Use extern c++ in the module and add GCC + MSVC runners

.github/workflows/ci.yml

@@ -1325,7 +1325,7 @@ jobs:
           git submodule update --init tools/boostdep
           python3 tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
 
-      - name: Build with the standard library module and int128 module, then test
+      - name: Configure
         run: |
           cd ../boost-root
           mkdir __build__ && cd __build__
@@ -1334,9 +1334,167 @@ jobs:
             -DBUILD_TESTING=ON \
             -DBOOST_INT128_BUILD_MODULE=ON \
             -DCMAKE_CXX_COMPILER=clang++-$LLVM_VERSION \
-            -DCMAKE_CXX_FLAGS="-stdlib=libc++" \
+            -DCMAKE_CXX_FLAGS="-stdlib=libc++ -Wno-reserved-module-identifier" \
             -DCMAKE_EXPERIMENTAL_CXX_IMPORT_STD=$IMPORT_STD_UUID \
             -DCMAKE_CXX_STDLIB_MODULES_JSON=/usr/lib/llvm-$LLVM_VERSION/lib/libc++.modules.json \
             ..
+
+      # Kept as a separate step so the boost.int128 module build time is visible on its own.
+      - name: Build the Boost.Int128 module
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target boost_int128_module
+
+      - name: Build and run the tests
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target tests
+          ctest --output-on-failure --no-tests=error
+
+  cmake-module-test-gcc:
+    strategy:
+      fail-fast: false
+
+    runs-on: ubuntu-latest
+
+    # GCC 15 (the first release with import std support and
+    # libstdc++.modules.json) is not packaged for the runner image, so the job
+    # runs in a container that ships it.
+    container: ubuntu:26.04
+
+    env:
+      GCC_VERSION: "15"
+      # Pinned so the experimental `import std` feature UUID below stays valid.
+      CMAKE_VERSION: 4.4.0
+      # CMake gates `import std` behind this UUID; it is specific to the CMake version.
+      IMPORT_STD_UUID: f35a9ac6-8463-4d38-8eec-5d6008153e7d
+
+    steps:
+      - name: Install toolchain
+        run: |
+          apt-get update
+          apt-get install -y g++-$GCC_VERSION gcc-$GCC_VERSION ninja-build wget ca-certificates git python3
+          wget -q https://github.com/Kitware/CMake/releases/download/v$CMAKE_VERSION/cmake-$CMAKE_VERSION-linux-x86_64.tar.gz
+          tar -xzf cmake-$CMAKE_VERSION-linux-x86_64.tar.gz -C /opt
+          echo "/opt/cmake-$CMAKE_VERSION-linux-x86_64/bin" >> "$GITHUB_PATH"
+
+      - uses: actions/checkout@v6
+
+      - 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 $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
+          cd boost-root
+          mkdir -p libs/$LIBRARY
+          cp -r $GITHUB_WORKSPACE/* libs/$LIBRARY
+          git submodule update --init tools/boostdep
+          python3 tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
+
+      - name: Configure
+        run: |
+          cd ../boost-root
+          mkdir __build__ && cd __build__
+          cmake -G Ninja \
+            -DBOOST_INCLUDE_LIBRARIES=$LIBRARY \
+            -DBUILD_TESTING=ON \
+            -DBOOST_INT128_BUILD_MODULE=ON \
+            -DCMAKE_C_COMPILER=gcc-$GCC_VERSION \
+            -DCMAKE_CXX_COMPILER=g++-$GCC_VERSION \
+            -DCMAKE_EXPERIMENTAL_CXX_IMPORT_STD=$IMPORT_STD_UUID \
+            ..
+
+      # Kept as a separate step so the boost.int128 module build time is visible on its own.
+      - name: Build the Boost.Int128 module
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target boost_int128_module
+
+      - name: Build and run the tests
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target tests
+          ctest --output-on-failure --no-tests=error
+
+  cmake-module-test-msvc:
+    strategy:
+      fail-fast: false
+
+    runs-on: windows-2025
+
+    env:
+      # Pinned so the experimental import std feature UUID below stays valid.
+      CMAKE_VERSION: 4.4.0
+      # CMake gates import std behind this UUID; it is specific to the CMake version.
+      IMPORT_STD_UUID: f35a9ac6-8463-4d38-8eec-5d6008153e7d
+
+    steps:
+      - uses: actions/checkout@v6
+
+      # Puts cl.exe and the MSVC INCLUDE/LIB environment on PATH for the later steps.
+      - name: Set up the MSVC environment
+        uses: ilammy/msvc-dev-cmd@v1
+
+      - name: Install toolchain
+        shell: pwsh
+        run: |
+          # A pinned CMake (the import std UUID is tied to the version) and Ninja
+          # (required for C++ modules).
+          curl.exe -L -o cmake.zip "https://github.com/Kitware/CMake/releases/download/v$env:CMAKE_VERSION/cmake-$env:CMAKE_VERSION-windows-x86_64.zip"
+          Expand-Archive -Path cmake.zip -DestinationPath C:\tools
+          "C:\tools\cmake-$env:CMAKE_VERSION-windows-x86_64\bin" | Out-File -FilePath $env:GITHUB_PATH -Append -Encoding utf8
+          choco install ninja -y
+
+      - name: Setup Boost
+        shell: pwsh
+        run: |
+          $LIBRARY = ($env:GITHUB_REPOSITORY -split '/')[-1]
+          "LIBRARY=$LIBRARY" | Out-File -FilePath $env:GITHUB_ENV -Append -Encoding utf8
+          $ref = if ($env:GITHUB_BASE_REF) { $env:GITHUB_BASE_REF } else { $env:GITHUB_REF }
+          $ref = $ref -replace '^refs/heads/', ''
+          $branch = if ($ref -eq 'master') { 'master' } else { 'develop' }
+          cd ..
+          git clone -b $branch --depth 1 https://github.com/boostorg/boost.git boost-root
+          cd boost-root
+          New-Item -ItemType Directory -Force -Path "libs/$LIBRARY" | Out-Null
+          Copy-Item -Recurse -Force "$env:GITHUB_WORKSPACE/*" "libs/$LIBRARY"
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
+
+      - name: Configure
+        shell: pwsh
+        run: |
+          cd ../boost-root
+          New-Item -ItemType Directory -Force -Path __build__ | Out-Null
+          cd __build__
+          cmake -G Ninja `
+            -DBOOST_INCLUDE_LIBRARIES="$env:LIBRARY" `
+            -DBUILD_TESTING=ON `
+            -DBOOST_INT128_BUILD_MODULE=ON `
+            -DCMAKE_CXX_COMPILER=cl `
+            -DCMAKE_EXPERIMENTAL_CXX_IMPORT_STD="$env:IMPORT_STD_UUID" `
+            ..
+
+      # Kept as a separate step so the boost.int128 module build time is visible on its own.
+      - name: Build the Boost.Int128 module
+        shell: pwsh
+        run: |
+          cd ../boost-root/__build__
+          cmake --build . --target boost_int128_module
+
+      - name: Build and run the tests
+        shell: pwsh
+        run: |
+          cd ../boost-root/__build__
           cmake --build . --target tests
           ctest --output-on-failure --no-tests=error

doc/modules/ROOT/pages/getting_started.adoc

@@ -77,7 +77,7 @@ When compiling with pass:[C++20] or newer, the library can be consumed as a name
 import boost.int128;
 ----
 
-Building the module requires defining `BOOST_INT128_BUILD_MODULE` and compiling `module/int128.cxx` with your toolchain's module support. See xref:config.adoc[Configuration Macros] for details.
+Building the module requires defining `BOOST_INT128_BUILD_MODULE` and compiling `module/int128.cppm` with your toolchain's module support. See xref:config.adoc[Configuration Macros] for details.
 
 == CUDA Support
 

module/CMakeLists.txt

@@ -16,12 +16,18 @@ endif()
 add_library(boost_int128_module)
 target_sources(boost_int128_module
         PUBLIC
-        FILE_SET int128_module TYPE CXX_MODULES FILES int128.cxx
+        FILE_SET int128_module TYPE CXX_MODULES FILES int128.cppm
 )
 target_link_libraries(boost_int128_module PUBLIC Boost::int128)
 target_compile_features(boost_int128_module PUBLIC cxx_std_23)
 
-# int128.cxx defines BOOST_INT128_BUILD_MODULE itself, but the tests link this
+# The purview includes the library's angled <boost/int128...> headers by design.
+# clang-scan-deps warns on that and ignores -Wno-include-angled-in-module-purview,
+# so -w keeps the dependency-scan output clean (this glue unit's code is fully
+# warned on in the header builds).
+target_compile_options(boost_int128_module PRIVATE $<$<CXX_COMPILER_ID:Clang>:-w>)
+
+# int128.cppm defines BOOST_INT128_BUILD_MODULE itself, but the tests link this
 # target and need the macro too so they import the module instead of including
 # the headers directly.
 target_compile_definitions(boost_int128_module PUBLIC BOOST_INT128_BUILD_MODULE)
@@ -68,6 +74,15 @@ set(BOOST_INT128_MODULE_TESTS
         test_powm
 )
 
+# GCC's libstdc++ cannot yet mix the test sources' textual standard library
+# includes with import std in one consumer translation unit, so only the quick
+# test runs there. MSVC is treated the same way until its full consumer suite has
+# been validated. The full list runs under clang/libc++.
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+    message(STATUS "Boost.Int128 module: ${CMAKE_CXX_COMPILER_ID} consumer tests limited to module_quick_test")
+    set(BOOST_INT128_MODULE_TESTS)
+endif()
+
 add_executable(module_quick_test "${CMAKE_CURRENT_SOURCE_DIR}/quick_test.cpp")
 target_link_libraries(module_quick_test PRIVATE boost_int128_module)
 # The superproject pins an older policy version (CMP0155), so C++ sources are not

module/Jamfile

@@ -11,6 +11,11 @@ import modules ;
 import testing ;
 import path ;
 import pch ;
+import type ;
+
+# b2 registers the standard C++ suffixes (cpp cxx cc) but not cppm, so teach it
+# that the .cppm module interface unit is a C++ source.
+type.register-suffixes cppm : CPP ;
 
 project
     : requirements
@@ -24,7 +29,7 @@ project
       <toolset>clang:<cxxstd>23
     ;
 
-obj int128 : int128.cxx : <define>BOOST_INT128_EXPORT_TESTING <toolset>msvc:<cxxflags>-interface ;
+obj int128 : int128.cppm : <define>BOOST_INT128_EXPORT_TESTING <toolset>msvc:<cxxflags>-interface ;
 
 run quick_test.cpp int128 : : : <dependency>int128 ;
 run ../test/test_bit.cpp int128 : : : <dependency>int128 ;

module/int128.cxx → module/int128.cppm

@@ -65,6 +65,10 @@ export module boost.int128;
 import std;
 #endif
 
+// Wrap the exported declarations in extern "C++" so they attach to the global
+// module (classic mangling), keeping the module and header forms ABI compatible.
+extern "C++" {
+
 export namespace boost::int128 {
 
 struct int128_t;
@@ -97,3 +101,5 @@ class numeric_limits<boost::int128::uint128_t>;
 #elif defined(__clang__)
 #  pragma clang diagnostic pop
 #endif
+
+} // extern "C++"