explicit changes to module support based on compiler versions

Author: JeffGarland

CMakeLists.txt

@@ -31,30 +31,54 @@ option(
     ${PROJECT_IS_TOP_LEVEL}
 )
 
-add_library(beman.scope)
-add_library(beman::scope ALIAS beman.scope)
-
 # gersemi: off
 
+message(
+    "Compiler is: ${CMAKE_CXX_COMPILER_ID} version:  ${CMAKE_CXX_COMPILER_VERSION}"
+)
+
+# Modules opt in only on compilers that support g++-15 and clang-20
+if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 20)
+    set(CMAKE_CXX_SCAN_FOR_MODULES 1)
+elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 15)
+    set(CMAKE_CXX_SCAN_FOR_MODULES 1)
+endif()
+
+if(CMAKE_CXX_SCAN_FOR_MODULES)
+    add_library(beman.scope)
+#    target_compile_features(beman.scope PRIVATE cxx_std_23)
+    target_sources(
+        beman.scope
+        PUBLIC
+            FILE_SET HEADERS
+            BASE_DIRS include
+            FILES include/beman/scope/scope.hpp
+        PUBLIC
+            FILE_SET CXX_MODULES
+            BASE_DIRS include
+            FILES include/beman/scope/beman.scope.cppm
+    )
+else()
+    add_library(beman.scope INTERFACE)
+#    target_compile_features(beman.scope INTERFACE cxx_std_20)
+    target_sources(
+        beman.scope
+        INTERFACE
+            FILE_SET HEADERS
+            BASE_DIRS include
+            FILES include/beman/scope/scope.hpp
+    )
+endif()
+
+add_library(beman::scope ALIAS beman.scope)
+
 set_target_properties(
     beman.scope
     PROPERTIES
         VERIFY_INTERFACE_HEADER_SETS ON
         EXPORT_NAME scope
 )
 
-target_sources(
-    beman.scope
-    PUBLIC
-        FILE_SET HEADERS
-        BASE_DIRS include
-        FILES include/beman/scope/scope.hpp
-    PUBLIC
-        FILE_SET CXX_MODULES
-        BASE_DIRS include
-        FILES include/beman/scope/beman.scope.cppm
-)
-
 include(GNUInstallDirs)
 
 install(

tests/CMakeLists.txt

@@ -9,14 +9,9 @@ FetchContent_Declare(
 )
 FetchContent_MakeAvailable(Catch2)
 
-message(
-    "Compiler is: ${CMAKE_CXX_COMPILER_ID} version:  ${CMAKE_CXX_COMPILER_VERSION}"
-)
 
 # module tests will only compile with gcc15 or clang20 and above
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 20)
-    set(ALL_TESTNAMES scope_success scope_exit scope_fail unique_resource module)
-elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 15)
+if(CMAKE_CXX_SCAN_FOR_MODULES)
     set(ALL_TESTNAMES scope_success scope_exit scope_fail unique_resource module)
 else()
     set(ALL_TESTNAMES scope_success scope_exit scope_fail unique_resource)