Fix C++20 modules BMI configuration and stabilize external build tests

- Resolved Linux configuration mismatches (GNU extensions) by propagating CMAKE_CXX_EXTENSIONS and HPX_CXX_STANDARD to external tests.
- Fixed Windows installation crashes by removing CXX_MODULES_DIRECTORY from install(EXPORT) and adding MSVC guards for Clang linker flags.
- Improved module metadata propagation by explicitly linking HPXInternal module targets in external examples.
- Added CXX_STANDARD support to hpx_setup_target and applied it to hello_world_component.
- Skipped pkg-config tests when modules are enabled due to lack of BMI discovery support.
- Fixed CMake quoting and logic bugs in standard header collection.
- Applied project-wide cmake-format and resolved CI linting failures.

Author: arpittkhandelwal

.cmake-format.py

@@ -586,7 +586,7 @@
                                                     'INCLUDE_DIRS': '+',
                                                     'FOUND_HEADERS': 1},
                           'pargs': { 'flags': [], 'nargs': '1+'}},
-    'hpx_configure_module_producer': { 'kwargs': { 'MODULE_OUT_DIR': 1},
+    'hpx_configure_module_producer': { 'kwargs': {},
                           'pargs': { 'flags': [], 'nargs': '1+'}}
   }
 

cmake/HPX_CXXModules.cmake

@@ -4,7 +4,6 @@
 # Distributed under the Boost Software License, Version 1.0. (See accompanying
 # file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
-include(HPX_AddCompileFlag)
 include(HPX_Message)
 
 macro(hpx_check_cxx_modules_support)
@@ -65,79 +64,39 @@ if(NOT HPX_WITH_CXX_MODULES)
   return()
 endif()
 
-# hpx_configure_module_producer(<producer> [MODULE_OUT_DIR <dir>])
+# hpx_configure_module_producer(<producer>)
 #
-# * Ensures a stable module output dir for producer target
-# * Adds compiler flags to write module cache there (Clang/GCC)
-# * Creates an interface target '<producer>_if' for consumers to link to
+# * Creates an interface target '<producer>_if' for consumers to link to.
+# * Sets INTERFACE_CXX_SCAN_FOR_MODULES ON so that CMake's native module
+#   dependency tracking propagates to all consumers.
+#
+# CMake 3.29+ with FILE_SET CXX_MODULES handles BMI generation and
+# -fmodule-output/-fprebuilt-module-path flags automatically. No manual compiler
+# flags are needed here.
 function(hpx_configure_module_producer producer)
   if(NOT TARGET ${producer})
     hpx_error("hpx_configure_module_producer: target '${producer}' not found")
   endif()
 
-  # parse optional args
-  set(options)
-  set(one_value_args MODULE_OUT_DIR)
-  set(multi_value_args)
-  cmake_parse_arguments(
-    _args "${options}" "${one_value_args}" "${multi_value_args}" ${ARGN}
-  )
-
-  if(_args_MODULE_OUT_DIR)
-    set(_moddir "${_args_MODULE_OUT_DIR}")
-  else()
-    set(_moddir "$<TARGET_FILE_DIR:${producer}>")
-  endif()
-
   set(_iface "${producer}_if")
   if(NOT TARGET ${_iface})
     add_library(${_iface} INTERFACE)
     target_link_libraries(${_iface} INTERFACE ${producer})
   endif()
 
-  # Set a property so consumers can query the BMI directory via
-  # get_target_property.
-  set_target_properties(
-    ${_iface} PROPERTIES INTERFACE_EXPORT_MODULE_DIR "${_moddir}"
-  )
-
-  # Make sure consumers scan for the BMI
-  set_target_properties(${_iface} PROPERTIES INTERFACE_CXX_SCAN_FOR_MODULES On)
-
-  if(MSVC)
-    # MSVC: CMake/MSVC handle IFCs automatically; create a target for
-    # convenience, consumers can link to this to get ordering and include info
-    return()
-  endif()
-
-  # Compiler-specific flags to instruct where to write module cache
-  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES
-                                              "AppleClang"
-  )
-    # Clang common flags
-    target_compile_options(${producer} PRIVATE "-fmodule-output=${_moddir}")
-  elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-    # GCC: modern flags
-    hpx_add_target_compile_option_if_available(
-      ${producer} PRIVATE "-fmodule-output=${_moddir}" RESULT ok
-    )
-    if(NOT ok)
-      hpx_error(
-        "hpx_configure_module_producer: the used version of gcc does not support '-fmodule-output'"
-      )
-    endif()
-  else()
-    hpx_warn(
-      "hpx_configure_module_producer: unknown compiler '${CMAKE_CXX_COMPILER_ID}'; "
-      "exposing EXPORT_MODULE_DIR='${_moddir}' for manual handling"
-    )
-  endif()
+  # Propagate scanning requirement to consumers via the interface target. CMake
+  # uses this to enable its native module dependency scanning for any target
+  # that links to this interface.
+  set_target_properties(${_iface} PROPERTIES INTERFACE_CXX_SCAN_FOR_MODULES ON)
 endfunction()
 
-# hpx_configure_module_consumer(<consumer> <producer>])
+# hpx_configure_module_consumer(<consumer> <producer>)
+#
+# * Links the consumer to the producer interface target.
+# * Enables CMake's native module scanning on the consumer.
 #
-# * propagates module-related properties from producer interface target
-# * sets necessary consumer compiler flags for clang and gcc
+# CMake 3.29+ automatically resolves the BMI location from the FILE_SET
+# CXX_MODULES declared on the producer. No -fprebuilt-module-path needed.
 function(hpx_configure_module_consumer consumer producer)
   if(NOT TARGET ${consumer})
     hpx_error("hpx_configure_module_consumer: target '${consumer}' not found")
@@ -146,41 +105,20 @@ function(hpx_configure_module_consumer consumer producer)
     hpx_error("hpx_configure_module_consumer: target '${producer}' not found")
   endif()
 
+  # Imported module metadata is only picked up from direct link dependencies.
+  # Link the underlying module target directly when the producer follows the
+  # '<module>_if' wrapper pattern.
+  if(producer MATCHES "_if$")
+    string(REGEX REPLACE "_if$" "" _producer_target "${producer}")
+    if(TARGET ${_producer_target})
+      target_link_libraries(${consumer} PRIVATE ${_producer_target})
+    endif()
+  endif()
+
   target_link_libraries(${consumer} PRIVATE ${producer})
+
   get_target_property(_scan ${producer} INTERFACE_CXX_SCAN_FOR_MODULES)
   if(_scan)
     set_target_properties(${consumer} PROPERTIES CXX_SCAN_FOR_MODULES ${_scan})
   endif()
-
-  get_target_property(_module_dir ${producer} INTERFACE_EXPORT_MODULE_DIR)
-  if(_module_dir)
-    if(MSVC)
-      return()
-    elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID
-                                                    MATCHES "AppleClang"
-    )
-      target_compile_options(
-        ${consumer} PRIVATE "-fprebuilt-module-path=${_module_dir}"
-      )
-      get_target_property(_type ${consumer} TYPE)
-      if((_type STREQUAL "SHARED_LIBRARY") OR (_type STREQUAL "EXECUTABLE"))
-        target_link_options(${consumer} PRIVATE "-fuse-ld=lld")
-        target_link_options(${consumer} PRIVATE "-Wl,--error-limit=0")
-      endif()
-    elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-      hpx_add_target_compile_option_if_available(
-        ${consumer} PRIVATE "-fprebuilt-module-path=${_module_dir}" RESULT ok
-      )
-      if(NOT ok)
-        hpx_error(
-          "hpx_configure_module_consumer: the used version of gcc does not "
-          "support '-fprebuilt-module-path='"
-        )
-      endif()
-    else()
-      hpx_warn(
-        "hpx_configure_module_consumer: unknown compiler '${CMAKE_CXX_COMPILER_ID}'"
-      )
-    endif()
-  endif()
 endfunction()

cmake/HPX_CollectStdHeaders.cmake

@@ -134,11 +134,10 @@ function(hpx_extract_includes_from_file module)
   foreach(include ${includes})
     string(REGEX REPLACE "#include (<[^>]+>)" "\\1" filename ${include})
 
-    if(NOT filename MATCHES "\\.|/")
-      # Check if the include is a standard library header
-      if(${filename} IN_LIST STANDARD_LIBRARY_HEADERS)
-        list(APPEND found_includes ${filename})
-      endif()
+    if("${filename}" IN_LIST STANDARD_LIBRARY_HEADERS)
+      # Capture standard library headers (with or without extensions, e.g.
+      # <vector>, <link.h>, <dlfcn.h>).
+      list(APPEND found_includes "${filename}")
     endif()
   endforeach()
 

cmake/HPX_GeneratePackage.cmake

@@ -19,11 +19,19 @@ write_basic_package_version_file(
   COMPATIBILITY AnyNewerVersion
 )
 
-# Export HPXInternalTargets in the build directory
+# CXX_MODULES_DIRECTORY was added in CMake 3.28
+set(_cxx_modules_directory_arg)
+if(CMAKE_VERSION VERSION_GREATER_EQUAL "3.28")
+  set(_cxx_modules_directory_arg CXX_MODULES_DIRECTORY cxx-modules)
+endif()
+
+# Export HPXInternalTargets in the build directory. Use the EXPORT signature so
+# CMake also generates the per-target C++ module metadata files.
 export(
   TARGETS ${HPX_EXPORT_INTERNAL_TARGETS}
   NAMESPACE HPXInternal::
   FILE "${CMAKE_CURRENT_BINARY_DIR}/lib/cmake/${HPX_PACKAGE_NAME}/HPXInternalTargets.cmake"
+       ${_cxx_modules_directory_arg}
 )
 
 # Export HPXInternalTargets in the install directory
@@ -35,11 +43,13 @@ install(
   COMPONENT cmake
 )
 
-# Export HPXTargets in the build directory
+# Export HPXTargets in the build directory. Use the EXPORT signature so CMake
+# also generates the per-target C++ module metadata files.
 export(
   TARGETS ${HPX_EXPORT_TARGETS}
   NAMESPACE HPX::
   FILE "${CMAKE_CURRENT_BINARY_DIR}/lib/cmake/${HPX_PACKAGE_NAME}/HPXTargets.cmake"
+       ${_cxx_modules_directory_arg}
 )
 
 # Add aliases with the namespace for use within HPX

cmake/HPX_SetupTarget.cmake

@@ -38,6 +38,7 @@ function(hpx_setup_target target)
       HPX_PREFIX
       HEADER_ROOT
       SCAN_FOR_MODULES
+      CXX_STANDARD
   )
   set(multi_value_args DEPENDENCIES COMPONENT_DEPENDENCIES COMPILE_FLAGS
                        LINK_FLAGS INSTALL_FLAGS INSTALL_PDB
@@ -87,6 +88,13 @@ function(hpx_setup_target target)
     hpx_debug("setup_target.${target}" "LINK_FLAGS: ${target_LINK_FLAGS}")
   endif()
 
+  if(target_CXX_STANDARD)
+    set_target_properties(
+      ${target} PROPERTIES CXX_STANDARD ${target_CXX_STANDARD}
+    )
+    hpx_debug("setup_target.${target}" "CXX_STANDARD: ${target_CXX_STANDARD}")
+  endif()
+
   if(target_NAME)
     set(name "${target_NAME}")
   else()
@@ -232,9 +240,21 @@ function(hpx_setup_target target)
   if(HPX_WITH_CXX_MODULES AND target_SCAN_FOR_MODULES)
     hpx_debug("setup_target.${target} SCAN_FOR_MODULES: ON")
 
-    hpx_configure_module_consumer(${target} hpx_core_module_if)
+    if(TARGET hpx_core_module_if)
+      hpx_configure_module_consumer(${target} hpx_core_module_if)
+    elseif(TARGET HPXInternal::hpx_core_module_if)
+      hpx_configure_module_consumer(${target} HPXInternal::hpx_core_module_if)
+    else()
+      hpx_error(
+        "setup_target.${target}: C++ modules scanning is enabled, but neither "
+        "hpx_core_module_if nor HPXInternal::hpx_core_module_if exists"
+      )
+    endif()
+
     if(TARGET hpx_full_module_if)
       hpx_configure_module_consumer(${target} hpx_full_module_if)
+    elseif(TARGET HPXInternal::hpx_full_module_if)
+      hpx_configure_module_consumer(${target} HPXInternal::hpx_full_module_if)
     endif()
   else()
     hpx_debug("setup_target.${target} SCAN_FOR_MODULES: OFF")
@@ -247,8 +267,9 @@ function(hpx_setup_target target)
 
       # If modules are enabled, Clang emits DWARF v5, which requires using lld
       # instead of ld.
-      if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES
-                                                  "AppleClang"
+      if((NOT MSVC)
+         AND (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID
+                                                       MATCHES "AppleClang")
       )
         get_target_property(_type ${target} TYPE)
         if((_type STREQUAL "SHARED_LIBRARY") OR (_type STREQUAL "EXECUTABLE"))

examples/gtest_emulation/CMakeLists.txt

@@ -14,10 +14,23 @@ if(EXISTS "${HPX_DIR}")
 
   if(HPX_WITH_CXX_MODULES)
     set(CMAKE_CXX_SCAN_FOR_MODULES ON)
+    # CMake only propagates imported C++ module metadata from targets that are
+    # linked directly by the consumer. The exported HPXInternal module targets
+    # are therefore linked explicitly for the external build tests.
+    set(hpx_cxx_module_targets)
+    if(TARGET HPXInternal::hpx_core_module)
+      list(APPEND hpx_cxx_module_targets HPXInternal::hpx_core_module)
+    endif()
+    if(TARGET HPXInternal::hpx_full_module)
+      list(APPEND hpx_cxx_module_targets HPXInternal::hpx_full_module)
+    endif()
   endif()
 
   # Add a static library which contains a main to emulate gtest_main
   add_library(static_main_lib STATIC static_main.cpp)
+  set_target_properties(
+    static_main_lib PROPERTIES CXX_STANDARD ${HPX_CXX_STANDARD}
+  )
 
   # /!\ This helper interface is needed to keep the right linking order
   add_library(hpx_helper_interface INTERFACE)
@@ -27,7 +40,14 @@ if(EXISTS "${HPX_DIR}")
 
   # Test with the main function in a separate static library
   add_executable(hpx_main_ext_main hpx_main_ext_main.cpp)
-  target_link_libraries(hpx_main_ext_main PRIVATE hpx_helper_interface)
+  # Keep the helper interface for link order, but link module targets directly
+  # to the executable so CMake can see the imported BMI metadata.
+  target_link_libraries(
+    hpx_main_ext_main PRIVATE hpx_helper_interface ${hpx_cxx_module_targets}
+  )
+  set_target_properties(
+    hpx_main_ext_main PROPERTIES CXX_STANDARD ${HPX_CXX_STANDARD}
+  )
 
   enable_testing()
   add_test(hello_world_test hpx_main_ext_main)

examples/hello_world_component/CMakeLists.txt

@@ -11,12 +11,22 @@ project(hello_world_client CXX)
 if(EXISTS "${HPX_DIR}")
   find_package(HPX REQUIRED)
 
-  if(HPX_WITH_DISTRIBUTED_RUNTIME)
-    add_library(hello_world_component SHARED hello_world_component.cpp)
-  endif()
-
   if(HPX_WITH_CXX_MODULES)
     set(CMAKE_CXX_SCAN_FOR_MODULES ON)
+    # CMake only propagates imported C++ module metadata from targets that are
+    # linked directly by the consumer. The exported HPXInternal module targets
+    # are therefore linked explicitly for the external build tests.
+    set(hpx_cxx_module_targets)
+    if(TARGET HPXInternal::hpx_core_module)
+      list(APPEND hpx_cxx_module_targets HPXInternal::hpx_core_module)
+    endif()
+    if(TARGET HPXInternal::hpx_full_module)
+      list(APPEND hpx_cxx_module_targets HPXInternal::hpx_full_module)
+    endif()
+  endif()
+
+  if(HPX_WITH_DISTRIBUTED_RUNTIME)
+    add_library(hello_world_component SHARED hello_world_component.cpp)
   endif()
 
   add_executable(hello_world_client hello_world_client.cpp)
@@ -26,11 +36,15 @@ if(EXISTS "${HPX_DIR}")
     if(HPX_WITH_DISTRIBUTED_RUNTIME)
       target_link_libraries(
         hello_world_component PUBLIC HPX::hpx HPX::iostreams_component
+                                     ${hpx_cxx_module_targets}
       )
       target_link_libraries(hello_world_component PRIVATE HPX::component)
       target_link_libraries(hello_world_client PRIVATE hello_world_component)
     endif()
-    target_link_libraries(hello_world_client PRIVATE HPX::hpx HPX::wrap_main)
+    target_link_libraries(
+      hello_world_client PRIVATE HPX::hpx HPX::wrap_main
+                                 ${hpx_cxx_module_targets}
+    )
 
     # We still support not linking to HPX::wrap_main when
     # HPX_WITH_DYNAMIC_HPX_MAIN=OFF for legacy use. This can only be done using
@@ -44,6 +58,7 @@ if(EXISTS "${HPX_DIR}")
 
       target_link_libraries(
         hello_world_client_only_hpx_init PRIVATE hello_world_component
+                                                 ${hpx_cxx_module_targets}
       )
     endif()
   elseif("${SETUP_TYPE}" STREQUAL "MACROS")
@@ -53,10 +68,20 @@ if(EXISTS "${HPX_DIR}")
         COMPONENT_DEPENDENCIES iostreams
         DEPENDENCIES HPX::wrap_main
         TYPE COMPONENT
+        SCAN_FOR_MODULES ${HPX_WITH_CXX_MODULES} CXX_STANDARD
+                                                 ${HPX_CXX_STANDARD}
+      )
+      hpx_setup_target(
+        hello_world_client
+        DEPENDENCIES hello_world_component
+        SCAN_FOR_MODULES ${HPX_WITH_CXX_MODULES} CXX_STANDARD
+                                                 ${HPX_CXX_STANDARD}
       )
-      hpx_setup_target(hello_world_client DEPENDENCIES hello_world_component)
     else()
-      hpx_setup_target(hello_world_client)
+      hpx_setup_target(
+        hello_world_client SCAN_FOR_MODULES ${HPX_WITH_CXX_MODULES}
+                                            CXX_STANDARD ${HPX_CXX_STANDARD}
+      )
     endif()
   else()
     message(FATAL_ERROR "Unknown SETUP_TYPE=\"${SETUP_TYPE}\"")