Add missing std::move

.drone.jsonnet

@@ -0,0 +1,263 @@
+# Copyright 2022 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+local library = "variant2";
+
+local triggers =
+{
+    branch: [ "master", "develop", "feature/*" ]
+};
+
+local ubsan = { UBSAN: '1', UBSAN_OPTIONS: 'print_stacktrace=1' };
+local asan = { ASAN: '1' };
+
+local linux_pipeline(name, image, environment, packages = "", sources = [], arch = "amd64") =
+{
+    name: name,
+    kind: "pipeline",
+    type: "docker",
+    trigger: triggers,
+    platform:
+    {
+        os: "linux",
+        arch: arch
+    },
+    steps:
+    [
+        {
+            name: "everything",
+            image: image,
+            environment: environment,
+            commands:
+            [
+                'set -e',
+                'wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -',
+            ] +
+            (if sources != [] then [ ('apt-add-repository "' + source + '"') for source in sources ] else []) +
+            (if packages != "" then [ 'apt-get update', 'apt-get -y install ' + packages ] else []) +
+            [
+                'export LIBRARY=' + library,
+                './.drone/drone.sh',
+            ]
+        }
+    ]
+};
+
+local macos_pipeline(name, environment, xcode_version = "12.2", osx_version = "catalina", arch = "amd64") =
+{
+    name: name,
+    kind: "pipeline",
+    type: "exec",
+    trigger: triggers,
+    platform: {
+        "os": "darwin",
+        "arch": arch
+    },
+    node: {
+        "os": osx_version
+    },
+    steps: [
+        {
+            name: "everything",
+            environment: environment + { "DEVELOPER_DIR": "/Applications/Xcode-" + xcode_version + ".app/Contents/Developer" },
+            commands:
+            [
+                'export LIBRARY=' + library,
+                './.drone/drone.sh',
+            ]
+        }
+    ]
+};
+
+local windows_pipeline(name, image, environment, arch = "amd64") =
+{
+    name: name,
+    kind: "pipeline",
+    type: "docker",
+    trigger: triggers,
+    platform:
+    {
+        os: "windows",
+        arch: arch
+    },
+    "steps":
+    [
+        {
+            name: "everything",
+            image: image,
+            environment: environment,
+            commands:
+            [
+                'cmd /C .drone\\\\drone.bat ' + library,
+            ]
+        }
+    ]
+};
+
+[
+    linux_pipeline(
+        "Linux 14.04 GCC 4.8*",
+        "cppalliance/droneubuntu1404:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11' },
+    ),
+
+    linux_pipeline(
+        "Linux 14.04 GCC 4.9",
+        "cppalliance/droneubuntu1404:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-4.9', CXXSTD: '03,11' },
+        "g++-4.9",
+        [ "ppa:ubuntu-toolchain-r/test" ],
+    ),
+
+    linux_pipeline(
+        "Linux 16.04 GCC 5*",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14' },
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 GCC 6",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-6', CXXSTD: '03,11,14' },
+        "g++-6",
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 GCC 7* 32/64",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14,17', ADDRMD: '32,64' },
+    ),
+
+    linux_pipeline(
+        "Linux 18.04 GCC 8",
+        "cppalliance/droneubuntu1804:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-8', CXXSTD: '03,11,14,17' },
+        "g++-8",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 9* 32",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14,17,2a', ADDRMD: '32' },
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 9* 64",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14,17,2a', ADDRMD: '64' },
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 9* ARM64",
+        "cppalliance/droneubuntu2004:multiarch",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14,17,2a' },
+        arch="arm64",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 9* S390x",
+        "cppalliance/droneubuntu2004:multiarch",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14,17,2a' },
+        arch="s390x",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 10 32 ASAN",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-10', CXXSTD: '03,11,14,17,20', ADDRMD: '32' } + asan,
+        "g++-10-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 GCC 10 64 ASAN",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-10', CXXSTD: '03,11,14,17,20', ADDRMD: '64' } + asan,
+        "g++-10-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 GCC 11* 32",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14,17,2a', ADDRMD: '32' },
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 GCC 11* 64",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '03,11,14,17,2a', ADDRMD: '64' },
+    ),
+
+    linux_pipeline(
+        "Linux 22.04 GCC 12",
+        "cppalliance/droneubuntu2204:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-12', CXXSTD: '03,11,14,17,20,2b' },
+        "g++-12",
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 13",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-13', CXXSTD: '03,11,14,17,20' },
+        "clang-13",
+        ["deb http://apt.llvm.org/focal/ llvm-toolchain-focal-13 main"],
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 14 UBSAN",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-14', CXXSTD: '03,11,14,17,20' } + ubsan,
+        "clang-14",
+        ["deb http://apt.llvm.org/focal/ llvm-toolchain-focal-14 main"],
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 14 ASAN",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-14', CXXSTD: '03,11,14,17,20' } + asan,
+        "clang-14",
+        ["deb http://apt.llvm.org/focal/ llvm-toolchain-focal-14 main"],
+    ),
+
+    linux_pipeline(
+        "Linux 20.04 Clang 15",
+        "cppalliance/droneubuntu2004:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-15', CXXSTD: '03,11,14,17,20,2b' },
+        "clang-15",
+        ["deb http://apt.llvm.org/focal/ llvm-toolchain-focal-15 main"],
+    ),
+
+    macos_pipeline(
+        "MacOS 10.15 Xcode 12.2 UBSAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,1z' } + ubsan,
+    ),
+
+    macos_pipeline(
+        "MacOS 10.15 Xcode 12.2 ASAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,1z' } + asan,
+    ),
+
+    windows_pipeline(
+        "Windows VS2015 msvc-14.0",
+        "cppalliance/dronevs2015",
+        { TOOLSET: 'msvc-14.0', CXXSTD: '14,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2017 msvc-14.1",
+        "cppalliance/dronevs2017",
+        { TOOLSET: 'msvc-14.1', CXXSTD: '14,17,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2019 msvc-14.2",
+        "cppalliance/dronevs2019",
+        { TOOLSET: 'msvc-14.2', CXXSTD: '14,17,20,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2022 msvc-14.3",
+        "cppalliance/dronevs2022:1",
+        { TOOLSET: 'msvc-14.3', CXXSTD: '14,17,20,latest' },
+    ),
+]

.drone/drone.bat

@@ -0,0 +1,23 @@
+@REM Copyright 2022 Peter Dimov
+@REM Distributed under the Boost Software License, Version 1.0.
+@REM https://www.boost.org/LICENSE_1_0.txt
+
+@ECHO ON
+
+set LIBRARY=%1
+set DRONE_BUILD_DIR=%CD%
+
+set BOOST_BRANCH=develop
+if "%DRONE_BRANCH%" == "master" set BOOST_BRANCH=master
+cd ..
+git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
+cd boost-root
+git submodule update --init tools/boostdep
+xcopy /s /e /q %DRONE_BUILD_DIR% libs\%LIBRARY%\
+python tools/boostdep/depinst/depinst.py %LIBRARY%
+cmd /c bootstrap
+b2 -d0 headers
+
+if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
+if not "%ADDRMD%" == "" set ADDRMD=address-model=%ADDRMD%
+b2 -j3 libs/%LIBRARY%/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release embed-manifest-via=linker

.drone/drone.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+# Copyright 2022 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+set -ex
+
+DRONE_BUILD_DIR=$(pwd)
+
+BOOST_BRANCH=develop
+if [ "$DRONE_BRANCH" = "master" ]; then BOOST_BRANCH=master; fi
+
+cd ..
+git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
+cd boost-root
+git submodule update --init tools/boostdep
+cp -r $DRONE_BUILD_DIR/* libs/$LIBRARY
+python tools/boostdep/depinst/depinst.py $LIBRARY
+./bootstrap.sh
+./b2 -d0 headers
+
+echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
+./b2 -j3 libs/$LIBRARY/test toolset=$TOOLSET cxxstd=$CXXSTD variant=debug,release ${ADDRMD:+address-model=$ADDRMD} ${UBSAN:+undefined-sanitizer=norecover debug-symbols=on} ${ASAN:+address-sanitizer=norecover debug-symbols=on} ${LINKFLAGS:+linkflags=$LINKFLAGS}

.github/workflows/ci.yml

@@ -19,22 +19,26 @@ jobs:
         include:
           - toolset: gcc-4.8
             cxxstd: "03,11"
-            os: ubuntu-18.04
+            os: ubuntu-latest
+            container: ubuntu:18.04
             install: g++-4.8
           - toolset: gcc-5
             cxxstd: "03,11,14,1z"
-            os: ubuntu-18.04
+            os: ubuntu-latest
+            container: ubuntu:18.04
             install: g++-5
           - toolset: gcc-6
             cxxstd: "03,11,14,1z"
-            os: ubuntu-18.04
+            os: ubuntu-latest
+            container: ubuntu:18.04
             install: g++-6
           - toolset: gcc-7
             cxxstd: "03,11,14,17"
-            os: ubuntu-18.04
+            os: ubuntu-latest
+            container: ubuntu:18.04
           - toolset: gcc-8
             cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             install: g++-8
           - toolset: gcc-9
             cxxstd: "03,11,14,17,2a"
@@ -47,30 +51,37 @@ jobs:
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-20.04
             install: g++-11
+          - toolset: gcc-12
+            cxxstd: "03,11,14,17,20,2b"
+            os: ubuntu-22.04
+            install: g++-12
           - toolset: clang
             compiler: clang++-3.9
             cxxstd: "03,11,14"
-            os: ubuntu-18.04
+            os: ubuntu-latest
+            container: ubuntu:18.04
             install: clang-3.9
           - toolset: clang
             compiler: clang++-4.0
             cxxstd: "03,11,14"
-            os: ubuntu-18.04
+            os: ubuntu-latest
+            container: ubuntu:18.04
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
             cxxstd: "03,11,14,1z"
-            os: ubuntu-18.04
+            os: ubuntu-latest
+            container: ubuntu:18.04
             install: clang-5.0
           - toolset: clang
             compiler: clang++-6.0
             cxxstd: "03,11,14,17"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             install: clang-6.0
           - toolset: clang
             compiler: clang++-7
             cxxstd: "03,11,14,17"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             install: clang-7
           - toolset: clang
             compiler: clang++-8
@@ -97,18 +108,39 @@ jobs:
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-20.04
             install: clang-12
+          - toolset: clang
+            compiler: clang++-13
+            cxxstd: "03,11,14,17,20,2b"
+            os: ubuntu-22.04
+            install: clang-13
+          - toolset: clang
+            compiler: clang++-14
+            cxxstd: "03,11,14,17,20,2b"
+            os: ubuntu-22.04
+            install: clang-14
           - toolset: clang
             cxxstd: "03,11,14,17,2a"
-            os: macos-10.15
+            os: macos-11
 
     runs-on: ${{matrix.os}}
+    container: ${{matrix.container}}
+
+    defaults:
+      run:
+        shell: bash
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
+
+      - name: Setup container environment
+        if: matrix.container
+        run: |
+          apt-get update
+          apt-get -y install sudo python git g++
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |
@@ -148,18 +180,18 @@ jobs:
       matrix:
         include:
           - toolset: msvc-14.0
-            cxxstd: "14"
+            cxxstd: "14,latest"
             addrmd: 32,64
             os: windows-2019
-          - toolset: msvc-14.1
-            cxxstd: "14,17,latest"
-            addrmd: 32,64
-            os: windows-2016
           - toolset: msvc-14.2
-            cxxstd: "14,17,latest"
+            cxxstd: "14,17,20,latest"
             addrmd: 32,64
             os: windows-2019
           - toolset: msvc-14.3
+            cxxstd: "14,17,20,latest"
+            addrmd: 32,64
+            os: windows-2022
+          - toolset: clang-win
             cxxstd: "14,17,latest"
             addrmd: 32,64
             os: windows-2022
@@ -171,7 +203,7 @@ jobs:
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Setup Boost
         shell: cmd
@@ -199,25 +231,25 @@ jobs:
         shell: cmd
         run: |
           cd ../boost-root
-          b2 -j3 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release
+          b2 -j3 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release embed-manifest-via=linker
 
   posix-cmake-subdir:
     strategy:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-18.04
           - os: ubuntu-20.04
-          - os: macos-10.15
+          - os: ubuntu-22.04
+          - os: macos-11
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |
@@ -252,18 +284,18 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-18.04
           - os: ubuntu-20.04
-          - os: macos-10.15
+          - os: ubuntu-22.04
+          - os: macos-11
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |
@@ -308,18 +340,18 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-18.04
           - os: ubuntu-20.04
-          - os: macos-10.15
+          - os: ubuntu-22.04
+          - os: macos-11
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: sudo apt-get -y install ${{matrix.install}}
 
       - name: Setup Boost
         run: |

doc/variant2/changelog.adoc

@@ -8,6 +8,10 @@ https://www.boost.org/LICENSE_1_0.txt
 # Revision History
 :idprefix: changelog_
 
+## Changes in 1.79.0
+
+* Added `operator<<` for `monostate`.
+
 ## Changes in 1.78.0
 
 * Added `<boost/variant2.hpp>`.

doc/variant2/reference.adoc

@@ -144,12 +144,6 @@ template<class... T>
 template<class... T>
   constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);
 
-// 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 );
-
 // swap
 
 template<class... T>
@@ -176,6 +170,16 @@ constexpr bool operator>(monostate, monostate) noexcept { return false; }
 constexpr bool operator<=(monostate, monostate) noexcept { return true; }
 constexpr bool operator>=(monostate, monostate) noexcept { return true; }
 
+// 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 );
+
+template<class Ch, class Tr>
+  std::basic_ostream<Ch, Tr>&
+    operator<<( std::basic_ostream<Ch, Tr>& os, monostate const& v );
+
 // bad_variant_access
 
 class bad_variant_access;
@@ -886,20 +890,6 @@ template<class... T>
 Returns: ::
   `w \<= v`.
 
-### 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()`.
-
 ### swap
 
 ```
@@ -946,6 +936,33 @@ Remarks: :: If `R` is given explicitly, as in `visit_by_index<int>`, the return
   of `Fi` to the corresponding variant alternatives must have the same return type
   for this deduction to succeed.
 
+### Stream Insertion (extension)
+```
+template<class Ch, class Tr, class... T>
+  std::basic_ostream<Ch, Tr>&
+    operator<<( std::basic_ostream<Ch, Tr>& os, variant<T...> const& v );
+```
+[none]
+* {blank}
++
+Requires: ::
+  `sizeof...(T) != 0`.
+Returns: ::
+  `os << get<I>(v)`, where `I` is `v.index()`.
+
+```
+template<class Ch, class Tr>
+  std::basic_ostream<Ch, Tr>&
+    operator<<( std::basic_ostream<Ch, Tr>& os, monostate const& v );
+```
+[none]
+* {blank}
++
+Effects: ::
+  `os << "monostate"`.
+Returns: ::
+  `os`.
+
 ### bad_variant_access
 
 ```

include/boost/variant2/variant.hpp

@@ -15,8 +15,9 @@
 
 #include <boost/mp11.hpp>
 #include <boost/assert.hpp>
+#include <boost/assert/source_location.hpp>
 #include <boost/config.hpp>
-#include <boost/detail/workaround.hpp>
+#include <boost/config/workaround.hpp>
 #include <boost/cstdint.hpp>
 #include <cstddef>
 #include <type_traits>
@@ -24,8 +25,9 @@
 #include <initializer_list>
 #include <utility>
 #include <functional> // std::hash
-#include <cstdint>
 #include <iosfwd>
+#include <cstdint>
+#include <cerrno>
 
 //
 
@@ -685,7 +687,17 @@ template<class T1, class... T> union variant_storage_impl<mp11::mp_true, T1, T..
 
     template<std::size_t I, class... A> BOOST_CXX14_CONSTEXPR void emplace_impl( mp11::mp_true, mp11::mp_size_t<I>, A&&... a )
     {
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+# pragma GCC diagnostic push
+// False positive in at least GCC 7 and GCC 10 ASAN triggered by monostate (via result<void>)
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
         *this = variant_storage_impl( mp11::mp_size_t<I>(), std::forward<A>(a)... );
+
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+# pragma GCC diagnostic pop
+#endif
     }
 
     template<std::size_t I, class... A> BOOST_CXX14_CONSTEXPR void emplace( mp11::mp_size_t<I>, A&&... a )
@@ -2263,6 +2275,13 @@ template<class Os, class T> struct is_output_streamable<Os, T, decltype( std::de
 
 } // 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 )
@@ -2385,6 +2404,104 @@ template<> struct hash< ::boost::variant2::monostate >
 
 } // namespace std
 
+// JSON support
+
+namespace boost
+{
+namespace json
+{
+
+class value;
+
+struct value_from_tag;
+
+template<class T>
+void value_from( T&& t, value& jv );
+
+template<class T>
+struct try_value_to_tag;
+
+template<class T1, class T2>
+struct result_for;
+
+template<class T>
+typename result_for<T, value>::type
+try_value_to( value const & jv );
+
+template<class T>
+typename result_for<T, value>::type
+result_from_errno( int e, boost::source_location const* loc ) noexcept;
+
+template<class T> struct is_null_like;
+
+template<> struct is_null_like<variant2::monostate>: std::true_type {};
+
+} // namespace json
+
+namespace variant2
+{
+
+namespace detail
+{
+
+struct tag_invoke_L1
+{
+    boost::json::value& v;
+
+    template<class T> void operator()( T const& t ) const
+    {
+        boost::json::value_from( t, v );
+    }
+};
+
+} // namespace detail
+
+template<class... T>
+    void tag_invoke( boost::json::value_from_tag const&, boost::json::value& v, variant<T...> const & w )
+{
+    visit( detail::tag_invoke_L1{ v }, w );
+}
+
+namespace detail
+{
+
+template<class V> struct tag_invoke_L2
+{
+    boost::json::value const& v;
+    typename boost::json::result_for<V, boost::json::value>::type& r;
+
+    template<class I> void operator()( I i ) const
+    {
+        if( !r )
+        {
+            using Ti = mp11::mp_at_c<V, i>;
+            auto r2 = boost::json::try_value_to<Ti>( v );
+
+            if( r2 )
+            {
+                r.emplace( in_place_index_t<i>{}, std::move( *r2 ) );
+            }
+        }
+    }
+};
+
+} // namespace detail
+
+template<class... T>
+    typename boost::json::result_for<variant<T...>, boost::json::value>::type
+    tag_invoke( boost::json::try_value_to_tag<variant<T...>> const&, boost::json::value const& v )
+{
+    static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
+    auto r = boost::json::result_from_errno< variant<T...> >( EINVAL, &loc );
+
+    mp11::mp_for_each<mp11::mp_iota_c<sizeof...(T)>>( detail::tag_invoke_L2< variant<T...> >{ v, r } );
+
+    return r;
+}
+
+} // namespace variant2
+} // namespace boost
+
 #undef BOOST_VARIANT2_CX14_ASSERT
 
 #if defined(_MSC_VER) && _MSC_VER < 1910

test/Jamfile

@@ -126,3 +126,8 @@ run variant_visit_by_index.cpp ;
 
 run variant_ostream_insert.cpp ;
 run is_output_streamable.cpp ;
+
+local JSON = <library>/boost//json/<warnings>off "<toolset>msvc-14.0:<build>no" "<toolset>msvc-14.2:<cxxflags>-wd5104" ;
+
+run variant_json_value_from.cpp : : : $(JSON) ;
+run variant_json_value_to.cpp : : : $(JSON) ;

test/is_output_streamable.cpp

@@ -30,9 +30,12 @@ int main()
     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/variant_emplace_index.cpp

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

test/variant_json_value_from.cpp

@@ -0,0 +1,41 @@
+// Copyright 2022 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/json/value_from.hpp>
+#include <boost/json/serialize.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <string>
+
+using namespace boost::variant2;
+namespace json = boost::json;
+
+int main()
+{
+    {
+        monostate m;
+        json::value w = json::value_from( m );
+        BOOST_TEST_EQ( w, json::value( nullptr ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v;
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( nullptr ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v( 17 );
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( 17 ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v( "test" );
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( "test" ) );
+    }
+
+    return boost::report_errors();
+}

test/variant_json_value_to.cpp

@@ -0,0 +1,42 @@
+// Copyright 2022 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/json/value_to.hpp>
+#include <boost/json/serialize.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::variant2;
+namespace json = boost::json;
+
+int main()
+{
+    {
+        json::value v;
+        auto r = json::try_value_to<monostate>( v );
+        BOOST_TEST( r.has_value() );
+    }
+
+    using V = variant<monostate, int, std::string>;
+
+    {
+        json::value v;
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V() );
+    }
+
+    {
+        json::value v( 12 );
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V(12) );
+    }
+
+    {
+        json::value v( "test" );
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V("test") );
+    }
+
+    return boost::report_errors();
+}

test/variant_ostream_insert.cpp

@@ -20,17 +20,24 @@ template<class T> std::string to_string( T const& t )
 
 int main()
 {
-    variant<int, float, std::string> v( 1 );
+    {
+        BOOST_TEST_EQ( to_string( monostate() ), "monostate" );
+    }
 
-    BOOST_TEST_EQ( to_string( v ), to_string( 1 ) );
+    {
+        variant<monostate, int, float, std::string> v;
 
-    v = 3.14f;
+        BOOST_TEST_EQ( to_string( v ), to_string( monostate() ) );
 
-    BOOST_TEST_EQ( to_string( v ), to_string( 3.14f ) );
+        v = 1;
+        BOOST_TEST_EQ( to_string( v ), to_string( 1 ) );
 
-    v = "test";
+        v = 3.14f;
+        BOOST_TEST_EQ( to_string( v ), to_string( 3.14f ) );
 
-    BOOST_TEST_EQ( to_string( v ), to_string( "test" ) );
+        v = "test";
+        BOOST_TEST_EQ( to_string( v ), to_string( "test" ) );
+    }
 
     return boost::report_errors();
 }