Disable module import test due to compiler stdlib issues

The test for direct 'import' statements in regular .cpp files is currently
disabled because GCC 15 and Clang 18 standard library modules expose
TU-local entities, causing compilation failures. This is a known upstream
issue, not specific to podio.

The test file and infrastructure remain in place as documentation of the
intended usage pattern when compiler support matures. It will be re-enabled
once standard library modules are stable.

Author: wdconinc

tests/unittests/CMakeLists.txt

@@ -49,32 +49,43 @@ if(PODIO_ENABLE_CXX_MODULES)
   # This test demonstrates the intended usage pattern when CMake support matures.
   # CMake 3.28+ added CXX_MODULE file sets, but consuming them in regular .cpp files
   # requires dependency scanning to work correctly, which is still evolving.
+  #
+  # KNOWN ISSUES (as of GCC 15 / CMake 3.31):
+  # - Standard library modules expose TU-local entities, causing compilation errors
+  # - Module dependency scanning doesn't fully work with imported modules in .cpp files
+  # - This test will fail to compile until these issues are resolved upstream
   if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.30)
-    message(STATUS "CMake ${CMAKE_VERSION} - adding module import demonstration test")
-    message(STATUS "  Note: Direct 'import' in .cpp files requires experimental CMake features")
-    message(STATUS "  This test documents the intended usage when CMake module support matures")
+    message(STATUS "CMake ${CMAKE_VERSION} - module import test available but expected to fail")
+    message(STATUS "  Note: Direct 'import' in .cpp files blocked by compiler/stdlib issues")
+    message(STATUS "  This test will be enabled once GCC/Clang stdlib modules are stable")
 
     # This test serves as:
-    # 1. Documentation of how modules SHOULD be consumed when CMake support is complete
-    # 2. A canary to detect when CMake module support reaches maturity
+    # 1. Documentation of how modules SHOULD be consumed when support is complete
+    # 2. A canary to detect when module support reaches maturity
     # 3. Validation that our module interfaces are correctly structured
-    add_executable(unittest_podio_import cxx_modules_import.cpp)
-    target_link_libraries(unittest_podio_import PRIVATE 
-      TestDataModel 
-      podio::podio 
-      Catch2::Catch2WithMain
-    )
-    target_compile_features(unittest_podio_import PRIVATE cxx_std_20)
-    
-    # Add the test but mark it as experimental
-    add_test(NAME unittest_import COMMAND unittest_podio_import)
-    PODIO_SET_TEST_ENV(unittest_import)
-    set_tests_properties(unittest_import PROPERTIES
-      LABELS "modules;import;experimental"
-      # This test is expected to fail until CMake module dependency scanning matures
-      # When it starts passing without code changes, CMake support has improved!
-      WILL_FAIL TRUE
-    )
+    #
+    # Currently disabled due to known compiler/stdlib issues
+    # Uncomment when GCC/Clang provide stable standard library module support
+    #
+    # add_executable(unittest_podio_import cxx_modules_import.cpp)
+    # target_link_libraries(unittest_podio_import PRIVATE 
+    #   TestDataModel 
+    #   podio::podio 
+    #   Catch2::Catch2WithMain
+    # )
+    # target_compile_features(unittest_podio_import PRIVATE cxx_std_20)
+    # 
+    # # Enable module scanning for this file
+    # set_source_files_properties(cxx_modules_import.cpp PROPERTIES
+    #   LANGUAGE CXX
+    #   CXX_SCAN_FOR_MODULES ON
+    # )
+    # 
+    # add_test(NAME unittest_import COMMAND unittest_podio_import)
+    # PODIO_SET_TEST_ENV(unittest_import)
+    # set_tests_properties(unittest_import PROPERTIES
+    #   LABELS "modules;import;experimental"
+    # )
   else()
     message(STATUS "CMake ${CMAKE_VERSION} - skipping module import test (requires 3.30+)")
   endif()

tests/unittests/cxx_modules_import.cpp

@@ -7,14 +7,16 @@
 // - Ninja or VS generator (Unix Makefiles doesn't support modules)
 // - GCC 14+ or Clang 18+ with -fmodules-ts support
 //
-// STATUS: As of CMake 3.31, consuming modules via 'import' in regular .cpp files
-// works when dependency scanning correctly identifies the module dependencies.
-// This requires proper build system configuration and may still have edge cases.
+// CURRENT STATUS (Dec 2024):
+// This test is DISABLED due to known issues with GCC 15 and Clang 18:
+// - Standard library modules expose TU-local entities causing compilation errors
+// - Module dependency scanning in CMake 3.31 doesn't fully handle std imports
+// - These are upstream compiler/stdlib issues, not podio-specific
 //
-// The goal of this test is to demonstrate the INTENDED usage pattern when
-// CMake module support becomes fully mature. For now, using traditional headers
-// alongside generated modules (as demonstrated in cxx_modules_import.cpp) is
-// the recommended approach.
+// FUTURE: This test demonstrates the INTENDED usage pattern when C++20 module
+// support matures. It will be enabled once compilers provide stable standard
+// library modules without TU-local entity issues. Until then, use traditional
+// headers alongside generated modules for maximum compatibility.
 
 import podio.core;
 import datamodel.datamodel;