[runtime-cxxmodules] Generate std_darwin.modulemap from active SDK

Dynamically extract and wrap the system modulemap from the active
SDK in a top-level 'std' module

Also remove 'std_ctype_h' module to break cyclic dependencies.

Author: devajithvs

cmake/scripts/std_modulemap_darwin_fix.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+import os
+import re
+import subprocess
+import sys
+
+
+def get_sdk_path():
+    result = subprocess.run(["xcrun", "--show-sdk-path"], capture_output=True, text=True)
+    if result.returncode != 0:
+        raise RuntimeError("Could not resolve SDK path")
+    return os.path.realpath(result.stdout.strip())
+
+
+def remove_ctype_module(content):
+    # Break cyclic module dependencies
+    # See: https://github.com/root-project/root/commit/8045591a17125b49c1007787c586868dea764479
+    pattern = re.compile(r"module\s+std_ctype_h\s+\[system\]\s*\{.*?\}", re.DOTALL)
+    return pattern.sub("", content)
+
+
+def main():
+    if len(sys.argv) < 2:
+        raise ValueError("Usage: std_modulemap_darwin_fix.py <output_path>")
+    output_path = sys.argv[1]
+    sdk = get_sdk_path()
+    cpp_modulemap = os.path.join(sdk, "usr/include/c++/v1/module.modulemap")
+
+    if not os.path.exists(cpp_modulemap):
+        raise FileNotFoundError(f"Cannot find libc++ modulemap at {cpp_modulemap}")
+
+    with open(cpp_modulemap, "r") as f:
+        original_content = f.read()
+
+    cleaned_content = remove_ctype_module(original_content)
+
+    os.makedirs(os.path.dirname(output_path), exist_ok=True)
+    with open(output_path, "w") as f:
+        f.write("// Auto-generated flat modulemap for Cling\n")
+        f.write("module std [system] {\n")
+        f.write("export *\n\n")
+        f.write("// Entire modulemap wrapped\n")
+        f.write(cleaned_content)
+        f.write("\n}\n")
+
+
+if __name__ == "__main__":
+    main()

core/clingutils/CMakeLists.txt

@@ -132,7 +132,18 @@ if (runtime_cxxmodules)
     elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 17.0.0.17000013)
       set(custom_modulemaps ${custom_modulemaps} std_darwin.MacOSX15.2.sdk.modulemap)
     else()
-      set(custom_modulemaps ${custom_modulemaps} std_darwin.modulemap)
+      set(GENERATED_MODULEMAP ${CMAKE_BINARY_DIR}/std_darwin.modulemap)
+
+      add_custom_command(
+        OUTPUT ${GENERATED_MODULEMAP}
+        COMMAND ${Python3_EXECUTABLE} ${CMAKE_SOURCE_DIR}/cmake/scripts/std_modulemap_darwin_fix.py ${GENERATED_MODULEMAP}
+        DEPENDS ${CMAKE_SOURCE_DIR}/cmake/scripts/std_modulemap_darwin_fix.py
+        COMMENT "Generating wrapped std modulemap for macOS SDK"
+      )
+      add_custom_target(generate_std_modulemap DEPENDS ${GENERATED_MODULEMAP})
+      list(APPEND copy_commands COMMAND ${CMAKE_COMMAND} -E copy ${GENERATED_MODULEMAP} ${CMAKE_BINARY_DIR}/etc/cling/std_darwin.modulemap)
+      list(APPEND files_to_copy ${GENERATED_MODULEMAP})
+      install(FILES ${CMAKE_BINARY_DIR}/etc/cling/std_darwin.modulemap DESTINATION ${CMAKE_INSTALL_SYSCONFDIR}/cling)
     endif()
   endif()
 
@@ -206,6 +217,9 @@ else()
         COMMENT "Copying LLVM resource and header files")
 endif()
 add_custom_target(LLVMRES DEPENDS ${stamp_file} CLING)
+if(GENERATED_MODULEMAP)
+  add_dependencies(LLVMRES generate_std_modulemap)
+endif()
 # CLING is a shorthand for CLING_LIBRARIES and some other clang-specific
 # dependencies which ensure the correct order of building. Then the cling header
 # files (such as RuntimeUniverse.h) are moved to a semi-private place in ROOT