Implement initial version of C++20 module boost.type_index (#15)

`#include <boost/type_index...` is now implicitly does `import boost.type_index` if the modules are supported 
All the library internals now have unconditional module level linkage.

Significant differences from https://anarthal.github.io/cppblog/modules3:
* `BOOST_TYPE_INDEX_USE_STD_MODULE` macro switch for `import std;` / `includes` while building module. This allows to use module in C++20 and even without usable  `std` module.

Author: apolukhin

.github/workflows/ci.yml

@@ -49,10 +49,10 @@ jobs:
             compiler: clang++-14
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-22.04
-          - toolset: clang
-            compiler: clang++-10
-            cxxstd: "03,11,14,17,2a"
-            os: ubuntu-20.04
+          - toolset: clang-19
+            cxxstd: "20,23"
+            os: ubuntu-24.04
+            install: clang-19 llvm-19 libclang-rt-19-dev libc++-19-dev libc++abi-19-dev clang-tools-19
 
     runs-on: ${{matrix.os}}
 
@@ -88,6 +88,31 @@ jobs:
           ./b2 -d0 headers
           ./b2 -j4 variant=debug tools/inspect/build
 
+      - name: Run modules tests wihtout 'import std;'
+        if: ${{matrix.toolset == 'clang-19'}}
+        run: |
+          cd ../boost-root/libs/type_index
+          mkdir build_module
+          cd build_module
+          cmake -DBOOST_USE_MODULES=1 -DBUILD_TESTING=1 -GNinja -DCMAKE_CXX_COMPILER=clang++-19 ../test/cmake_subdir_test/
+          cmake --build .
+          ctest -V
+          cd ..
+          rm -rf build_module
+
+      - name: Run modules tests
+        if: false
+        #if: ${{matrix.toolset == 'clang-19'}}
+        run: |
+          cd ../boost-root/libs/type_index
+          mkdir build_module
+          cd build_module
+          cmake -DBUILD_TESTING=1 -DBOOST_USE_MODULES=1 -DCMAKE_CXX_COMPILER=clang++-19 -DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-stdlib=libc++ -DCMAKE_CXX_STANDARD=23 -DCMAKE_EXPERIMENTAL_CXX_IMPORT_STD=0e5b6991-d74f-4b3d-a41c-cf096e0b2508 -G Ninja ../test/cmake_subdir_test/
+          cmake --build .
+          ctest -V
+          cd ..
+          rm -rf build_module
+
       - name: Run tests
         run: |
           cd ../boost-root
@@ -175,6 +200,36 @@ jobs:
           cmd /c bootstrap
           b2 -d0 headers
 
+      - name: Run modules tests
+        if: ${{matrix.toolset == 'msvc-14.3'}}
+        shell: cmd
+        run: |
+          choco install --no-progress ninja
+          call "C:/Program Files/Microsoft Visual Studio/2022/Enterprise/VC/Auxiliary/Build/vcvarsall.bat" x64
+          cd ../boost-root/libs/type_index
+          mkdir build_module
+          cd build_module
+          cmake -DBOOST_USE_MODULES=1 -DBUILD_TESTING=1 -DCMAKE_CXX_STANDARD=23 -DCMAKE_EXPERIMENTAL_CXX_IMPORT_STD=0e5b6991-d74f-4b3d-a41c-cf096e0b2508 -G Ninja ../test/cmake_subdir_test/
+          cmake --build .
+          ctest --no-tests=error -V
+          cd ..
+          rm -rf build_module
+
+      - name: Run modules tests without 'import std;'
+        if: ${{matrix.toolset == 'msvc-14.3'}}
+        shell: cmd
+        run: |
+          choco install --no-progress ninja
+          call "C:/Program Files/Microsoft Visual Studio/2022/Enterprise/VC/Auxiliary/Build/vcvarsall.bat" x64
+          cd ../boost-root/libs/type_index
+          mkdir build_module
+          cd build_module
+          cmake -DBOOST_USE_MODULES=1 -DBUILD_TESTING=1 -DCMAKE_CXX_STANDARD=20 -G Ninja ../test/cmake_subdir_test/
+          cmake --build .
+          ctest --no-tests=error -V
+          cd ..
+          rm -rf build_module
+
       - name: Run tests
         shell: cmd
         run: |

CMakeLists.txt

@@ -3,17 +3,35 @@
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
 
-cmake_minimum_required(VERSION 3.5...3.20)
+cmake_minimum_required(VERSION 3.5...3.31)
 
 project(boost_type_index VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
 
-add_library(boost_type_index INTERFACE)
-add_library(Boost::type_index ALIAS boost_type_index)
+if (BOOST_USE_MODULES)
+    add_library(boost_type_index)
+    target_sources(boost_type_index PUBLIC
+        FILE_SET modules_public TYPE CXX_MODULES FILES modules/boost_type_index.cppm
+    )
+
+    target_compile_features(boost_type_index PUBLIC cxx_std_20)
+    target_compile_definitions(boost_type_index PUBLIC BOOST_USE_MODULES)
+    if (CMAKE_CXX_COMPILER_IMPORT_STD)
+        target_compile_definitions(boost_type_index PRIVATE BOOST_TYPE_INDEX_USE_STD_MODULE)
+        message(STATUS "Using `import std;`")
+    else()
+        message(STATUS "`import std;` is not awailable")
+    endif()
+    set(__scope PUBLIC)
+else()
+    add_library(boost_type_index INTERFACE)
+    set(__scope INTERFACE)
+endif()
 
-target_include_directories(boost_type_index INTERFACE include)
+target_include_directories(boost_type_index ${__scope} include)
+add_library(Boost::type_index ALIAS boost_type_index)
 
 target_link_libraries(boost_type_index
-  INTERFACE
+  ${__scope}
     Boost::config
     Boost::container_hash
     Boost::core

doc/type_index.qbk

@@ -1,6 +1,6 @@
 [library Boost.TypeIndex
     [quickbook 1.6]
-    [version 4.1]
+    [version 4.2]
     [copyright 2012-2025 Antony Polukhin]
     [category Language Features Emulation]
     [license
@@ -314,6 +314,40 @@ Sometimes there may be a need to create your own type info system. This may be u
 
 [endsect]
 
+[section C++20 module]
+
+[caution C++20 module support is on early stage, targets, flags and behavior may change in the future]
+
+If using modern CMake define CMake option `-DBOOST_USE_MODULES=1` to build a C++20 module and
+make the `Boost::type_index` CMake target provide it. After that an explicit usage of C++20 module `boost.type_index` is allowed:
+
+[import ../modules/usage_sample.cpp]
+[type_index_module_example]
+
+The `Boost::type_index` CMake target gives an ability to mix includes and imports of the library in different translation units. Moreover,
+if `BOOST_USE_MODULES` macro is defined then all the `boost/type_index/...` includes implicilty do `import boost.type_index;` to give all the
+benifits of modules without changing the existing code.
+
+[note For better compile times make sure that `import std;` is available when building the `boost.type_index` module (in CMake logs there should be
+      a 'Using `import std;`' message). ]
+
+If not using CMake, then the module could be build manually from the `modules/boost_type_index.cppm` file.
+
+For manual module build the following commands can be used for clang compiler:
+
+```
+cd type_index/modules
+clang++ -I ../include -std=c++20 --precompile -x c++-module boost_type_index.cppm
+```
+
+After that, the module could be used in the following way:
+
+```
+clang++ -std=c++20 -fmodule-file=boost_type_index.pcm boost_type_index.pcm usage_sample.cpp
+```
+
+[endsect]
+
 
 [section Space and Performance]
 

include/boost/type_index.hpp

@@ -14,7 +14,7 @@
 /// By inclusion of this file most optimal type index classes will be included and used 
 /// as a boost::typeindex::type_index and boost::typeindex::type_info.
 
-#include <boost/config.hpp>
+#include <boost/type_index/detail/config.hpp>
 
 #ifdef BOOST_HAS_PRAGMA_ONCE
 # pragma once
@@ -49,8 +49,12 @@
 #define BOOST_TYPE_INDEX_REGISTER_CLASS
 #endif
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+
 namespace boost { namespace typeindex {
 
+BOOST_TYPE_INDEX_BEGIN_MODULE_EXPORT
+
 #if defined(BOOST_TYPE_INDEX_DOXYGEN_INVOKED)
 
 /// \def BOOST_TYPE_INDEX_FUNCTION_SIGNATURE
@@ -257,9 +261,11 @@ inline type_index type_id_runtime(const T& runtime_val) noexcept {
     return type_index::type_id_runtime(runtime_val);
 }
 
-}} // namespace boost::typeindex
+BOOST_TYPE_INDEX_END_MODULE_EXPORT
 
+}} // namespace boost::typeindex
 
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
 
 #endif // BOOST_TYPE_INDEX_HPP
 

include/boost/type_index/ctti_type_index.hpp

@@ -18,12 +18,19 @@
 /// It is used in situations when typeid() method is not available or 
 /// BOOST_TYPE_INDEX_FORCE_NO_RTTI_COMPATIBILITY macro is defined.
 
+#include <boost/type_index/detail/config.hpp>
+
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+
 #include <boost/type_index/type_index_facade.hpp>
 #include <boost/type_index/detail/compile_time_type_info.hpp>
 
+#if !defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
 #include <cstring>
 #include <type_traits>
+
 #include <boost/container_hash/hash.hpp>
+#endif
 
 #ifdef BOOST_HAS_PRAGMA_ONCE
 # pragma once
@@ -64,6 +71,8 @@ class ctti_data {
 
 } // namespace detail
 
+BOOST_TYPE_INDEX_BEGIN_MODULE_EXPORT
+
 /// Helper method for getting detail::ctti_data of a template parameter T.
 template <class T>
 inline const detail::ctti_data& ctti_construct() noexcept {
@@ -132,6 +141,8 @@ class ctti_type_index: public type_index_facade<ctti_type_index, detail::ctti_da
     inline static ctti_type_index type_id_runtime(const T& variable) noexcept;
 };
 
+BOOST_TYPE_INDEX_END_MODULE_EXPORT
+
 
 inline const ctti_type_index::type_info_t& ctti_type_index::type_info() const noexcept {
     return *reinterpret_cast<const detail::ctti_data*>(data_);
@@ -197,8 +208,9 @@ inline std::size_t ctti_type_index::hash_code() const noexcept {
     return boost::hash_range(raw_name(), raw_name() + get_raw_name_length());
 }
 
-
 }} // namespace boost::typeindex
 
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+
 #endif // BOOST_TYPE_INDEX_CTTI_TYPE_INDEX_HPP
 

include/boost/type_index/detail/compile_time_type_info.hpp

@@ -13,10 +13,12 @@
 /// \brief Contains helper macros and implementation details of boost::typeindex::ctti_type_index.
 /// Not intended for inclusion from user's code.
 
-#include <boost/config.hpp>
+#include <boost/type_index/detail/config.hpp>
 
+#if !defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
 #include <cstring>
 #include <type_traits>
+#endif
 
 #ifdef BOOST_HAS_PRAGMA_ONCE
 # pragma once

include/boost/type_index/detail/config.hpp

@@ -0,0 +1,32 @@
+//
+// Copyright 2013-2025 Antony Polukhin.
+//
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BOOST_TYPE_INDEX_DETAIL_CONFIG_HPP
+#define BOOST_TYPE_INDEX_DETAIL_CONFIG_HPP
+
+#if !defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+#include <boost/config.hpp>
+#ifdef BOOST_HAS_PRAGMA_ONCE
+# pragma once
+#endif
+#endif
+
+#ifdef BOOST_TYPE_INDEX_INTERFACE_UNIT
+#   define BOOST_TYPE_INDEX_BEGIN_MODULE_EXPORT export {
+#   define BOOST_TYPE_INDEX_END_MODULE_EXPORT }
+#else
+#   define BOOST_TYPE_INDEX_BEGIN_MODULE_EXPORT
+#   define BOOST_TYPE_INDEX_END_MODULE_EXPORT
+#endif
+
+#if defined(BOOST_USE_MODULES) && !defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+import boost.type_index;
+#endif
+
+#endif // BOOST_TYPE_INDEX_DETAIL_CONFIG_HPP
+

include/boost/type_index/detail/ctti_register_class.hpp

@@ -19,15 +19,23 @@
 # pragma once
 #endif
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+
 namespace boost { namespace typeindex { namespace detail {
 
+BOOST_TYPE_INDEX_BEGIN_MODULE_EXPORT
+
 template <class T>
 inline const ctti_data& ctti_construct_typeid_ref(const T*) noexcept {
     return boost::typeindex::ctti_construct<T>();
 }
 
+BOOST_TYPE_INDEX_END_MODULE_EXPORT
+
 }}} // namespace boost::typeindex::detail
 
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+
 /// @cond
 #define BOOST_TYPE_INDEX_REGISTER_CLASS                                                                       \
     virtual const boost::typeindex::detail::ctti_data& boost_type_index_type_id_runtime_() const noexcept {   \

include/boost/type_index/detail/stl_register_class.hpp

@@ -19,15 +19,23 @@
 # pragma once
 #endif
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+
 namespace boost { namespace typeindex { namespace detail {
 
+BOOST_TYPE_INDEX_BEGIN_MODULE_EXPORT
+
 template <class T>
 inline const stl_type_index::type_info_t& stl_construct_typeid_ref(const T*) noexcept {
     return typeid(T);
 }
 
+BOOST_TYPE_INDEX_END_MODULE_EXPORT
+
 }}} // namespace boost::typeindex::detail
 
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_TYPE_INDEX_INTERFACE_UNIT)
+
 /// @cond
 #define BOOST_TYPE_INDEX_REGISTER_CLASS                                                                               \
     virtual const boost::typeindex::stl_type_index::type_info_t& boost_type_index_type_id_runtime_() const noexcept { \

include/boost/type_index/runtime_cast.hpp

@@ -9,6 +9,8 @@
 #ifndef BOOST_TYPE_INDEX_RUNTIME_CAST_HPP
 #define BOOST_TYPE_INDEX_RUNTIME_CAST_HPP
 
+#include <boost/type_index/detail/config.hpp>
+
 /// \file runtime_cast.hpp
 /// \brief Contains the basic utilities necessary to fully emulate
 /// dynamic_cast for language level constructs (raw pointers and references).