Experimental/CXXModules: Implement EcoStd Module Metadata parser

Adds a parser and serializer for the EcoStd Module Metadata format

RFC: https://github.com/ecostd/rfcs/pull/3

This adapts the existing experimental support for import std; to use
the new parser. The CMAKE_CXX_STDLIB_MODULES_JSON is now the canonical
variable for controlling how CMake discovers the stdlib module metadata,
and is used directly by compiler detection.

Toolchains can still override the __CMAKE::CXX## targets if they wish,
either in conjunction with CMAKE_CXX_STDLIB_MODULE_JSON or not. It is
possible to disable automatic detection of
CMAKE_CXX_STDLIB_MODULE_JSON by setting it to the empty string.

When available, the CMAKE_CXX_STDLIB_MODULE_JSON will be used to create
all requested C++ stdlibs which do not already have targets.

Modules/CMakeCXXCompiler.cmake.in

@@ -96,5 +96,8 @@ set(CMAKE_CXX_IMPLICIT_LINK_DIRECTORIES "@CMAKE_CXX_IMPLICIT_LINK_DIRECTORIES@")
 set(CMAKE_CXX_IMPLICIT_LINK_FRAMEWORK_DIRECTORIES "@CMAKE_CXX_IMPLICIT_LINK_FRAMEWORK_DIRECTORIES@")
 set(CMAKE_CXX_COMPILER_CLANG_RESOURCE_DIR "@CMAKE_CXX_COMPILER_CLANG_RESOURCE_DIR@")
 
-set(CMAKE_CXX_COMPILER_IMPORT_STD "")
-@CMAKE_CXX_IMPORT_STD@
+set(CMAKE_CXX_COMPILER_IMPORT_STD "@CMAKE_CXX_COMPILER_IMPORT_STD@")
+set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE  "@CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE@")
+if(CMAKE_CXX_STDLIB_MODULES_JSON)
+  set(CMAKE_CXX_STDLIB_MODULES_JSON "@CMAKE_CXX_STDLIB_MODULES_JSON@")
+endif()

Modules/CMakeDetermineCompilerSupport.cmake

@@ -105,18 +105,21 @@ function(cmake_determine_compiler_support lang)
       )
     endif()
 
-    # Create targets for use with `import std;` here.
-    set(CMAKE_CXX_IMPORT_STD "")
-    foreach (_cmake_import_std_version IN ITEMS 23 26)
-      if (CMAKE_CXX${_cmake_import_std_version}_COMPILE_FEATURES)
-        set(_cmake_cxx_import_std "")
-        cmake_create_cxx_import_std("${_cmake_import_std_version}" _cmake_cxx_import_std)
-        if (_cmake_cxx_import_std)
-          string(APPEND CMAKE_CXX_IMPORT_STD "### Imported target for C++${_cmake_import_std_version} standard library\n")
-          string(APPEND CMAKE_CXX_IMPORT_STD "${_cmake_cxx_import_std}\n\n")
-        endif ()
-      endif ()
-    endforeach ()
+    # Find the module metadata for import std
+    set(CMAKE_CXX_COMPILER_IMPORT_STD "")
+    cmake_cxx_find_modules_json()
+    foreach(_cmake_import_std_version IN ITEMS 23 26)
+      if(CMAKE_CXX${_cmake_import_std_version}_COMPILE_FEATURES)
+        # Modules JSON covers all versions, otherwise rely on toolchain targets
+        if(CMAKE_CXX_STDLIB_MODULES_JSON OR (TARGET "__CMAKE:CXX${_cmake_import_std_version}"))
+          list(APPEND CMAKE_CXX_COMPILER_IMPORT_STD ${_cmake_import_std_version})
+        endif()
+      endif()
+    endforeach()
+
+    set(CMAKE_CXX_COMPILER_IMPORT_STD ${CMAKE_CXX_COMPILER_IMPORT_STD} PARENT_SCOPE)
+    set(CMAKE_CXX_STDLIB_MODULES_JSON ${CMAKE_CXX_STDLIB_MODULES_JSON} PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "${CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE}" PARENT_SCOPE)
 
     set(CMAKE_CXX_COMPILE_FEATURES ${CMAKE_CXX_COMPILE_FEATURES} PARENT_SCOPE)
     set(CMAKE_CXX98_COMPILE_FEATURES ${CMAKE_CXX98_COMPILE_FEATURES} PARENT_SCOPE)
@@ -126,7 +129,6 @@ function(cmake_determine_compiler_support lang)
     set(CMAKE_CXX20_COMPILE_FEATURES ${CMAKE_CXX20_COMPILE_FEATURES} PARENT_SCOPE)
     set(CMAKE_CXX23_COMPILE_FEATURES ${CMAKE_CXX23_COMPILE_FEATURES} PARENT_SCOPE)
     set(CMAKE_CXX26_COMPILE_FEATURES ${CMAKE_CXX26_COMPILE_FEATURES} PARENT_SCOPE)
-    set(CMAKE_CXX_IMPORT_STD ${CMAKE_CXX_IMPORT_STD} PARENT_SCOPE)
 
     message(CHECK_PASS "done")
 

Modules/Compiler/CMakeCommonCompilerMacros.cmake

@@ -202,43 +202,21 @@ macro(cmake_record_hip_compile_features)
   _has_compiler_features_hip(98)
 endmacro()
 
-function(cmake_create_cxx_import_std std variable)
-  set(_cmake_supported_import_std_features
-    # Compilers support `import std` in C++20 as an extension. Skip
-    # for now.
-    # 20
-    23
-    26)
-  list(FIND _cmake_supported_import_std_features "${std}" _cmake_supported_import_std_idx)
-  if (_cmake_supported_import_std_idx EQUAL "-1")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Unsupported C++ standard: C++${std}\")\n"
-      PARENT_SCOPE)
-    return ()
-  endif ()
-  # If the target exists, skip. A toolchain file may have provided it.
-  if (TARGET "__CMAKE::CXX${std}")
-    return ()
-  endif ()
+function(cmake_cxx_find_modules_json)
   # The generator must support imported C++ modules.
   if (NOT CMAKE_GENERATOR MATCHES "Ninja")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Unsupported generator: ${CMAKE_GENERATOR}\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Unsupported generator: ${CMAKE_GENERATOR}" PARENT_SCOPE)
     return ()
   endif ()
+
   # Check if the compiler understands how to `import std;`.
   include("${CMAKE_ROOT}/Modules/Compiler/${CMAKE_CXX_COMPILER_ID}-CXX-CXXImportStd.cmake" OPTIONAL RESULT_VARIABLE _cmake_import_std_res)
   if (NOT _cmake_import_std_res)
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Toolchain does not support discovering `import std` support\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Toolchain does not support discovering module metadata" PARENT_SCOPE)
     return ()
   endif ()
-  if (NOT COMMAND _cmake_cxx_import_std)
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Toolchain does not provide `import std` discovery command\")\n"
-      PARENT_SCOPE)
+  if (NOT COMMAND _cmake_cxx_find_modules_json)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Toolchain does not provide module metadata discovery command" PARENT_SCOPE)
     return ()
   endif ()
 
@@ -249,19 +227,11 @@ function(cmake_create_cxx_import_std std variable)
     "CxxImportStd"
     _cmake_supported_import_std_experimental)
   if (NOT _cmake_supported_import_std_experimental)
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled (usually a `project()` call)\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled (usually a `project()` call)" PARENT_SCOPE)
     return ()
   endif ()
 
-  _cmake_cxx_import_std("${std}" target_definition)
-  string(CONCAT guarded_target_definition
-    "if (NOT TARGET \"__CMAKE::CXX${std}\")\n"
-    "${target_definition}"
-    "endif ()\n"
-    "if (TARGET \"__CMAKE::CXX${std}\")\n"
-    "  list(APPEND CMAKE_CXX_COMPILER_IMPORT_STD \"${std}\")\n"
-    "endif ()\n")
-  set("${variable}" "${guarded_target_definition}" PARENT_SCOPE)
+  _cmake_cxx_find_modules_json()
+  set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "${CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE}" PARENT_SCOPE)
+  set(CMAKE_CXX_STDLIB_MODULES_JSON "${CMAKE_CXX_STDLIB_MODULES_JSON}" PARENT_SCOPE)
 endfunction()

Modules/Compiler/Clang-CXX-CXXImportStd.cmake

@@ -1,18 +1,14 @@
-function (_cmake_cxx_import_std std variable)
+function (_cmake_cxx_find_modules_json)
   if (CMAKE_CXX_STANDARD_LIBRARY STREQUAL "libc++")
     set(_clang_modules_json_impl "libc++")
   elseif (CMAKE_CXX_STANDARD_LIBRARY STREQUAL "libstdc++")
     set(_clang_modules_json_impl "libstdc++")
   else ()
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Only `libc++` and `libstdc++` are supported\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Only `libc++` and `libstdc++` are supported" PARENT_SCOPE)
     return ()
   endif ()
 
-  if (CMAKE_CXX_STDLIB_MODULES_JSON)
-    set(_clang_libcxx_modules_json_file "${CMAKE_CXX_STDLIB_MODULES_JSON}")
-  else ()
+  if (NOT DEFINED CMAKE_CXX_STDLIB_MODULES_JSON)
     execute_process(
       COMMAND
         "${CMAKE_CXX_COMPILER}"
@@ -24,19 +20,10 @@ function (_cmake_cxx_import_std std variable)
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_STRIP_TRAILING_WHITESPACE)
     if (_clang_libcxx_modules_json_file_res)
-      set("${variable}"
-        "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Could not find `${_clang_modules_json_impl}.modules.json` resource\")\n"
-        PARENT_SCOPE)
+      set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Could not find `${_clang_modules_json_impl}.modules.json` resource" PARENT_SCOPE)
       return ()
     endif ()
-  endif ()
-
-  # Without this file, we do not have modules installed.
-  if (NOT EXISTS "${_clang_libcxx_modules_json_file}")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`${_clang_modules_json_impl}.modules.json` resource does not exist\")\n"
-      PARENT_SCOPE)
-    return ()
+    set(CMAKE_CXX_STDLIB_MODULES_JSON "${_clang_libcxx_modules_json_file}" PARENT_SCOPE)
   endif ()
 
   if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS "18.1.2" AND
@@ -44,145 +31,7 @@ function (_cmake_cxx_import_std std variable)
     # The original PR had a key spelling mismatch internally. Do not support it
     # and instead require a release known to have the fix.
     # https://github.com/llvm/llvm-project/pull/83036
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"LLVM 18.1.2 is required for `${_clang_modules_json_impl}.modules.json` format fix\")\n"
-      PARENT_SCOPE)
-    return ()
-  endif ()
-
-  file(READ "${_clang_libcxx_modules_json_file}" _clang_libcxx_modules_json)
-  string(JSON _clang_modules_json_version GET "${_clang_libcxx_modules_json}" "version")
-  string(JSON _clang_modules_json_revision GET "${_clang_libcxx_modules_json}" "revision")
-  # Require version 1.
-  if (NOT _clang_modules_json_version EQUAL "1")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libc++.modules.json` version ${_clang_modules_json_version}.${_clang_modules_json_revision} is not recognized\")\n"
-      PARENT_SCOPE)
-    return ()
-  endif ()
-
-  string(JSON _clang_modules_json_nmodules LENGTH "${_clang_libcxx_modules_json}" "modules")
-  # Don't declare the target without any modules.
-  if (NOT _clang_modules_json_nmodules)
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libc++.modules.json` does not list any available modules\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "LLVM 18.1.2 is required for `${_clang_modules_json_impl}.modules.json` format fix" PARENT_SCOPE)
     return ()
   endif ()
-
-  # Declare the target.
-  set(_clang_libcxx_target "")
-  # Clang 18 does not provide the module initializer for the `std` modules.
-  # Create a static library to hold these. Hope that Clang 19 can provide this,
-  # but never run the code.
-  string(APPEND _clang_libcxx_target
-    "add_library(__cmake_cxx${std} STATIC)\n")
-  string(APPEND _clang_libcxx_target
-    "target_sources(__cmake_cxx${std} INTERFACE \"$<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,STATIC_LIBRARY>:$<TARGET_OBJECTS:__cmake_cxx${std}>>\")\n")
-  string(APPEND _clang_libcxx_target
-    "set_property(TARGET __cmake_cxx${std} PROPERTY EXCLUDE_FROM_ALL 1)\n")
-  string(APPEND _clang_libcxx_target
-    "set_property(TARGET __cmake_cxx${std} PROPERTY CXX_SCAN_FOR_MODULES 1)\n")
-  string(APPEND _clang_libcxx_target
-    "set_property(TARGET __cmake_cxx${std} PROPERTY CXX_MODULE_STD 0)\n")
-  string(APPEND _clang_libcxx_target
-    "target_compile_features(__cmake_cxx${std} PUBLIC cxx_std_${std})\n")
-
-  set(_clang_modules_is_stdlib 0)
-  set(_clang_modules_include_dirs_list "")
-  set(_clang_modules_module_paths "")
-  get_filename_component(_clang_modules_dir "${_clang_libcxx_modules_json_file}" DIRECTORY)
-
-  # Add module sources.
-  math(EXPR _clang_modules_json_nmodules_range "${_clang_modules_json_nmodules} - 1")
-  foreach (_clang_modules_json_modules_idx RANGE 0 "${_clang_modules_json_nmodules_range}")
-    string(JSON _clang_modules_json_module GET "${_clang_libcxx_modules_json}" "modules" "${_clang_modules_json_modules_idx}")
-
-    string(JSON _clang_modules_json_module_source GET "${_clang_modules_json_module}" "source-path")
-    string(JSON _clang_modules_json_module_is_stdlib GET "${_clang_modules_json_module}" "is-std-library")
-    string(JSON _clang_modules_json_module_local_arguments ERROR_VARIABLE _clang_modules_json_module_local_arguments_error GET "${_clang_modules_json_module}" "local-arguments")
-    string(JSON _clang_modules_json_module_nsystem_include_directories ERROR_VARIABLE _clang_modules_json_module_nsystem_include_directories_error LENGTH "${_clang_modules_json_module_local_arguments}" "system-include-directories")
-
-    if (_clang_modules_json_module_local_arguments_error)
-      set(_clang_modules_json_module_local_arguments "")
-    endif ()
-    if (_clang_modules_json_module_nsystem_include_directories_error)
-      set(_clang_modules_json_module_nsystem_include_directories 0)
-    endif ()
-    if (NOT IS_ABSOLUTE "${_clang_modules_json_module_source}")
-      string(PREPEND _clang_modules_json_module_source "${_clang_modules_dir}/")
-    endif ()
-    list(APPEND _clang_modules_module_paths
-      "${_clang_modules_json_module_source}")
-
-    if (_clang_modules_json_module_is_stdlib)
-      set(_clang_modules_is_stdlib 1)
-    endif ()
-
-    if (_clang_modules_json_module_nsystem_include_directories)
-      math(EXPR _clang_modules_json_module_nsystem_include_directories_range "${_clang_modules_json_module_nsystem_include_directories} - 1")
-      foreach (_clang_modules_json_modules_system_include_directories_idx RANGE 0 "${_clang_modules_json_module_nsystem_include_directories_range}")
-        string(JSON _clang_modules_json_module_system_include_directory GET "${_clang_modules_json_module_local_arguments}" "system-include-directories" "${_clang_modules_json_modules_system_include_directories_idx}")
-
-        if (NOT IS_ABSOLUTE "${_clang_modules_json_module_system_include_directory}")
-          string(PREPEND _clang_modules_json_module_system_include_directory "${_clang_modules_dir}/")
-        endif ()
-        list(APPEND _clang_modules_include_dirs_list
-          "${_clang_modules_json_module_system_include_directory}")
-      endforeach ()
-    endif ()
-  endforeach ()
-
-  # Split the paths into basedirs and module paths.
-  set(_clang_modules_base_dirs_list "")
-  set(_clang_modules_files "")
-  foreach (_clang_modules_module_path IN LISTS _clang_modules_module_paths)
-    get_filename_component(_clang_module_dir "${_clang_modules_module_path}" DIRECTORY)
-
-    list(APPEND _clang_modules_base_dirs_list
-      "${_clang_module_dir}")
-    string(APPEND _clang_modules_files
-      " \"${_clang_modules_module_path}\"")
-  endforeach ()
-  list(REMOVE_DUPLICATES _clang_modules_base_dirs_list)
-  set(_clang_modules_base_dirs "")
-  foreach (_clang_modules_base_dir IN LISTS _clang_modules_base_dirs_list)
-    string(APPEND _clang_modules_base_dirs
-      " \"${_clang_modules_base_dir}\"")
-  endforeach ()
-
-  # If we have a standard library module, suppress warnings about reserved
-  # module names.
-  if (_clang_modules_is_stdlib)
-    string(APPEND _clang_libcxx_target
-      "target_compile_options(__cmake_cxx${std} PRIVATE -Wno-reserved-module-identifier)\n")
-  endif ()
-
-  # Set up include directories.
-  list(REMOVE_DUPLICATES _clang_modules_include_dirs_list)
-  set(_clang_modules_include_dirs "")
-  foreach (_clang_modules_include_dir IN LISTS _clang_modules_include_dirs_list)
-    string(APPEND _clang_modules_include_dirs
-      " \"${_clang_modules_include_dir}\"")
-  endforeach ()
-  string(APPEND _clang_libcxx_target
-    "target_include_directories(__cmake_cxx${std} PRIVATE ${_clang_modules_include_dirs})\n")
-
-  # Create the file set for the modules.
-  string(APPEND _clang_libcxx_target
-    "target_sources(__cmake_cxx${std}
-  PUBLIC
-  FILE_SET std TYPE CXX_MODULES
-    BASE_DIRS ${_clang_modules_base_dirs}
-    FILES ${_clang_modules_files})\n")
-
-  # Wrap the `__cmake_cxx${std}` target in a check.
-  string(PREPEND _clang_libcxx_target
-    "if (NOT TARGET \"__cmake_cxx${std}\")\n")
-  string(APPEND _clang_libcxx_target
-    "endif ()\n")
-  string(APPEND _clang_libcxx_target
-    "add_library(__CMAKE::CXX${std} ALIAS __cmake_cxx${std})\n")
-
-  set("${variable}" "${_clang_libcxx_target}" PARENT_SCOPE)
 endfunction ()

Modules/Compiler/GNU-CXX-CXXImportStd.cmake

@@ -1,14 +1,10 @@
-function (_cmake_cxx_import_std std variable)
+function (_cmake_cxx_find_modules_json)
   if (NOT CMAKE_CXX_STANDARD_LIBRARY STREQUAL "libstdc++")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Only `libstdc++` is supported\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Only `libstdc++` is supported" PARENT_SCOPE)
     return ()
   endif ()
 
-  if (CMAKE_CXX_STDLIB_MODULES_JSON)
-    set(_gnu_libstdcxx_modules_json_file "${CMAKE_CXX_STDLIB_MODULES_JSON}")
-  else ()
+  if (NOT DEFINED CMAKE_CXX_STDLIB_MODULES_JSON)
     execute_process(
       COMMAND
         "${CMAKE_CXX_COMPILER}"
@@ -20,119 +16,9 @@ function (_cmake_cxx_import_std std variable)
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_STRIP_TRAILING_WHITESPACE)
     if (_gnu_libstdcxx_modules_json_file_res)
-      set("${variable}"
-        "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Could not find `libstdc++.modules.json` resource\")\n"
-        PARENT_SCOPE)
+      set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Could not find `libstdc++.modules.json` resource" PARENT_SCOPE)
       return ()
     endif ()
+    set(CMAKE_CXX_STDLIB_MODULES_JSON "${_gnu_libstdcxx_modules_json_file}" PARENT_SCOPE)
   endif ()
-
-  # Without this file, we do not have modules installed.
-  if (NOT EXISTS "${_gnu_libstdcxx_modules_json_file}")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libstdc++.modules.json` resource does not exist\")\n"
-      PARENT_SCOPE)
-    return ()
-  endif ()
-
-  file(READ "${_gnu_libstdcxx_modules_json_file}" _gnu_libstdcxx_modules_json)
-  string(JSON _gnu_modules_json_version GET "${_gnu_libstdcxx_modules_json}" "version")
-  string(JSON _gnu_modules_json_revision GET "${_gnu_libstdcxx_modules_json}" "revision")
-  # Require version 1.
-  if (NOT _gnu_modules_json_version EQUAL "1")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libstdc++.modules.json` version ${_gnu_modules_json_version}.${_gnu_modules_json_revision} is not recognized\")\n"
-      PARENT_SCOPE)
-    return ()
-  endif ()
-
-  string(JSON _gnu_modules_json_nmodules LENGTH "${_gnu_libstdcxx_modules_json}" "modules")
-  # Don't declare the target without any modules.
-  if (NOT _gnu_modules_json_nmodules)
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libstdc++.modules.json` does not list any available modules\")\n"
-      PARENT_SCOPE)
-    return ()
-  endif ()
-
-  # Declare the target.
-  set(_gnu_libstdcxx_target "")
-  string(APPEND _gnu_libstdcxx_target
-    "add_library(__CMAKE::CXX${std} IMPORTED INTERFACE)\n")
-  string(APPEND _gnu_libstdcxx_target
-    "target_compile_features(__CMAKE::CXX${std} INTERFACE cxx_std_${std})\n")
-
-  set(_gnu_modules_is_stdlib 0)
-  set(_gnu_modules_include_dirs_list "")
-  set(_gnu_modules_module_paths "")
-  get_filename_component(_gnu_modules_dir "${_gnu_libstdcxx_modules_json_file}" DIRECTORY)
-
-  # Add module sources.
-  math(EXPR _gnu_modules_json_nmodules_range "${_gnu_modules_json_nmodules} - 1")
-  foreach (_gnu_modules_json_modules_idx RANGE 0 "${_gnu_modules_json_nmodules_range}")
-    string(JSON _gnu_modules_json_module GET "${_gnu_libstdcxx_modules_json}" "modules" "${_gnu_modules_json_modules_idx}")
-
-    string(JSON _gnu_modules_json_module_source GET "${_gnu_modules_json_module}" "source-path")
-    string(JSON _gnu_modules_json_module_is_stdlib GET "${_gnu_modules_json_module}" "is-std-library")
-    string(JSON _gnu_modules_json_module_local_arguments ERROR_VARIABLE _gnu_modules_json_module_local_arguments_error GET "${_gnu_modules_json_module}" "local-arguments")
-    string(JSON _gnu_modules_json_module_nsystem_include_directories ERROR_VARIABLE _gnu_modules_json_module_nsystem_include_directories_error LENGTH "${_gnu_modules_json_module_local_arguments}" "system-include-directories")
-
-    if (_gnu_modules_json_module_local_arguments_error STREQUAL "NOTFOUND")
-      set(_gnu_modules_json_module_local_arguments "")
-    endif ()
-    if (_gnu_modules_json_module_nsystem_include_directories_error STREQUAL "NOTFOUND")
-      set(_gnu_modules_json_module_nsystem_include_directories 0)
-    endif ()
-
-    if (NOT IS_ABSOLUTE "${_gnu_modules_json_module_source}")
-      string(PREPEND _gnu_modules_json_module_source "${_gnu_modules_dir}/")
-    endif ()
-    list(APPEND _gnu_modules_module_paths
-      "${_gnu_modules_json_module_source}")
-
-    if (_gnu_modules_json_module_is_stdlib)
-      set(_gnu_modules_is_stdlib 1)
-    endif ()
-
-    if (_gnu_modules_json_module_nsystem_include_directories)
-      math(EXPR _gnu_modules_json_module_nsystem_include_directories_range "${_gnu_modules_json_module_nsystem_include_directories} - 1")
-      foreach (_gnu_modules_json_modules_system_include_directories_idx RANGE 0 "${_gnu_modules_json_module_nsystem_include_directories_range}")
-        string(JSON _gnu_modules_json_module_system_include_directory GET "${_gnu_modules_json_module_local_arguments}" "system-include-directories" "${_gnu_modules_json_modules_system_include_directories_idx}")
-
-        if (NOT IS_ABSOLUTE "${_gnu_modules_json_module_system_include_directory}")
-          string(PREPEND _gnu_modules_json_module_system_include_directory "${_gnu_modules_dir}/")
-        endif ()
-        list(APPEND _gnu_modules_include_dirs_list
-          "${_gnu_modules_json_module_system_include_directory}")
-      endforeach ()
-    endif ()
-  endforeach ()
-
-  # Split the paths into basedirs and module paths.
-  set(_gnu_modules_base_dirs_list "")
-  set(_gnu_modules_files "")
-  foreach (_gnu_modules_module_path IN LISTS _gnu_modules_module_paths)
-    get_filename_component(_gnu_module_dir "${_gnu_modules_module_path}" DIRECTORY)
-
-    list(APPEND _gnu_modules_base_dirs_list
-      "${_gnu_module_dir}")
-    string(APPEND _gnu_modules_files
-      " \"${_gnu_modules_module_path}\"")
-  endforeach ()
-  list(REMOVE_DUPLICATES _gnu_modules_base_dirs_list)
-  set(_gnu_modules_base_dirs "")
-  foreach (_gnu_modules_base_dir IN LISTS _gnu_modules_base_dirs_list)
-    string(APPEND _gnu_modules_base_dirs
-      " \"${_gnu_modules_base_dir}\"")
-  endforeach ()
-
-  # Create the file set for the modules.
-  string(APPEND _gnu_libstdcxx_target
-    "target_sources(__CMAKE::CXX${std}
-  INTERFACE
-  FILE_SET std TYPE CXX_MODULES
-    BASE_DIRS ${_gnu_modules_base_dirs}
-    FILES ${_gnu_modules_files})\n")
-
-  set("${variable}" "${_gnu_libstdcxx_target}" PARENT_SCOPE)
 endfunction ()

Modules/Compiler/MSVC-CXX-CXXImportStd.cmake

@@ -1,119 +1,104 @@
-function (_cmake_cxx_import_std std variable)
-  if (CMAKE_CXX_STDLIB_MODULES_JSON)
-    set(_msvc_modules_json_file "${CMAKE_CXX_STDLIB_MODULES_JSON}")
-  else ()
-    find_file(_msvc_modules_json_file
-      NAME modules.json
-      HINTS
-        "$ENV{VCToolsInstallDir}/modules"
-      PATHS
-        "$ENV{INCLUDE}"
-        "${CMAKE_CXX_COMPILER}/../../.."
-        "${CMAKE_CXX_COMPILER}/../.."    # msvc-wine layout
-      PATH_SUFFIXES
-        ../modules
-      NO_CACHE)
-    # Without this file, we do not have modules installed.
-    if (NOT EXISTS "${_msvc_modules_json_file}")
-      set("${variable}"
-        "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Could not find `modules.json` resource\")\n"
-        PARENT_SCOPE)
-      return ()
-    endif ()
+function (_cmake_cxx_find_modules_json)
+  if (DEFINED CMAKE_CXX_STDLIB_MODULES_JSON)
+    return ()
+  endif ()
+
+  find_file(_msvc_modules_json_file
+    NAME modules.json
+    HINTS
+      "$ENV{VCToolsInstallDir}/modules"
+    PATHS
+      "$ENV{INCLUDE}"
+      "${CMAKE_CXX_COMPILER}/../../.."
+      "${CMAKE_CXX_COMPILER}/../.."    # msvc-wine layout
+    PATH_SUFFIXES
+      ../modules
+    NO_CACHE)
+
+  # Without this file, we do not have modules installed.
+  if (NOT EXISTS "${_msvc_modules_json_file}")
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Could not find `modules.json` resource" PARENT_SCOPE)
+    return ()
   endif ()
 
   file(READ "${_msvc_modules_json_file}" _msvc_modules_json)
+  string(JSON _msvc_json_modules ERROR_VARIABLE _msvc_json_err GET "${_msvc_modules_json}" "modules")
+
+  # This is probably a conforming module metadata file, use it as is
+  if (_msvc_json_modules)
+    set(CMAKE_CXX_STDLIB_MODULES_JSON "${_msvc_modules_json_file}" PARENT_SCOPE)
+    return ()
+  endif ()
+
+  # Otherwise it's a Microsoft STL-style modules.json, check if we recognize it
   string(JSON _msvc_json_version GET "${_msvc_modules_json}" "version")
   string(JSON _msvc_json_revision GET "${_msvc_modules_json}" "revision")
   # Require version 1.
   if (NOT _msvc_json_version EQUAL "1")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`modules.json` version ${_msvc_json_version}.${_msvc_json_revision} is not recognized\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "`modules.json` version ${_msvc_json_version}.${_msvc_json_revision} is not recognized" PARENT_SCOPE)
     return ()
   endif ()
 
   string(JSON _msvc_json_library GET "${_msvc_modules_json}" "library")
   # Bail if we don't understand the library.
   if (NOT _msvc_json_library STREQUAL "microsoft/STL")
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`modules.json` library `${_msvc_json_library}` is not recognized\")\n"
-      PARENT_SCOPE)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "`modules.json` library `${_msvc_json_library}` is not recognized" PARENT_SCOPE)
     return ()
   endif ()
 
-  string(JSON _msvc_json_nmodules LENGTH "${_msvc_modules_json}" "module-sources")
-  # Don't declare the target without any modules.
-  if (NOT _msvc_json_nmodules)
-    set("${variable}"
-      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`modules.json` does not list any available modules\")\n"
-      PARENT_SCOPE)
+  string(JSON _msvc_json_sources_len LENGTH "${_msvc_modules_json}" "module-sources")
+  # Bail if there aren't any sources
+  if (NOT _msvc_json_sources_len)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "`modules.json` does not list any available sources" PARENT_SCOPE)
     return ()
   endif ()
 
-  # Declare the target.
-  set(_msvc_std_target "")
-  string(APPEND _msvc_std_target
-    "add_library(__cmake_cxx${std} STATIC)\n")
-  string(APPEND _msvc_std_target
-    "target_sources(__cmake_cxx${std} INTERFACE \"$<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,STATIC_LIBRARY>:$<TARGET_OBJECTS:__cmake_cxx${std}>>\")\n")
-  string(APPEND _msvc_std_target
-    "set_property(TARGET __cmake_cxx${std} PROPERTY EXCLUDE_FROM_ALL 1)\n")
-  string(APPEND _msvc_std_target
-    "set_property(TARGET __cmake_cxx${std} PROPERTY CXX_SCAN_FOR_MODULES 1)\n")
-  string(APPEND _msvc_std_target
-    "set_property(TARGET __cmake_cxx${std} PROPERTY CXX_MODULE_STD 0)\n")
-  string(APPEND _msvc_std_target
-    "target_compile_features(__cmake_cxx${std} PUBLIC cxx_std_${std})\n")
+  set(_msvc_module_metadata [=[{
+    "version": 1,
+    "revision": 1,
+    "modules": []
+  }]=])
 
-  set(_msvc_modules_module_paths "")
-  get_filename_component(_msvc_modules_dir "${_msvc_modules_json_file}" DIRECTORY)
+  cmake_path(GET _msvc_modules_json_file PARENT_PATH _msvc_base_dir)
+  math(EXPR _msvc_json_sources_len "${_msvc_json_sources_len}-1")
+  foreach (idx RANGE ${_msvc_json_sources_len})
+    string(JSON _msvc_source GET "${_msvc_modules_json}" "module-sources" ${idx})
 
-  # Add module sources.
-  math(EXPR _msvc_modules_json_nmodules_range "${_msvc_json_nmodules} - 1")
-  foreach (_msvc_modules_json_modules_idx RANGE 0 "${_msvc_modules_json_nmodules_range}")
-    string(JSON _msvc_modules_json_module_source GET "${_msvc_modules_json}" "module-sources" "${_msvc_modules_json_modules_idx}")
-
-    if (NOT IS_ABSOLUTE "${_msvc_modules_json_module_source}")
-      string(PREPEND _msvc_modules_json_module_source "${_msvc_modules_dir}/")
+    set(_msvc_path ${_msvc_source})
+    cmake_path(IS_RELATIVE _msvc_path _msvc_is_rel)
+    if (_msvc_is_rel)
+      cmake_path(ABSOLUTE_PATH _msvc_path BASE_DIRECTORY "${_msvc_base_dir}")
     endif ()
-    list(APPEND _msvc_modules_module_paths
-      "${_msvc_modules_json_module_source}")
-  endforeach ()
-
-  # Split the paths into basedirs and module paths.
-  set(_msvc_modules_base_dirs_list "")
-  set(_msvc_modules_files "")
-  foreach (_msvc_modules_module_path IN LISTS _msvc_modules_module_paths)
-    get_filename_component(_msvc_module_dir "${_msvc_modules_module_path}" DIRECTORY)
 
-    list(APPEND _msvc_modules_base_dirs_list
-      "${_msvc_module_dir}")
-    string(APPEND _msvc_modules_files
-      " \"${_msvc_modules_module_path}\"")
-  endforeach ()
-  list(REMOVE_DUPLICATES _msvc_modules_base_dirs_list)
-  set(_msvc_modules_base_dirs "")
-  foreach (_msvc_modules_base_dir IN LISTS _msvc_modules_base_dirs_list)
-    string(APPEND _msvc_modules_base_dirs
-      " \"${_msvc_modules_base_dir}\"")
-  endforeach ()
+    if (_msvc_source MATCHES "std.ixx")
+      string(JSON _msvc_module_metadata
+        SET "${_msvc_module_metadata}" "modules" ${idx}
+        "{
+            \"logical-name\": \"std\",
+            \"source-path\": \"${_msvc_path}\",
+            \"is-std-library\": true
+        }"
+      )
+    elseif (_msvc_source MATCHES "std.compat.ixx")
+      string(JSON _msvc_module_metadata
+        SET "${_msvc_module_metadata}" "modules" ${idx}
+        "{
+            \"logical-name\": \"std.compat\",
+            \"source-path\": \"${_msvc_path}\",
+            \"is-std-library\": true
+        }"
+      )
+    endif ()
+  endforeach()
 
-  # Create the file set for the modules.
-  string(APPEND _msvc_std_target
-    "target_sources(__cmake_cxx${std}
-  PUBLIC
-  FILE_SET std TYPE CXX_MODULES
-    BASE_DIRS ${_msvc_modules_base_dirs}
-    FILES ${_msvc_modules_files})\n")
+  string(JSON _msvc_module_metadata_modules_len LENGTH ${_msvc_module_metadata} "modules")
 
-  # Wrap the `__cmake_cxx${std}` target in a check.
-  string(PREPEND _msvc_std_target
-    "if (NOT TARGET \"__cmake_cxx${std}\")\n")
-  string(APPEND _msvc_std_target
-    "endif ()\n")
-  string(APPEND _msvc_std_target
-    "add_library(__CMAKE::CXX${std} ALIAS __cmake_cxx${std})\n")
+  # Bail if we didn't recognize any of the modules
+  if (NOT _msvc_module_metadata_modules_len)
+    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "`modules.json` did not contain any recognized sources (std.ixx, std.compat.ixx)" PARENT_SCOPE)
+    return ()
+  endif ()
 
-  set("${variable}" "${_msvc_std_target}" PARENT_SCOPE)
+  file(WRITE "${CMAKE_PLATFORM_INFO_DIR}/msvcstl.modules.json" "${_msvc_module_metadata}")
+  set(CMAKE_CXX_STDLIB_MODULES_JSON "${CMAKE_PLATFORM_INFO_DIR}/msvcstl.modules.json" PARENT_SCOPE)
 endfunction ()

Source/CMakeLists.txt

@@ -165,6 +165,8 @@ add_library(
   cmCustomCommandTypes.h
   cmCxxModuleMapper.cxx
   cmCxxModuleMapper.h
+  cmCxxModuleMetadata.cxx
+  cmCxxModuleMetadata.h
   cmCxxModuleUsageEffects.cxx
   cmCxxModuleUsageEffects.h
   cmDefinitions.cxx

Source/cmCxxModuleMetadata.cxx

@@ -0,0 +1,460 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file LICENSE.rst or https://cmake.org/licensing for details.  */
+
+#include "cmCxxModuleMetadata.h"
+
+#include <algorithm>
+#include <set>
+#include <string>
+#include <utility>
+
+#include <cmext/string_view>
+
+#include <cm3p/json/value.h>
+#include <cm3p/json/writer.h>
+
+#include "cmsys/FStream.hxx"
+
+#include "cmFileSet.h"
+#include "cmJSONState.h"
+#include "cmListFileCache.h"
+#include "cmStringAlgorithms.h"
+#include "cmSystemTools.h"
+#include "cmTarget.h"
+
+namespace {
+
+bool JsonIsStringArray(Json::Value const& v)
+{
+  return v.isArray() &&
+    std::all_of(v.begin(), v.end(),
+                [](Json::Value const& it) { return it.isString(); });
+}
+
+bool ParsePreprocessorDefine(Json::Value& dval,
+                             cmCxxModuleMetadata::PreprocessorDefineData& out,
+                             cmJSONState* state)
+{
+  if (!dval.isObject()) {
+    state->AddErrorAtValue("each entry in 'definitions' must be an object",
+                           &dval);
+    return false;
+  }
+
+  if (!dval.isMember("name") || !dval["name"].isString() ||
+      dval["name"].asString().empty()) {
+    state->AddErrorAtValue(
+      "preprocessor definition requires a non-empty 'name'", &dval["name"]);
+    return false;
+  }
+  out.Name = dval["name"].asString();
+
+  if (dval.isMember("value")) {
+    if (dval["value"].isString()) {
+      out.Value = dval["value"].asString();
+    } else if (!dval["value"].isNull()) {
+      state->AddErrorAtValue(
+        "'value' in preprocessor definition must be string or null",
+        &dval["value"]);
+      return false;
+    }
+  }
+
+  if (dval.isMember("undef")) {
+    if (!dval["undef"].isBool()) {
+      state->AddErrorAtValue(
+        "'undef' in preprocessor definition must be boolean", &dval["undef"]);
+      return false;
+    }
+    out.Undef = dval["undef"].asBool();
+  }
+
+  if (dval.isMember("vendor")) {
+    out.Vendor = std::move(dval["vendor"]);
+  }
+
+  return true;
+}
+
+bool ParseLocalArguments(Json::Value& lav,
+                         cmCxxModuleMetadata::LocalArgumentsData& out,
+                         cmJSONState* state)
+{
+  if (!lav.isObject()) {
+    state->AddErrorAtValue("'local-arguments' must be an object", &lav);
+    return false;
+  }
+
+  if (lav.isMember("include-directories")) {
+    if (!JsonIsStringArray(lav["include-directories"])) {
+      state->AddErrorAtValue(
+        "'include-directories' must be an array of strings",
+        &lav["include-directories"]);
+      return false;
+    }
+    for (auto const& s : lav["include-directories"]) {
+      out.IncludeDirectories.push_back(s.asString());
+    }
+  }
+
+  if (lav.isMember("system-include-directories")) {
+    if (!JsonIsStringArray(lav["system-include-directories"])) {
+      state->AddErrorAtValue(
+        "'system-include-directories' must be an array of strings",
+        &lav["system-include-directories"]);
+      return false;
+    }
+    for (auto const& s : lav["system-include-directories"]) {
+      out.SystemIncludeDirectories.push_back(s.asString());
+    }
+  }
+
+  if (lav.isMember("definitions")) {
+    if (!lav["definitions"].isArray()) {
+      state->AddErrorAtValue("'definitions' must be an array",
+                             &lav["definitions"]);
+      return false;
+    }
+    for (Json::Value& dval : lav["definitions"]) {
+      out.Definitions.emplace_back();
+      if (!ParsePreprocessorDefine(dval, out.Definitions.back(), state)) {
+        return false;
+      }
+    }
+  }
+
+  if (lav.isMember("vendor")) {
+    out.Vendor = std::move(lav["vendor"]);
+  }
+
+  return true;
+}
+
+bool ParseModule(Json::Value& mval, cmCxxModuleMetadata::ModuleData& mod,
+                 cmJSONState* state)
+{
+  if (!mval.isObject()) {
+    state->AddErrorAtValue("each entry in 'modules' must be an object", &mval);
+    return false;
+  }
+
+  if (!mval.isMember("logical-name") || !mval["logical-name"].isString() ||
+      mval["logical-name"].asString().empty()) {
+    state->AddErrorAtValue(
+      "module entries require a non-empty 'logical-name' string",
+      &mval["logical-name"]);
+    return false;
+  }
+  mod.LogicalName = mval["logical-name"].asString();
+
+  if (!mval.isMember("source-path") || !mval["source-path"].isString() ||
+      mval["source-path"].asString().empty()) {
+    state->AddErrorAtValue(
+      "module entries require a non-empty 'source-path' string",
+      &mval["source-path"]);
+    return false;
+  }
+  mod.SourcePath = mval["source-path"].asString();
+
+  if (mval.isMember("is-interface")) {
+    if (!mval["is-interface"].isBool()) {
+      state->AddErrorAtValue("'is-interface' must be boolean",
+                             &mval["is-interface"]);
+      return false;
+    }
+    mod.IsInterface = mval["is-interface"].asBool();
+  } else {
+    mod.IsInterface = true;
+  }
+
+  if (mval.isMember("is-std-library")) {
+    if (!mval["is-std-library"].isBool()) {
+      state->AddErrorAtValue("'is-std-library' must be boolean",
+                             &mval["is-std-library"]);
+      return false;
+    }
+    mod.IsStdLibrary = mval["is-std-library"].asBool();
+  } else {
+    mod.IsStdLibrary = false;
+  }
+
+  if (mval.isMember("local-arguments")) {
+    mod.LocalArguments.emplace();
+    if (!ParseLocalArguments(mval["local-arguments"], *mod.LocalArguments,
+                             state)) {
+      return false;
+    }
+  }
+
+  if (mval.isMember("vendor")) {
+    mod.Vendor = std::move(mval["vendor"]);
+  }
+
+  return true;
+}
+
+bool ParseRoot(Json::Value& root, cmCxxModuleMetadata& meta,
+               cmJSONState* state)
+{
+  if (!root.isMember("version") || !root["version"].isInt()) {
+    state->AddErrorAtValue(
+      "Top-level member 'version' is required and must be an integer", &root);
+    return false;
+  }
+  meta.Version = root["version"].asInt();
+
+  if (root.isMember("revision")) {
+    if (!root["revision"].isInt()) {
+      state->AddErrorAtValue("'revision' must be an integer",
+                             &root["revision"]);
+      return false;
+    }
+    meta.Revision = root["revision"].asInt();
+  }
+
+  if (root.isMember("modules")) {
+    if (!root["modules"].isArray()) {
+      state->AddErrorAtValue("'modules' must be an array", &root["modules"]);
+      return false;
+    }
+    for (Json::Value& mval : root["modules"]) {
+      meta.Modules.emplace_back();
+      if (!ParseModule(mval, meta.Modules.back(), state)) {
+        return false;
+      }
+    }
+  }
+
+  for (std::string& key : root.getMemberNames()) {
+    if (key == "version" || key == "revision" || key == "modules") {
+      continue;
+    }
+    meta.Extensions.emplace(std::move(key), std::move(root[key]));
+  }
+
+  return true;
+}
+
+} // namespace
+
+cmCxxModuleMetadata::ParseResult cmCxxModuleMetadata::LoadFromFile(
+  std::string const& path)
+{
+  ParseResult res;
+
+  Json::Value root;
+  cmJSONState parseState(path, &root);
+  if (!parseState.errors.empty()) {
+    res.Error = parseState.GetErrorMessage();
+    return res;
+  }
+
+  cmCxxModuleMetadata meta;
+  if (!ParseRoot(root, meta, &parseState)) {
+    res.Error = parseState.GetErrorMessage();
+    return res;
+  }
+
+  meta.MetadataFilePath = path;
+  res.Meta = std::move(meta);
+  return res;
+}
+
+namespace {
+
+Json::Value SerializePreprocessorDefine(
+  cmCxxModuleMetadata::PreprocessorDefineData const& d)
+{
+  Json::Value dv(Json::objectValue);
+  dv["name"] = d.Name;
+  if (d.Value) {
+    dv["value"] = *d.Value;
+  } else {
+    dv["value"] = Json::Value::null;
+  }
+  dv["undef"] = d.Undef;
+  if (d.Vendor) {
+    dv["vendor"] = *d.Vendor;
+  }
+  return dv;
+}
+
+Json::Value SerializeLocalArguments(
+  cmCxxModuleMetadata::LocalArgumentsData const& la)
+{
+  Json::Value lav(Json::objectValue);
+
+  if (!la.IncludeDirectories.empty()) {
+    Json::Value& inc = lav["include-directories"] = Json::arrayValue;
+    for (auto const& s : la.IncludeDirectories) {
+      inc.append(s);
+    }
+  }
+
+  if (!la.SystemIncludeDirectories.empty()) {
+    Json::Value& sinc = lav["system-include-directories"] = Json::arrayValue;
+    for (auto const& s : la.SystemIncludeDirectories) {
+      sinc.append(s);
+    }
+  }
+
+  if (!la.Definitions.empty()) {
+    Json::Value& defs = lav["definitions"] = Json::arrayValue;
+    for (auto const& d : la.Definitions) {
+      defs.append(SerializePreprocessorDefine(d));
+    }
+  }
+
+  if (la.Vendor) {
+    lav["vendor"] = *la.Vendor;
+  }
+
+  return lav;
+}
+
+Json::Value SerializeModule(cmCxxModuleMetadata::ModuleData const& m)
+{
+  Json::Value mv(Json::objectValue);
+  mv["logical-name"] = m.LogicalName;
+  mv["source-path"] = m.SourcePath;
+  mv["is-interface"] = m.IsInterface;
+  mv["is-std-library"] = m.IsStdLibrary;
+
+  if (m.LocalArguments) {
+    mv["local-arguments"] = SerializeLocalArguments(*m.LocalArguments);
+  }
+
+  if (m.Vendor) {
+    mv["vendor"] = *m.Vendor;
+  }
+
+  return mv;
+}
+
+} // namespace
+
+Json::Value cmCxxModuleMetadata::ToJsonValue(cmCxxModuleMetadata const& meta)
+{
+  Json::Value root(Json::objectValue);
+
+  root["version"] = meta.Version;
+  root["revision"] = meta.Revision;
+
+  Json::Value& modules = root["modules"] = Json::arrayValue;
+  for (auto const& m : meta.Modules) {
+    modules.append(SerializeModule(m));
+  }
+
+  for (auto const& kv : meta.Extensions) {
+    root[kv.first] = kv.second;
+  }
+
+  return root;
+}
+
+cmCxxModuleMetadata::SaveResult cmCxxModuleMetadata::SaveToFile(
+  std::string const& path, cmCxxModuleMetadata const& meta)
+{
+  SaveResult st;
+
+  cmsys::ofstream ofs(path.c_str());
+  if (!ofs.is_open()) {
+    st.Error = cmStrCat("Unable to open file for writing: "_s, path);
+    return st;
+  }
+
+  Json::StreamWriterBuilder wbuilder;
+  wbuilder["indentation"] = "  ";
+  ofs << Json::writeString(wbuilder, ToJsonValue(meta));
+
+  if (!ofs.good()) {
+    st.Error = cmStrCat("Write failed for file: "_s, path);
+    return st;
+  }
+
+  st.Ok = true;
+  return st;
+}
+
+void cmCxxModuleMetadata::PopulateTarget(
+  cmTarget& target, cmCxxModuleMetadata const& meta,
+  std::vector<std::string> const& configs)
+{
+  std::vector<cm::string_view> allIncludeDirectories;
+  std::vector<std::string> allCompileDefinitions;
+  std::set<std::string> baseDirs;
+
+  std::string metadataDir =
+    cmSystemTools::GetFilenamePath(meta.MetadataFilePath);
+
+  auto fileSet = target.GetOrCreateFileSet("CXX_MODULES", "CXX_MODULES",
+                                           cmFileSetVisibility::Interface);
+
+  for (auto const& module : meta.Modules) {
+    std::string sourcePath = module.SourcePath;
+    if (!cmSystemTools::FileIsFullPath(sourcePath)) {
+      sourcePath = cmStrCat(metadataDir, '/', sourcePath);
+    }
+
+    // Module metadata files can reference files in different roots,
+    // just use the immediate parent directory as a base directory
+    baseDirs.insert(cmSystemTools::GetFilenamePath(sourcePath));
+
+    fileSet.first->AddFileEntry(sourcePath);
+
+    if (module.LocalArguments) {
+      for (auto const& incDir : module.LocalArguments->IncludeDirectories) {
+        allIncludeDirectories.push_back(incDir);
+      }
+      for (auto const& sysIncDir :
+           module.LocalArguments->SystemIncludeDirectories) {
+        allIncludeDirectories.push_back(sysIncDir);
+      }
+
+      for (auto const& def : module.LocalArguments->Definitions) {
+        if (!def.Undef) {
+          if (def.Value) {
+            allCompileDefinitions.push_back(
+              cmStrCat(def.Name, "="_s, *def.Value));
+          } else {
+            allCompileDefinitions.push_back(def.Name);
+          }
+        }
+      }
+    }
+  }
+
+  for (auto const& baseDir : baseDirs) {
+    fileSet.first->AddDirectoryEntry(baseDir);
+  }
+
+  if (!allIncludeDirectories.empty()) {
+    target.SetProperty("IMPORTED_CXX_MODULES_INCLUDE_DIRECTORIES",
+                       cmJoin(allIncludeDirectories, ";"));
+  }
+
+  if (!allCompileDefinitions.empty()) {
+    target.SetProperty("IMPORTED_CXX_MODULES_COMPILE_DEFINITIONS",
+                       cmJoin(allCompileDefinitions, ";"));
+  }
+
+  for (auto const& config : configs) {
+    std::vector<std::string> moduleList;
+    for (auto const& module : meta.Modules) {
+      if (module.IsInterface) {
+        std::string sourcePath = module.SourcePath;
+        if (!cmSystemTools::FileIsFullPath(sourcePath)) {
+          sourcePath = cmStrCat(metadataDir, '/', sourcePath);
+        }
+        moduleList.push_back(cmStrCat(module.LogicalName, "="_s, sourcePath));
+      }
+    }
+
+    if (!moduleList.empty()) {
+      std::string upperConfig = cmSystemTools::UpperCase(config);
+      std::string propertyName =
+        cmStrCat("IMPORTED_CXX_MODULES_"_s, upperConfig);
+      target.SetProperty(propertyName, cmJoin(moduleList, ";"));
+    }
+  }
+}

Source/cmCxxModuleMetadata.h

@@ -0,0 +1,84 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file LICENSE.rst or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include <cm/optional>
+#include <cm/string_view>
+
+#include <cm3p/json/value.h>
+
+class cmTarget;
+
+class cmCxxModuleMetadata
+{
+public:
+  struct PreprocessorDefineData
+  {
+    std::string Name;
+    cm::optional<std::string> Value;
+    bool Undef = false;
+    cm::optional<Json::Value> Vendor;
+  };
+
+  struct LocalArgumentsData
+  {
+    std::vector<std::string> IncludeDirectories;
+    std::vector<std::string> SystemIncludeDirectories;
+    std::vector<PreprocessorDefineData> Definitions;
+    cm::optional<Json::Value> Vendor;
+  };
+
+  struct ModuleData
+  {
+    std::string LogicalName;
+    std::string SourcePath;
+    bool IsInterface = true;
+    bool IsStdLibrary = false;
+    cm::optional<LocalArgumentsData> LocalArguments;
+    cm::optional<Json::Value> Vendor;
+  };
+
+  int Version = 0;
+  int Revision = 0;
+  std::vector<ModuleData> Modules;
+  std::string MetadataFilePath;
+
+  std::unordered_map<std::string, Json::Value> Extensions;
+
+  struct ParseResult;
+
+  struct SaveResult
+  {
+    bool Ok = false;
+    std::string Error;
+    explicit operator bool() const { return Ok; }
+  };
+
+  static ParseResult LoadFromFile(std::string const& path);
+  static Json::Value ToJsonValue(cmCxxModuleMetadata const& meta);
+  static SaveResult SaveToFile(std::string const& path,
+                               cmCxxModuleMetadata const& meta);
+  static void PopulateTarget(cmTarget& target, cmCxxModuleMetadata const& meta,
+                             std::vector<std::string> const& configs);
+  static void PopulateTarget(cmTarget& target, cmCxxModuleMetadata const& meta,
+                             cm::string_view config)
+  {
+    PopulateTarget(target, meta,
+                   std::vector<std::string>{ std::string(config) });
+  }
+  static void PopulateTarget(cmTarget& target, cmCxxModuleMetadata const& meta)
+  {
+    PopulateTarget(target, meta, "NOCONFIG");
+  }
+};
+
+struct cmCxxModuleMetadata::ParseResult
+{
+  cm::optional<cmCxxModuleMetadata> Meta;
+  std::string Error;
+  explicit operator bool() const { return static_cast<bool>(Meta); }
+};

Source/cmGeneratorTarget.cxx

@@ -22,6 +22,7 @@
 #include "cmAlgorithms.h"
 #include "cmComputeLinkInformation.h" // IWYU pragma: keep
 #include "cmCryptoHash.h"
+#include "cmCxxModuleMetadata.h"
 #include "cmCxxModuleUsageEffects.h"
 #include "cmExperimental.h"
 #include "cmFileSet.h"
@@ -5126,6 +5127,70 @@ bool cmGeneratorTarget::IsNullImpliedByLinkLibraries(
   return cm::contains(this->LinkImplicitNullProperties, p);
 }
 
+namespace {
+bool CreateCxxStdlibTarget(cmMakefile* makefile, cmLocalGenerator* lg,
+                           std::string const& targetName,
+                           std::string const& cxxTargetName,
+                           std::string const& stdLevel,
+                           std::vector<std::string> const& configs)
+{
+#ifndef CMAKE_BOOTSTRAP
+
+  static cm::optional<cmCxxModuleMetadata> metadata;
+
+  // Load metadata only when we need to create a target
+  if (!metadata) {
+    auto errorMessage =
+      makefile->GetDefinition("CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE");
+    if (!errorMessage.IsEmpty()) {
+      makefile->IssueMessage(
+        MessageType::FATAL_ERROR,
+        cmStrCat(R"(The "CXX_MODULE_STD" property on target ")", targetName,
+                 "\" requires toolchain support, but it was not provided.  "
+                 "Reason:\n  ",
+                 *errorMessage));
+      return false;
+    }
+
+    auto metadataPath =
+      makefile->GetDefinition("CMAKE_CXX_STDLIB_MODULES_JSON");
+    if (metadataPath.IsEmpty()) {
+      makefile->IssueMessage(
+        MessageType::FATAL_ERROR,
+        cmStrCat(
+          R"("The "CXX_MODULE_STD" property on target ")", targetName,
+          "\" requires CMAKE_CXX_STDLIB_MODULES_JSON be set, but it was not "
+          "provided by the toolchain."));
+      return false;
+    }
+
+    auto parseResult = cmCxxModuleMetadata::LoadFromFile(*metadataPath);
+    if (!parseResult) {
+      makefile->IssueMessage(
+        MessageType::FATAL_ERROR,
+        cmStrCat("Failed to load C++ standard library modules metadata "
+                 "from \"",
+                 *metadataPath, "\": ", parseResult.Error));
+      return false;
+    }
+
+    metadata = std::move(*parseResult.Meta);
+  }
+
+  auto* stdlibTgt = makefile->AddImportedTarget(
+    cxxTargetName, cmStateEnums::INTERFACE_LIBRARY, true);
+  cmCxxModuleMetadata::PopulateTarget(*stdlibTgt, *metadata, configs);
+  stdlibTgt->AppendProperty("IMPORTED_CXX_MODULES_COMPILE_FEATURES",
+                            cmStrCat("cxx_std_", stdLevel));
+
+  lg->AddGeneratorTarget(cm::make_unique<cmGeneratorTarget>(stdlibTgt, lg));
+
+#endif // CMAKE_BOOTSTRAP
+
+  return true;
+}
+} // namespace
+
 bool cmGeneratorTarget::ApplyCXXStdTargets()
 {
   cmStandardLevelResolver standardResolver(this->Makefile);
@@ -5207,35 +5272,33 @@ bool cmGeneratorTarget::ApplyCXXStdTargets()
 
     auto const stdLevel =
       standardResolver.GetLevelString("CXX", *explicitLevel);
-    auto const targetName = cmStrCat("__CMAKE::CXX", stdLevel);
-    if (!this->Makefile->FindTargetToUse(targetName)) {
-      auto basicReason = this->Makefile->GetDefinition(cmStrCat(
-        "CMAKE_CXX", stdLevel, "_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE"));
-      std::string reason;
-      if (!basicReason.IsEmpty()) {
-        reason = cmStrCat("  Reason:\n  ", basicReason);
-      }
-      this->Makefile->IssueMessage(
-        MessageType::FATAL_ERROR,
-        cmStrCat(R"(The "CXX_MODULE_STD" property on the target ")",
-                 this->GetName(), "\" requires that the \"", targetName,
-                 "\" target exist, but it was not provided by the toolchain.",
-                 reason));
-      break;
-    }
-
-    // Check the experimental feature here as well. A toolchain may have
-    // provided the target and skipped the check in the toolchain preparation
-    // logic.
-    if (!cmExperimental::HasSupportEnabled(
-          *this->Makefile, cmExperimental::Feature::CxxImportStd)) {
-      break;
+    auto const cxxTargetName = cmStrCat("__CMAKE::CXX", stdLevel);
+
+    // Create the __CMAKE::CXX## IMPORTED interface target if it doesn't
+    // already exist
+    if (!this->Makefile->FindTargetToUse(cxxTargetName) &&
+        !CreateCxxStdlibTarget(this->Makefile, this->LocalGenerator,
+                               this->GetName(), cxxTargetName, stdLevel,
+                               configs)) {
+      return false;
     }
 
     this->Target->AppendProperty(
       "LINK_LIBRARIES",
-      cmStrCat("$<BUILD_LOCAL_INTERFACE:$<$<CONFIG:", config, ">:", targetName,
-               ">>"));
+      cmStrCat("$<BUILD_LOCAL_INTERFACE:$<$<CONFIG:", config,
+               ">:", cxxTargetName, ">>"));
+  }
+
+  // Check the experimental feature here. A toolchain may have
+  // skipped the check in the toolchain preparation logic.
+  if (!cmExperimental::HasSupportEnabled(
+        *this->Makefile, cmExperimental::Feature::CxxImportStd)) {
+    this->Makefile->IssueMessage(
+      MessageType::FATAL_ERROR,
+      "Experimental `import std` support not enabled when detecting "
+      "toolchain; it must be set before `CXX` is enabled (usually a "
+      "`project()` call).");
+    return false;
   }
 
   return true;
@@ -5335,7 +5398,9 @@ bool cmGeneratorTarget::DiscoverSyntheticTargets(cmSyntheticTargetCache& cache,
         }
         // See `cmGlobalGenerator::ApplyCXXStdTargets` in
         // `cmGlobalGenerator::Compute` for non-synthetic target resolutions.
-        gtp->ApplyCXXStdTargets();
+        if (!gtp->ApplyCXXStdTargets()) {
+          return false;
+        }
 
         gtp->DiscoverSyntheticTargets(cache, config);
 

Source/cmGlobalGenerator.cxx

@@ -1787,7 +1787,17 @@ void cmGlobalGenerator::ComputeTargetOrder(cmGeneratorTarget const* gt,
 bool cmGlobalGenerator::ApplyCXXStdTargets()
 {
   for (auto const& gen : this->LocalGenerators) {
-    for (auto const& tgt : gen->GetGeneratorTargets()) {
+
+    // tgt->ApplyCXXStd can create targets itself, so we need iterators which
+    // won't be invalidated by that target creation
+    auto const& genTgts = gen->GetGeneratorTargets();
+    std::vector<cmGeneratorTarget*> existingTgts;
+    existingTgts.reserve(genTgts.size());
+    for (auto const& tgt : genTgts) {
+      existingTgts.push_back(tgt.get());
+    }
+
+    for (auto const& tgt : existingTgts) {
       if (!tgt->ApplyCXXStdTargets()) {
         return false;
       }

Tests/RunCMake/CXXModules/Inspect.cmake

@@ -37,6 +37,7 @@ set(CMAKE_CXX_COMPILE_FEATURES \"${CMAKE_CXX_COMPILE_FEATURES}\")
 set(CMAKE_MAKE_PROGRAM \"${CMAKE_MAKE_PROGRAM}\")
 set(forced_cxx_standard \"${forced_cxx_standard}\")
 set(have_cxx23_import_std \"${have_cxx23_import_std}\")
+set(have_cxx26_import_std \"${have_cxx26_import_std}\")
 set(CMAKE_CXX_COMPILER_VERSION \"${CMAKE_CXX_COMPILER_VERSION}\")
 set(CMAKE_CXX_OUTPUT_EXTENSION \"${CMAKE_CXX_OUTPUT_EXTENSION}\")
 set(CXXModules_default_build_type \"${CMAKE_BUILD_TYPE}\")

Tests/RunCMake/CXXModules/NoCXX23TargetRequired-stderr.txt

@@ -1,9 +1,8 @@
 CMake Error in CMakeLists\.txt:
-  The "CXX_MODULE_STD" property on the target "nocxx23target" requires that
-  the "__CMAKE::CXX23" target exist, but it was not provided by the
-  toolchain\.  Reason:
+  The "CXX_MODULE_STD" property on target "nocxx23target" requires toolchain
+  support, but it was not provided.  Reason:
 
-    (Toolchain does not support discovering `import std` support|Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled \(usually a `project\(\)` call\)|Unsupported generator: [^\n]*)
+    (Toolchain does not support discovering module metadata|Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled \(usually a `project\(\)` call\)|Unsupported generator: [^\n]*)
 
 
 CMake Generate step failed\.  Build files cannot be regenerated correctly\.

Tests/RunCMake/CXXModules/NoCXX26TargetRequired-stderr.txt

@@ -1,9 +1,8 @@
 CMake Error in CMakeLists\.txt:
-  The "CXX_MODULE_STD" property on the target "nocxx26target" requires that
-  the "__CMAKE::CXX26" target exist, but it was not provided by the
-  toolchain\.  Reason:
+  The "CXX_MODULE_STD" property on target "nocxx26target" requires toolchain
+  support, but it was not provided.  Reason:
 
-    (Toolchain does not support discovering `import std` support|Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled \(usually a `project\(\)` call\)|Unsupported generator: [^\n]*)
+    (Toolchain does not support discovering module metadata|Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled \(usually a `project\(\)` call\)|Unsupported generator: [^\n]*)
 
 
 CMake Generate step failed\.  Build files cannot be regenerated correctly\.

Tests/RunCMake/CXXModulesCompile/imp-std-exp-no-std-build-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-exp-no-std-install-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-no-std-prop-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-not-in-exp-build-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-not-in-exp-install-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-trans-exp-no-std-build-stderr.txt

@@ -0,0 +1,8 @@
+CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0-9]* \(cmake_language\):
+  CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
+  is meant only for experimentation and feedback to CMake developers\.
+Call Stack \(most recent call first\):
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
+  .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
+  CMakeLists\.txt:[0-9]* \(project\)
+This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-trans-exp-no-std-install-stderr.txt

@@ -0,0 +1,8 @@
+CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0-9]* \(cmake_language\):
+  CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
+  is meant only for experimentation and feedback to CMake developers\.
+Call Stack \(most recent call first\):
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
+  .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
+  CMakeLists\.txt:[0-9]* \(project\)
+This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-trans-not-in-exp-build-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CXXModulesCompile/imp-std-trans-not-in-exp-install-stderr.txt

@@ -2,7 +2,7 @@ CMake Warning \(dev\) at .*/Modules/Compiler/CMakeCommonCompilerMacros\.cmake:[0
   CMake's support for `import std;` in C\+\+23 and newer is experimental\.  It
   is meant only for experimentation and feedback to CMake developers\.
 Call Stack \(most recent call first\):
-  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_create_cxx_import_std\)
+  .*/Modules/CMakeDetermineCompilerSupport\.cmake:[0-9]* \(cmake_cxx_find_modules_json\)
   .*/Modules/CMakeTestCXXCompiler\.cmake:[0-9]* \(CMAKE_DETERMINE_COMPILER_SUPPORT\)
   CMakeLists\.txt:[0-9]* \(project\)
 This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/property_init/CompileSources.cmake

@@ -102,7 +102,6 @@ set(properties
   "C_LINKER_LAUNCHER"                       "ccache"            "<SAME>"
   ### C++
   "CXX_LINKER_LAUNCHER"                     "ccache"            "<SAME>"
-  "CXX_MODULE_STD"                          "ON"                "<SAME>"
   ### CUDA
   "CUDA_RESOLVE_DEVICE_SYMBOLS"             "ON"                "<SAME>"
   "CUDA_RUNTIME_LIBRARY"                    "Static"            "<SAME>"
@@ -229,6 +228,15 @@ macro (add_language_properties lang std)
     )
 endmacro ()
 
+set(_cmake_supported_import_std_experimental "")
+cmake_language(GET_EXPERIMENTAL_FEATURE_ENABLED "CxxImportStd" _cmake_supported_import_std_experimental)
+if(_cmake_supported_import_std_experimental)
+  list(APPEND properties
+    # property                      expected  alias
+    "CXX_MODULE_STD"                "ON"      "<SAME>"
+  )
+endif()
+
 # Mock up knowing the standard flag. This doesn't actually build, so nothing
 # should care at this point.
 set(CMAKE_Cc_std_11_STANDARD_COMPILE_OPTION "-std=c11")