That works at least on Darwin with g++-15

Author: ClausKlein

CMakeLists.txt

@@ -11,11 +11,12 @@ if(CMAKE_CXX_SCAN_FOR_MODULES)
     set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
         "d0edc3af-4c50-42ea-a356-e2862fe7a444"
     )
-    if(APPLE)
+    if(APPLE AND $ENV{CXX} STREQUAL g++-15)
         set(CMAKE_CXX_STDLIB_MODULES_JSON
             /usr/local/Cellar/gcc/15.2.0/lib/gcc/current/libstdc++.modules.json
         )
-        # set(CMAKE_CXX_STDLIB_MODULES_JSON $ENV{LLVM_DIR}/lib/c++/libc++.modules.json)
+    else()
+        set(CMAKE_CXX_STDLIB_MODULES_JSON $ENV{LLVM_DIR}/lib/c++/libc++.modules.json)
     endif()
 endif()
 
@@ -79,11 +80,11 @@ if(CMAKE_GENERATOR STREQUAL "Ninja" AND FMT_USE_MODULES)
         set(LLVM_DIR ${LLVM_DIR} CACHE PATH "")
         message(STATUS "LLVM_DIR=${LLVM_DIR}")
 
-        if(LINUX AND EXISTS ${LLVM_LIBC_SOURCE}/std.cppm)
-            message(WARNING "LLVM_LIBC_SOURCE=${LLVM_LIBC_SOURCE}")
-            set(ADD_STDLIB_MODULES stdc++)
-            list(APPEND CMAKE_CXX_COMPILER_IMPORT_STD 23)
-        endif()
+        # if(LINUX AND EXISTS ${LLVM_LIBC_SOURCE}/std.cppm)
+        #     message(WARNING "LLVM_LIBC_SOURCE=${LLVM_LIBC_SOURCE}")
+        #     set(ADD_STDLIB_MODULES stdc++)
+        #     list(APPEND CMAKE_CXX_COMPILER_IMPORT_STD 23)
+        # endif()
 
         # Build the stdlib module
         function(add_stdlib_module NAME)
@@ -122,7 +123,7 @@ if(CMAKE_GENERATOR STREQUAL "Ninja" AND FMT_USE_MODULES)
 
     # Tell CMake that we explicitly want `import std`.
     # This will initialize the property on all targets declared after this to 1
-    if(23 IN_LIST CMAKE_CXX_COMPILER_IMPORT_STD)
+    if(NOT APPLE AND 23 IN_LIST CMAKE_CXX_COMPILER_IMPORT_STD)
         set(CMAKE_CXX_MODULE_STD ON)
     endif()
 

module/tests/CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 4.0...4.2)
 
 project(fmt_moddule_examples LANGUAGES CXX)
 
-set(USE_MODULES ${CMAKE_CXX_MODULE_STD})
+set(USE_MODULES ON)
 
 if(PROJECT_IS_TOP_LEVEL)
     enable_testing()

tests/CMakeLists.txt

@@ -16,8 +16,8 @@ if(PROJECT_IS_TOP_LEVEL)
     set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
     #XXX find_package(fmt 12.0.0.2 EXACT REQUIRED HINT ../stagedir/lib)
+    #XXX set(ADD_STDLIB_MODULES stdc++)
 
-    set(ADD_STDLIB_MODULES stdc++)
     add_subdirectory(.. fmt_module)
 endif()