I ) const -> Vret<R, F, V1>
     {
         return std::forward<F>(f)( unsafe_get<I::value>( std::forward<V1>(v1) ) );
     }
@@ -2034,9 +2069,9 @@ template<class F, class V1> struct visit_L1
 
 } // namespace detail
 
-template<class F, class V1> constexpr auto visit( F&& f, V1&& v1 ) -> detail::Vret<F, V1>
+template<class R = detail::deduced, class F, class V1> constexpr auto visit( F&& f, V1&& v1 ) -> detail::Vret<R, F, V1>
 {
-    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L1<F, V1>{ std::forward<F>(f), std::forward<V1>(v1) } );
+    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L1<R, F, V1>{ std::forward<F>(f), std::forward<V1>(v1) } );
 }
 
 #if defined(BOOST_NO_CXX14_GENERIC_LAMBDAS) || BOOST_WORKAROUND( BOOST_MSVC, < 1920 )
@@ -2060,31 +2095,31 @@ template<class F, class A> bind_front_<F, A> bind_front( F&& f, A&& a )
     return bind_front_<F, A>{ std::forward<F>(f), std::forward<A>(a) };
 }
 
-template<class F, class V1, class V2> struct visit_L2
+template<class R, class F, class V1, class V2> struct visit_L2
 {
     F&& f;
 
     V1&& v1;
     V2&& v2;
 
-    template<class I> auto operator()( I ) const -> Vret<F, V1, V2>
+    template<class I> auto operator()( I ) const -> Vret<R, F, V1, V2>
     {
         auto f2 = bind_front( std::forward<F>(f), unsafe_get<I::value>( std::forward<V1>(v1) ) );
-        return visit( f2, std::forward<V2>(v2) );
+        return visit<R>( f2, std::forward<V2>(v2) );
     }
 };
 
 } // namespace detail
 
-template<class F, class V1, class V2> constexpr auto visit( F&& f, V1&& v1, V2&& v2 ) -> detail::Vret<F, V1, V2>
+template<class R = detail::deduced, class F, class V1, class V2> constexpr auto visit( F&& f, V1&& v1, V2&& v2 ) -> detail::Vret<R, F, V1, V2>
 {
-    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L2<F, V1, V2>{ std::forward<F>(f), std::forward<V1>(v1), std::forward<V2>(v2) } );
+    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L2<R, F, V1, V2>{ std::forward<F>(f), std::forward<V1>(v1), std::forward<V2>(v2) } );
 }
 
 namespace detail
 {
 
-template<class F, class V1, class V2, class V3> struct visit_L3
+template<class R, class F, class V1, class V2, class V3> struct visit_L3
 {
     F&& f;
 
@@ -2092,24 +2127,24 @@ template<class F, class V1, class V2, class V3> struct visit_L3
     V2&& v2;
     V3&& v3;
 
-    template<class I> auto operator()( I ) const -> Vret<F, V1, V2, V3>
+    template<class I> auto operator()( I ) const -> Vret<R, F, V1, V2, V3>
     {
         auto f2 = bind_front( std::forward<F>(f), unsafe_get<I::value>( std::forward<V1>(v1) ) );
-        return visit( f2, std::forward<V2>(v2), std::forward<V3>(v3) );
+        return visit<R>( f2, std::forward<V2>(v2), std::forward<V3>(v3) );
     }
 };
 
 } // namespace detail
 
-template<class F, class V1, class V2, class V3> constexpr auto visit( F&& f, V1&& v1, V2&& v2, V3&& v3 ) -> detail::Vret<F, V1, V2, V3>
+template<class R = detail::deduced, class F, class V1, class V2, class V3> constexpr auto visit( F&& f, V1&& v1, V2&& v2, V3&& v3 ) -> detail::Vret<R, F, V1, V2, V3>
 {
-    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L3<F, V1, V2, V3>{ std::forward<F>(f), std::forward<V1>(v1), std::forward<V2>(v2), std::forward<V3>(v3) } );
+    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L3<R, F, V1, V2, V3>{ std::forward<F>(f), std::forward<V1>(v1), std::forward<V2>(v2), std::forward<V3>(v3) } );
 }
 
 namespace detail
 {
 
-template<class F, class V1, class V2, class V3, class V4> struct visit_L4
+template<class R, class F, class V1, class V2, class V3, class V4> struct visit_L4
 {
     F&& f;
 
@@ -2118,28 +2153,28 @@ template<class F, class V1, class V2, class V3, class V4> struct visit_L4
     V3&& v3;
     V4&& v4;
 
-    template<class I> auto operator()( I ) const -> Vret<F, V1, V2, V3, V4>
+    template<class I> auto operator()( I ) const -> Vret<R, F, V1, V2, V3, V4>
     {
         auto f2 = bind_front( std::forward<F>(f), unsafe_get<I::value>( std::forward<V1>(v1) ) );
-        return visit( f2, std::forward<V2>(v2), std::forward<V3>(v3), std::forward<V4>(v4) );
+        return visit<R>( f2, std::forward<V2>(v2), std::forward<V3>(v3), std::forward<V4>(v4) );
     }
 };
 
 } // namespace detail
 
-template<class F, class V1, class V2, class V3, class V4> constexpr auto visit( F&& f, V1&& v1, V2&& v2, V3&& v3, V4&& v4 ) -> detail::Vret<F, V1, V2, V3, V4>
+template<class R = detail::deduced, class F, class V1, class V2, class V3, class V4> constexpr auto visit( F&& f, V1&& v1, V2&& v2, V3&& v3, V4&& v4 ) -> detail::Vret<R, F, V1, V2, V3, V4>
 {
-    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L4<F, V1, V2, V3, V4>{ std::forward<F>(f), std::forward<V1>(v1), std::forward<V2>(v2), std::forward<V3>(v3), std::forward<V4>(v4) } );
+    return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), detail::visit_L4<R, F, V1, V2, V3, V4>{ std::forward<F>(f), std::forward<V1>(v1), std::forward<V2>(v2), std::forward<V3>(v3), std::forward<V4>(v4) } );
 }
 
 #else
 
-template<class F, class V1, class V2, class... V> constexpr auto visit( F&& f, V1&& v1, V2&& v2, V&&... v ) -> detail::Vret<F, V1, V2, V...>
+template<class R = detail::deduced, class F, class V1, class V2, class... V> constexpr auto visit( F&& f, V1&& v1, V2&& v2, V&&... v ) -> detail::Vret<R, F, V1, V2, V...>
 {
     return mp11::mp_with_index<detail::variant_base_size<V1>>( v1.index(), [&]( auto I ){
 
         auto f2 = [&]( auto&&... a ){ return std::forward<F>(f)( detail::unsafe_get<I.value>( std::forward<V1>(v1) ), std::forward<decltype(a)>(a)... ); };
-        return visit( f2, std::forward<V2>(v2), std::forward<V>(v)... );
+        return visit<R>( f2, std::forward<V2>(v2), std::forward<V>(v)... );
 
     });
 }

meta/libraries.json

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

test/Jamfile

@@ -114,3 +114,10 @@ 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 ;

test/variant_derived_construct.cpp

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

test/variant_emplace_index.cpp

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

test/variant_eq_ne.cpp

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

test/variant_eq_ne_cx.cpp

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

test/variant_in_place_index_construct_cx.cpp

@@ -105,7 +105,7 @@ int main()
         STATIC_ASSERT( v.index() == 4 );
     }
 
-#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 100200)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 110000)
 
     // no idea why this fails on g++ 10
 

test/variant_in_place_type_construct_cx.cpp

@@ -102,7 +102,7 @@ int main()
         STATIC_ASSERT( holds_alternative<X>(v) );
     }
 
-#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 100200)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 110000)
 
     // no idea why this fails on g++ 10
 

test/variant_lt_gt.cpp

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

test/variant_lt_gt_cx.cpp

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

test/variant_visit.cpp

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

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();
+}