glob headers

.github/workflows/ci.yml

@@ -19,18 +19,15 @@ jobs:
         include:
           - toolset: gcc-4.8
             cxxstd: "03,11"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             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
+            os: ubuntu-18.04
+            install: g++-5
           - toolset: gcc-6
             cxxstd: "03,11,14,1z"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             install: g++-6
           - toolset: gcc-7
             cxxstd: "03,11,14,17"
@@ -41,44 +38,29 @@ jobs:
             install: g++-8
           - toolset: gcc-9
             cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
           - toolset: gcc-10
             cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
-          - toolset: clang
+            install: g++-10
-            compiler: clang++-3.5
+          - toolset: gcc-11
-            cxxstd: "03,11,14"
+            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-16.04
+            os: ubuntu-20.04
-            install: clang-3.5
+            install: g++-11
-          - 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
+            os: ubuntu-18.04
             install: clang-3.9
           - toolset: clang
             compiler: clang++-4.0
             cxxstd: "03,11,14"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
             cxxstd: "03,11,14,1z"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             install: clang-5.0
           - toolset: clang
             compiler: clang++-6.0
@@ -92,21 +74,29 @@ jobs:
             install: clang-7
           - toolset: clang
             compiler: clang++-8
-            cxxstd: "03,11,14,17,2a"
+            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
@@ -157,6 +147,10 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          - toolset: msvc-14.0
+            cxxstd: "14"
+            addrmd: 32,64
+            os: windows-2019
           - toolset: msvc-14.1
             cxxstd: "14,17,latest"
             addrmd: 32,64
@@ -165,6 +159,10 @@ jobs:
             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
@@ -202,3 +200,159 @@ jobs:
         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

CMakeLists.txt

@@ -1,8 +1,11 @@
-# Copyright 2018, 2019 Peter Dimov
+# Generated by `boostdep --cmake variant2`
+# Copyright 2020, 2021 Peter Dimov
 # Distributed under the Boost Software License, Version 1.0.
-# http://www.boost.org/LICENSE_1_0.txt
+# https://www.boost.org/LICENSE_1_0.txt
 
-cmake_minimum_required(VERSION 3.5...3.16)
+if(NOT DEFINED IDF_TARGET)
+
+cmake_minimum_required(VERSION 3.8...3.20)
 
 project(boost_variant2 VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
 
@@ -13,12 +16,34 @@ target_include_directories(boost_variant2 INTERFACE include)
 
 target_link_libraries(boost_variant2
   INTERFACE
+    Boost::assert
     Boost::config
     Boost::mp11
 )
 
-if(BUILD_TESTING)
+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()
+
+

README.md

@@ -1,21 +1,18 @@
-# variant2
+# Boost.Variant2
 
 This repository contains a never-valueless, strong guarantee, C++11/14/17
 implementation of [std::variant](http://en.cppreference.com/w/cpp/utility/variant).
 See [the documentation](https://www.boost.org/libs/variant2)
 for more information.
 
-The code requires [Boost.Mp11](https://github.com/boostorg/mp11) and
+The library is part of Boost, starting from release 1.71. It depends on
-Boost.Config.
+Boost.Mp11, Boost.Config, and Boost.Assert.
-
-The library is part of Boost, starting from release 1.71, but the header
-`variant.hpp` will also work [standalone](https://godbolt.org/z/nVUNKX).
 
 Supported compilers:
 
 * g++ 4.8 or later with `-std=c++11` or above
-* clang++ 3.5 or later with `-std=c++11` or above
+* clang++ 3.9 or later with `-std=c++11` or above
 * Visual Studio 2015, 2017, 2019
 
-Tested on [Travis](https://travis-ci.org/boostorg/variant2/) and
+Tested on [Github Actions](https://github.com/boostorg/variant2/actions) and
 [Appveyor](https://ci.appveyor.com/project/pdimov/variant2-fkab9).

appveyor.yml

@@ -1,4 +1,4 @@
-# Copyright 2016-2020 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)
 
@@ -29,6 +29,10 @@ environment:
       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
@@ -47,4 +51,4 @@ build: off
 test_script:
   - if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
   - if not "%ADDRMD%" == "" set ADDRMD=address-model=%ADDRMD%
-  - b2 -j3 libs/variant2/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release
+  - b2 -j3 libs/variant2/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release embed-manifest-via=linker

doc/variant2.adoc

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

doc/variant2/changelog.adoc

@@ -1,16 +1,24 @@
 ////
 Copyright 2019-2021 Peter Dimov
-
 Distributed under the Boost Software License, Version 1.0.
-
+https://www.boost.org/LICENSE_1_0.txt
-See accompanying file LICENSE_1_0.txt or copy at
-http://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.

doc/variant2/design.adoc

@@ -169,6 +169,10 @@ The main differences between this implementation and `std::variant` are:
   `variant<int, float>` is provided as the member function `subset<U...>`.
   (This operation can throw if the current state of the variant cannot be
   represented.)
+* `unsafe_get`, an unchecked alternative to `get` and `get_if`, is provided
+  as an extension.
+* `visit_by_index`, a visitation function that takes a single variant and a
+  number of function objects, one per alternative, is provided as an extension.
 * The {cpp}20 additions and changes to `std::variant` have not yet been
   implemented.
 

doc/variant2/implementation.adoc

@@ -1,10 +1,7 @@
 ////
 Copyright 2019 Peter Dimov
-
 Distributed under the Boost Software License, Version 1.0.
-
+https://www.boost.org/LICENSE_1_0.txt
-See accompanying file LICENSE_1_0.txt or copy at
-http://www.boost.org/LICENSE_1_0.txt
 ////
 
 [#implementation]
@@ -13,13 +10,13 @@ http://www.boost.org/LICENSE_1_0.txt
 
 ## Dependencies
 
-This implementation only depends on Boost.Config and Boost.Mp11.
+This implementation only depends on Boost.Config, Boost.Assert, and Boost.Mp11.
 
 ## Supported Compilers
 
 * GCC 4.8 or later with `-std=c++11` or above
-* Clang 3.5 or later with `-std=c++11` or above
+* Clang 3.9 or later with `-std=c++11` or above
 * Visual Studio 2015, 2017, 2019
 
-Tested on https://travis-ci.org/boostorg/variant2/[Travis] and
+Tested on https://github.com/boostorg/variant2/actions[Github Actions] and
 https://ci.appveyor.com/project/pdimov/variant2-fkab9[Appveyor].

doc/variant2/reference.adoc

@@ -1,10 +1,7 @@
 ////
-Copyright 2018, 2019 Peter Dimov
+Copyright 2018-2021 Peter Dimov
-
 Distributed under the Boost Software License, Version 1.0.
-
+https://www.boost.org/LICENSE_1_0.txt
-See accompanying file LICENSE_1_0.txt or copy at
-http://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 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>
+template<class Ch, class Tr, class... T>
-  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
+  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
 
@@ -775,6 +802,34 @@ Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
 Effects:  :: Equivalent to: `return get_if<I>(v);` with `I` being
   the zero-based index of `U` in `T...`.
 
+### unsafe_get (extension)
+
+```
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&
+    unsafe_get(variant<T...>& v);
+```
+```
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&&
+    unsafe_get(variant<T...>&& v);
+```
+```
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&
+    unsafe_get(const variant<T...>& v);
+```
+```
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&&
+    unsafe_get(const variant<T...>&& v);
+```
+[none]
+* {blank}
++
+Requires: :: `v.index() == I`.
+Returns: :: a reference to the object stored in the variant.
+
 ### Relational Operators
 
 ```
@@ -835,6 +890,18 @@ template<class... T>
 Returns: ::
   `w \<= v`.
 
+### swap
+
+```
+template<class... T>
+  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
+```
+[none]
+* {blank}
++
+Effects: ::
+  Equivalent to `v.swap(w)`.
+
 ### visit
 
 ```
@@ -851,17 +918,50 @@ Remarks: :: If `R` is given explicitly, as in `visit<int>`, the return
   of `F` to the variant alternatives must have the same return type for
   this deduction to succeed.
 
-### swap
+### visit_by_index (extension)
 
 ```
-template<class... T>
+template<class R = /*unspecified*/, class V, class... F>
-  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
+  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: ::
-  Equivalent to `v.swap(w)`.
+  `os << "monostate"`.
+Returns: ::
+  `os`.
 
 ### bad_variant_access
 
@@ -878,3 +978,7 @@ public:
     }
 };
 ```
+
+## <boost/variant2.hpp>
+
+This convenience header includes `<boost/variant2/variant.hpp>`.

include/boost/variant2.hpp

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

include/boost/variant2/variant.hpp

@@ -13,20 +13,19 @@
 # pragma warning( disable: 4521 4522 ) // multiple copy operators
 #endif
 
-#ifndef BOOST_MP11_HPP_INCLUDED
 #include <boost/mp11.hpp>
-#endif
+#include <boost/assert.hpp>
 #include <boost/config.hpp>
 #include <boost/detail/workaround.hpp>
 #include <boost/cstdint.hpp>
 #include <cstddef>
 #include <type_traits>
 #include <exception>
-#include <cassert>
 #include <initializer_list>
 #include <utility>
 #include <functional> // std::hash
 #include <cstdint>
+#include <iosfwd>
 
 //
 
@@ -367,37 +366,50 @@ template<std::size_t I, class... T> constexpr variant_alternative_t<I, variant<T
 #endif
 }
 
-// detail::unsafe_get (for visit)
+// unsafe_get
 
-namespace detail
+#if !defined(BOOST_NO_CXX14_CONSTEXPR)
-{
+# define BOOST_VARIANT2_CX14_ASSERT(expr) BOOST_ASSERT(expr);
+#else
+# define BOOST_VARIANT2_CX14_ASSERT(expr)
+#endif
 
 template<std::size_t I, class... T> constexpr variant_alternative_t<I, variant<T...>>& unsafe_get(variant<T...>& v)
 {
     static_assert( I < sizeof...(T), "Index out of bounds" );
+
+    BOOST_VARIANT2_CX14_ASSERT( v.index() == I )
+
     return v._get_impl( mp11::mp_size_t<I>() );
 }
 
 template<std::size_t I, class... T> constexpr variant_alternative_t<I, variant<T...>>&& unsafe_get(variant<T...>&& v)
 {
     static_assert( I < sizeof...(T), "Index out of bounds" );
+
+    BOOST_VARIANT2_CX14_ASSERT( v.index() == I )
+
     return std::move( v._get_impl( mp11::mp_size_t<I>() ) );
 }
 
 template<std::size_t I, class... T> constexpr variant_alternative_t<I, variant<T...>> const& unsafe_get(variant<T...> const& v)
 {
     static_assert( I < sizeof...(T), "Index out of bounds" );
+
+    BOOST_VARIANT2_CX14_ASSERT( v.index() == I )
+
     return v._get_impl( mp11::mp_size_t<I>() );
 }
 
 template<std::size_t I, class... T> constexpr variant_alternative_t<I, variant<T...>> const&& unsafe_get(variant<T...> const&& v)
 {
     static_assert( I < sizeof...(T), "Index out of bounds" );
+
+    BOOST_VARIANT2_CX14_ASSERT( v.index() == I )
+
     return std::move( v._get_impl( mp11::mp_size_t<I>() ) );
 }
 
-} // namespace detail
-
 // get (type)
 
 template<class U, class... T> constexpr U& get(variant<T...>& v)
@@ -830,14 +842,14 @@ struct none {};
 // trivially destructible, single buffered
 template<class... T> struct variant_base_impl<true, true, T...>
 {
-    unsigned ix_;
     variant_storage<none, T...> st_;
+    unsigned ix_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): st_( mp11::mp_size_t<0>() ), ix_( 0 )
     {
     }
 
-    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)... )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): st_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), ix_( I::value + 1 )
     {
     }
 
@@ -857,7 +869,7 @@ template<class... T> struct variant_base_impl<true, true, T...>
     {
         size_t const J = I+1;
 
-        assert( ix_ == J );
+        BOOST_ASSERT( ix_ == J );
 
         return st_.get( mp11::mp_size_t<J>() );
     }
@@ -865,7 +877,8 @@ template<class... T> struct variant_base_impl<true, true, T...>
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
     {
         // size_t const J = I+1;
-        // assert( ix_ == I+1 );
+
+        BOOST_VARIANT2_CX14_ASSERT( ix_ == I+1 )
 
         return st_.get( mp11::mp_size_t<I+1>() );
     }
@@ -900,14 +913,14 @@ template<class... T> struct variant_base_impl<true, true, T...>
 // trivially destructible, double buffered
 template<class... T> struct variant_base_impl<true, false, T...>
 {
-    unsigned ix_;
     variant_storage<none, T...> st_[ 2 ];
+    unsigned ix_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }
+    constexpr variant_base_impl(): st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }, ix_( 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>() } }
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): st_{ { mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... }, { mp11::mp_size_t<0>() } }, ix_( ( I::value + 1 ) * 2 )
     {
     }
 
@@ -925,7 +938,7 @@ template<class... T> struct variant_base_impl<true, false, T...>
 
     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 );
+        BOOST_ASSERT( index() == I );
 
         size_t const J = I+1;
 
@@ -935,7 +948,8 @@ template<class... T> struct variant_base_impl<true, false, T...>
 
     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 );
+        BOOST_VARIANT2_CX14_ASSERT( index() == I )
+
         // size_t const J = I+1;
         // constexpr mp_size_t<J> j{};
 
@@ -957,14 +971,14 @@ template<class... T> struct variant_base_impl<true, false, T...>
 // not trivially destructible, single buffered
 template<class... T> struct variant_base_impl<false, true, T...>
 {
-    unsigned ix_;
     variant_storage<none, T...> st_;
+    unsigned ix_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): st_( mp11::mp_size_t<0>() ), ix_( 0 )
     {
     }
 
-    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)... )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): st_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), ix_( I::value + 1 )
     {
     }
 
@@ -1013,7 +1027,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
     {
         size_t const J = I+1;
 
-        assert( ix_ == J );
+        BOOST_ASSERT( ix_ == J );
 
         return st_.get( mp11::mp_size_t<J>() );
     }
@@ -1021,7 +1035,8 @@ template<class... T> struct variant_base_impl<false, true, T...>
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
     {
         // size_t const J = I+1;
-        // assert( ix_ == J );
+
+        BOOST_VARIANT2_CX14_ASSERT( ix_ == I+1 )
 
         return st_.get( mp11::mp_size_t<I+1>() );
     }
@@ -1046,19 +1061,18 @@ 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...>
 {
-    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_;
+    unsigned ix_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() ), st2_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): st1_( mp11::mp_size_t<0>() ), st2_( mp11::mp_size_t<0>() ), ix_( 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>() )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), st2_( mp11::mp_size_t<0>() ), ix_( ( I::value + 1 ) * 2 )
     {
     }
 
@@ -1075,12 +1089,13 @@ template<class... T> struct variant_base_impl<false, false, T...>
 #else
 
     variant_storage<none, T...> st_[ 2 ];
+    unsigned ix_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }
+    constexpr variant_base_impl(): st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }, ix_( 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>() } }
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): st_{ { mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... }, { mp11::mp_size_t<0>() } }, ix_( ( I::value + 1 ) * 2 )
     {
     }
 
@@ -1137,7 +1152,7 @@ template<class... T> struct variant_base_impl<false, false, T...>
 
     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 );
+        BOOST_ASSERT( index() == I );
 
         size_t const J = I+1;
 
@@ -1147,7 +1162,8 @@ template<class... T> struct variant_base_impl<false, false, T...>
 
     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 );
+        BOOST_VARIANT2_CX14_ASSERT( index() == I )
+
         // size_t const J = I+1;
         // constexpr mp_size_t<J> j{};
 
@@ -2176,7 +2192,7 @@ template<class R = detail::deduced, class F, class V1, class V2, class... V> con
 {
     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)... ); };
+        auto f2 = [&]( auto&&... a ){ return std::forward<F>(f)( unsafe_get<I.value>( std::forward<V1>(v1) ), std::forward<decltype(a)>(a)... ); };
         return visit<R>( f2, std::forward<V2>(v2), std::forward<V>(v)... );
 
     });
@@ -2193,6 +2209,75 @@ void swap( variant<T...> & v, variant<T...> & w )
     v.swap( w );
 }
 
+// visit_by_index
+
+namespace detail
+{
+
+template<class R, class V, class... F> using Vret2 = mp11::mp_eval_if_not< std::is_same<R, deduced>, R, front_if_same, mp11::mp_transform<mp11::mp_invoke_q, mp11::mp_list<Qret<F>...>, apply_cv_ref<V>> >;
+
+template<class R, class V, class... F> struct visit_by_index_L
+{
+    V&& v;
+    std::tuple<F&&...> tp;
+
+    template<class I> constexpr detail::Vret2<R, V, F...> operator()( I ) const
+    {
+        return std::get<I::value>( std::move(tp) )( unsafe_get<I::value>( std::forward<V>(v) ) );
+    }
+};
+
+} // namespace detail
+
+template<class R = detail::deduced, class V, class... F> constexpr auto visit_by_index( V&& v, F&&... f ) -> detail::Vret2<R, V, F...>
+{
+    static_assert( variant_size<V>::value == sizeof...(F), "Incorrect number of function objects" );
+
+    return mp11::mp_with_index<variant_size<V>::value>( v.index(),
+        detail::visit_by_index_L<R, V, F...>{ std::forward<V>(v), std::tuple<F&&...>( std::forward<F>(f)... ) } );
+}
+
+// output streaming
+
+namespace detail
+{
+
+template<class Ch, class Tr, class... T> struct ostream_insert_L
+{
+    std::basic_ostream<Ch, Tr>& os;
+    variant<T...> const& v;
+
+    template<class I> std::basic_ostream<Ch, Tr>& operator()( I ) const
+    {
+        return os << unsafe_get<I::value>( v );
+    }
+};
+
+template<class Os, class T, class E = void> struct is_output_streamable: std::false_type
+{
+};
+
+template<class Os, class T> struct is_output_streamable<Os, T, decltype( std::declval<Os&>() << std::declval<T const&>(), (void)0 )>: std::true_type
+{
+};
+
+} // namespace detail
+
+template<class Ch, class Tr>
+std::basic_ostream<Ch, Tr>& operator<<( std::basic_ostream<Ch, Tr>& os, monostate const& )
+{
+    os << "monostate";
+    return os;
+}
+
+template<class Ch, class Tr, class T1, class... T,
+    class E = typename std::enable_if< mp11::mp_all< detail::is_output_streamable<std::basic_ostream<Ch, Tr>, T>... >::value >::type >
+std::basic_ostream<Ch, Tr>& operator<<( std::basic_ostream<Ch, Tr>& os, variant<T1, T...> const& v )
+{
+    return mp11::mp_with_index<1 + sizeof...(T)>( v.index(),
+        detail::ostream_insert_L<Ch, Tr, T1, T...>{ os, v } );
+}
+
 // hashing support
 
 namespace detail
@@ -2307,6 +2392,8 @@ template<> struct hash< ::boost::variant2::monostate >
 
 } // namespace std
 
+#undef BOOST_VARIANT2_CX14_ASSERT
+
 #if defined(_MSC_VER) && _MSC_VER < 1910
 # pragma warning( pop )
 #endif

test/Jamfile

@@ -121,3 +121,8 @@ run variant_visit_r.cpp : : :
     ;
 
 compile variant_derived_construct.cpp ;
+
+run variant_visit_by_index.cpp ;
+
+run variant_ostream_insert.cpp ;
+run is_output_streamable.cpp ;

test/cmake_subdir_test/CMakeLists.txt

@@ -7,6 +7,8 @@ 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)
 

test/is_output_streamable.cpp

@@ -0,0 +1,41 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <ostream>
+#include <string>
+
+struct X
+{
+};
+
+struct Y
+{
+};
+
+std::ostream& operator<<( std::ostream& os, Y const& /*y*/ )
+{
+    os << "Y()";
+    return os;
+}
+
+int main()
+{
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, int>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, float>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, std::string>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, void>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, X>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, Y>));
+
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::monostate>));
+
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, std::string>>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, X>>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, Y>>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<boost::variant2::monostate, int, float>>));
+
+    return boost::report_errors();
+}

test/quick.cpp

@@ -1,10 +1,8 @@
-
 // Copyright 2019 Peter Dimov.
-//
 // Distributed under the Boost Software License, Version 1.0.
-// http://www.boost.org/LICENSE_1_0.txt
+// https://www.boost.org/LICENSE_1_0.txt
 
-#include <boost/variant2/variant.hpp>
+#include <boost/variant2.hpp>
 
 using namespace boost::variant2;
 

test/variant_in_place_index_construct_cx.cpp

@@ -105,9 +105,9 @@ int main()
         STATIC_ASSERT( v.index() == 4 );
     }
 
-#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 110000)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 120000)
 
-    // no idea why this fails on g++ 10
+    // no idea why this fails on g++ 10/11
 
 #else
 

test/variant_in_place_type_construct_cx.cpp

@@ -102,9 +102,9 @@ int main()
         STATIC_ASSERT( holds_alternative<X>(v) );
     }
 
-#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 110000)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 120000)
 
-    // no idea why this fails on g++ 10
+    // no idea why this fails on g++ 10/11
 
 #else
 

test/variant_ostream_insert.cpp

@@ -0,0 +1,43 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <sstream>
+#include <string>
+
+using namespace boost::variant2;
+
+template<class T> std::string to_string( T const& t )
+{
+    std::ostringstream os;
+
+    os << t;
+
+    return os.str();
+}
+
+int main()
+{
+    {
+        BOOST_TEST_EQ( to_string( monostate() ), "monostate" );
+    }
+
+    {
+        variant<monostate, int, float, std::string> v;
+
+        BOOST_TEST_EQ( to_string( v ), to_string( monostate() ) );
+
+        v = 1;
+        BOOST_TEST_EQ( to_string( v ), to_string( 1 ) );
+
+        v = 3.14f;
+        BOOST_TEST_EQ( to_string( v ), to_string( 3.14f ) );
+
+        v = "test";
+        BOOST_TEST_EQ( to_string( v ), to_string( "test" ) );
+    }
+
+    return boost::report_errors();
+}

test/variant_visit_by_index.cpp

@@ -0,0 +1,126 @@
+// Copyright 2017, 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/mp11.hpp>
+#include <boost/config.hpp>
+
+using namespace boost::variant2;
+using boost::mp11::mp_int;
+
+struct X
+{
+};
+
+struct F1
+{
+    int operator()( X& ) const { return 1; }
+    int operator()( X const& ) const { return 2; }
+    int operator()( X&& ) const { return 3; }
+    int operator()( X const&& ) const { return 4; }
+};
+
+struct F2
+{
+    mp_int<1> operator()( X& ) const { return {}; }
+    mp_int<2> operator()( X const& ) const { return {}; }
+    mp_int<3> operator()( X&& ) const { return {}; }
+    mp_int<4> operator()( X const&& ) const { return {}; }
+};
+
+int main()
+{
+    {
+        variant<int, int, float> v;
+
+        visit_by_index( v,
+            []( int& x ){ BOOST_TEST_EQ( x, 0 ); },
+            []( int&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> v( in_place_index_t<0>(), 1 );
+
+        visit_by_index( v,
+            []( int const& x ){ BOOST_TEST_EQ( x, 1 ); },
+            []( int&         ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int, int, float> const v( in_place_index_t<1>(), 2 );
+
+        visit_by_index( v,
+            []( int const&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const& x ){ BOOST_TEST_EQ( x, 2 ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> const v( 3.14f );
+
+        visit_by_index( v,
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& x ){ BOOST_TEST_EQ( x, 3.14f ); } );
+    }
+
+    {
+        variant<int, float> const v( 7 );
+
+        auto r = visit_by_index( v,
+            []( int const& x   ) -> double { return x; },
+            []( float const& x ) -> double { return x; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 7.0 );
+    }
+
+    {
+        variant<int, float> const v( 2.0f );
+
+        auto r = visit_by_index( v,
+            []( int const& x   ) { return x + 0.0; },
+            []( float const& x ) { return x + 0.0; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 2.0 );
+    }
+
+    {
+        variant<int, float, double> const v( 3.0 );
+
+        auto r = visit_by_index<double>( v,
+            []( int const& x    ) { return x; },
+            []( float const& x  ) { return x; },
+            []( double const& x ) { return x; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 3.0 );
+    }
+
+    {
+        variant<X> v;
+        variant<X> const cv;
+
+        F1 f1;
+
+        BOOST_TEST_EQ( visit_by_index( v, f1 ), 1 );
+        BOOST_TEST_EQ( visit_by_index( cv, f1 ), 2 );
+        BOOST_TEST_EQ( visit_by_index( std::move( v ), f1 ), 3 );
+        BOOST_TEST_EQ( visit_by_index( std::move( cv ), f1 ), 4 );
+
+        F2 f2;
+
+        BOOST_TEST_EQ( visit_by_index<int>( v, f2 ), 1 );
+        BOOST_TEST_EQ( visit_by_index<int>( cv, f2 ), 2 );
+        BOOST_TEST_EQ( visit_by_index<int>( std::move( v ), f2 ), 3 );
+        BOOST_TEST_EQ( visit_by_index<int>( std::move( cv ), f2 ), 4 );
+    }
+
+    return boost::report_errors();
+}

test/variant_visit_derived.cpp

@@ -1,4 +1,3 @@
-
 // Copyright 2017, 2020 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
@@ -15,7 +14,7 @@
 
 struct X: boost::variant2::variant<int, float>
 {
-#if BOOST_WORKAROUND( BOOST_MSVC, < 1930 )
+#if BOOST_WORKAROUND( BOOST_MSVC, < 1940 )
 
     template<class T> explicit X( T&& t ): variant( std::forward<T>( t ) ) {};