Add support for CMake 3.28 C++ modules (fmtlib#3679)

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.8...3.26)
+cmake_minimum_required(VERSION 3.8...3.28)
 
 # Fallback for using newer policies on CMake <3.12.
 if(${CMAKE_VERSION} VERSION_LESS 3.12)
@@ -26,7 +26,7 @@ endfunction()
 
 # DEPRECATED! Should be merged into add_module_library.
 function(enable_module target)
-  if (MSVC)
+  if (MSVC AND CMAKE_VERSION VERSION_LESS 3.28)
     set(BMI ${CMAKE_CURRENT_BINARY_DIR}/${target}.ifc)
     target_compile_options(${target}
       PRIVATE /interface /ifcOutput ${BMI}
@@ -65,7 +65,7 @@ function(add_module_library name)
   # `std` is affected by CMake options and may be higher than C++20.
   get_target_property(std ${name} CXX_STANDARD)
 
-  if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+  if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND CMAKE_VERSION VERSION_LESS 3.28)
     set(pcms)
     foreach (src ${sources})
       get_filename_component(pcm ${src} NAME_WE)
@@ -103,7 +103,11 @@ function(add_module_library name)
         DEPENDS ${pcm})
     endforeach ()
   endif ()
-  target_sources(${name} PRIVATE ${sources})
+  if(CMAKE_VERSION VERSION_LESS 3.28)
+    target_sources(${name} PRIVATE ${sources})
+  else()
+    target_sources(${name} PUBLIC FILE_SET fmt_module TYPE CXX_MODULES FILES ${sources})
+  endif()
 endfunction()
 
 include(CMakeParseArguments)
@@ -392,8 +396,14 @@ if (FMT_INSTALL)
           LIBRARY DESTINATION ${FMT_LIB_DIR}
           ARCHIVE DESTINATION ${FMT_LIB_DIR}
           PUBLIC_HEADER DESTINATION "${FMT_INC_DIR}/fmt"
+          FILE_SET fmt_module DESTINATION "${FMT_LIB_DIR}/cxx/miu"
           RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
 
+  if(FMT_MODULE AND CMAKE_VERSION VERSION_GREATER_EQUAL 3.28)
+    #Install format.cc and os.cc which are #included by the fmt.cc module interface file
+    install(FILES src/format.cc src/os.cc DESTINATION "${FMT_LIB_DIR}/cxx/miu/src")
+  endif()
+
   # Use a namespace because CMake provides better diagnostics for namespaced
   # imported targets.
   export(TARGETS ${INSTALL_TARGETS} NAMESPACE fmt::