Fix C++ modules BMI installation and re-enable external build tests

[arpittkhandelwal]

Summary
This PR standardizes the export of C++ module BMI files to external projects and officially re-enables the external build tests when HPX_WITH_CXX_MODULES=ON.

Fixes #7197
Closes #7202

Problem
Previously, external build tests were disabled (#7190) because HPX lacked the CMake infrastructure to correctly export and install the Binary Module Interface (BMI) generation paths downstream. Without these paths, any external project trying to import HPX modules structurally failed.

Root Cause
CMake 3.28 natively supports C++20 module dependency tracking via the FILE_SET CXX_MODULES feature, generating proper module interfaces. However, the custom macros hpx_configure_module_producer and hpx_configure_module_consumer were overriding this by manually injecting -fmodule-output= and -fprebuilt-module-path= compiler flags. This hardcoded manipulation fundamentally conflicts with modern CMake's internal module tracking and installation processes, corrupting the exported target configuration (HPXTargets.cmake).

What changed
Since HPX_WITH_CXX_MODULES now enforces CMAKE_VERSION >= 3.29, we can entirely rely on native functionality:

• Stripped the manual -fmodule-output= flags from HPX_CXXModules.cmake.
• Stripped the manual -fprebuilt-module-path= flags from HPX_CXXModules.cmake (safely retained fuse-ld=lld specifically for Clang).
• Removed the return() check block from tests/unit/build/CMakeLists.txt that bypassed downstream tests.

Why this fixes the issue
The native CMake target definitions now assume full control over BMI file sets, successfully resolving paths dynamically. When an external project consumes HPX via find_package(HPX) through the CI pipeline, CMake inherently handles the BMI dependencies.

[codacy-production]

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[Copilot]

Pull request overview

This PR updates HPX’s CMake module support to rely on CMake’s native C++20 modules/BMI handling (requiring CMake ≥ 3.29) and re-enables the external dependent-project build tests when HPX_WITH_CXX_MODULES=ON.

Changes:

• Removed manual compiler flag injection for module BMI output (-fmodule-output=...) and prebuilt module search paths (-fprebuilt-module-path=...) in HPX_CXXModules.cmake.
• Re-enabled tests/unit/build external build tests for module-enabled builds by removing the early return() guard.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tests/unit/build/CMakeLists.txt	Re-enables external build tests when modules are enabled.
cmake/HPX_CXXModules.cmake	Drops manual module BMI flag management to let modern CMake manage BMI generation/consumption.

[Copilot]

Pull request overview

This PR updates HPX’s CMake C++20 modules support to rely on CMake’s native module/BMI handling (CMake ≥ 3.29) and re-enables downstream external-build CI coverage when HPX_WITH_CXX_MODULES=ON.

Changes:

• Re-enabled external build tests by removing the HPX_WITH_CXX_MODULES early-return in tests/unit/build/CMakeLists.txt.
• Simplified module producer configuration to stop overriding CMake’s BMI/output handling, leaning on native FILE_SET ... TYPE CXX_MODULES / CXX_MODULES_BMI.
• Updated cmake-format command metadata to match the updated hpx_configure_module_producer signature.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/unit/build/CMakeLists.txt	Removes the guard that skipped external build tests when modules are enabled.
libs/CMakeLists.txt	Updates module producer invocation to match the simplified macro API.
cmake/HPX_CXXModules.cmake	Removes manual module output/prebuilt-path flags and relies on CMake-native scanning/export behavior.
.cmake-format.py	Adjusts formatting metadata for the updated CMake function signature.

[Copilot]

Pull request overview

This PR updates HPX’s CMake support for C++20 modules to rely on CMake’s native FILE_SET CXX_MODULES/BMI handling (now that HPX_WITH_CXX_MODULES requires newer CMake), and re-enables external build tests that were previously disabled for modules-enabled builds.

Changes:

• Remove manual compiler flag injection for module BMI output/consumption from HPX_CXXModules.cmake and simplify the producer macro API.
• Re-enable tests/unit/build external build tests when C++ modules are enabled, with a pkg-config build tweak to force header fallback where needed.
• Make module-consumer setup more robust across build-tree vs installed-target naming by checking for HPXInternal:: targets.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/unit/build/CMakeLists.txt	Re-enables external build tests with modules enabled; adjusts pkg-config build flags to disable module imports for that test path.
libs/CMakeLists.txt	Updates module producer setup to match the simplified hpx_configure_module_producer API.
cmake/HPX_SetupTarget.cmake	Adds support for consuming either build-tree or HPXInternal::-namespaced module interface targets when configuring module scanning.
cmake/HPX_CXXModules.cmake	Removes manual -fmodule-output/-fprebuilt-module-path flag management, relying on native CMake module handling.
.cmake-format.py	Updates cmake-format parser config for the updated hpx_configure_module_producer signature.

Comments suppressed due to low confidence (1)

cmake/HPX_CXXModules.cmake:91

• The doc comment for hpx_configure_module_consumer says it "sets necessary consumer compiler flags for clang and gcc", but the function now only propagates CXX_SCAN_FOR_MODULES and adds Clang linker flags (-fuse-ld=lld, --error-limit=0). Please update the comment to reflect the current behavior (or reintroduce the described compiler-flag propagation if still required).

# hpx_configure_module_consumer(<consumer> <producer>])
#
# * propagates module-related properties from producer interface target
# * sets necessary consumer compiler flags for clang and gcc
function(hpx_configure_module_consumer consumer producer)

[hkaiser]

@arpittkhandelwal The changes to the build system seem not to have broken anything - good. However, they have notfixed anything either :/ Also, I think the copilot comments bear some truths with them.

[Copilot]

Pull request overview

This PR updates HPX’s CMake infrastructure for C++20 modules to rely on CMake’s native BMI/module handling, and re-enables external build tests when HPX_WITH_CXX_MODULES=ON so downstream find_package(HPX) consumers are exercised again.

Changes:

• Remove manual compiler flag injection for module BMI output/consumption and rely on CMake’s native module scanning/FILE_SET behavior.
• Re-enable external build tests for module-enabled builds and adjust pkg-config external test flags.
• Improve module-consumer setup logic to handle both in-tree and exported HPXInternal:: module interface targets.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/unit/build/CMakeLists.txt	Re-enables external build tests with modules and adjusts pkg-config CXX flags.
libs/CMakeLists.txt	Updates module producer configuration call to match the simplified producer macro.
examples/hello_world_component/CMakeLists.txt	Adds module-aware linking/scanning for the external hello-world example.
cmake/HPX_SetupTarget.cmake	Makes module consumer configuration robust to HPXInternal:: exported target names.
cmake/HPX_CXXModules.cmake	Removes manual BMI path flags and simplifies producer/consumer module configuration.
.cmake-format.py	Updates formatting metadata for the changed CMake macro signature.

[hkaiser]

@arpittkhandelwal I like your solution to the issue a lot. Thanks. However, while some of the external target now build, some others do not build yet. Please have a look at #7202, which applies other changes that may be helpful in resolving the issue.

Could you please tend to the cmake-format issue reported? Also, please either accept the copilot changes or mark them as being irrelevant.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

[arpittkhandelwal]

@hkaiser sir I found the root cause of the HPX.Core not found CI failures.

We had a conflict between our manual compiler flags and CMake 3.28+'s native module tracking. CMake automatically generates BMIs to CMakeFiles/.dir/ via FILE_SET CXX_MODULES, but our manual -fmodule-output flags were forcing Clang to write to a completely different directory. This caused a path mismatch when consumers tried to import HPX.Core.

Fix: I completely removed the manual -fmodule-output and -fprebuilt-module-path flags from HPX_CXXModules.cmake. We now rely 100% on CMake's native FILE_SET CXX_MODULES and INTERFACE_CXX_SCAN_FOR_MODULES ON to automatically map dependencies and paths for both internal and external builds.

[hkaiser]

@arpittkhandelwal Wow, this seems to have done the trick! Thank you so much! I think we can go ahead and merge this PR now.

[hkaiser]

Otherwise, LGTM! Thanks!

[arpittkhandelwal]

@hkaiser Fixed both I removed the redundant CMake version guard in HPX_GeneratePackage.cmake (since modules already require CMake 3.28+) and corrected the misleading comment in HPX_CollectStdHeaders.cmake to accurately describe the STANDARD_LIBRARY_HEADERS filtering.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[hkaiser]

The failing ARM cross-compiling CIs point towards a missing CMake version check (that CI currently uses CMake V3.22.1). We need to update that docker image (unrelated to this PR), but should also make sure we see a proper error messages if CMake doesn't support C++ modules yet and those are enabled. For this PR here we should make sure that C++ module CMake constructs are being used only if HPX_WITH_CXX_MODULES=ON.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 duplication

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TIP This summary will be updated as you push new changes.}

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

cmake/HPX_GeneratePackage.cmake:35

• CXX_MODULES_DIRECTORY is only being passed to the build-tree export() calls. The corresponding install(EXPORT ...) steps for HPXInternalTargets and HPXTargets don't pass CXX_MODULES_DIRECTORY, which can result in the installed package missing the per-target C++ module metadata directory needed for downstream find_package(HPX) consumption with modules enabled. Consider forwarding the same ${_cxx_modules_directory_arg} to both install(EXPORT ...) calls as well (guarded by HPX_WITH_CXX_MODULES).

# Export HPXInternalTargets in the build directory. Use the EXPORT signature so
# CMake also generates the per-target C++ module metadata files. Note:
# CXX_MODULES_DIRECTORY requires CMake 3.28+, which is already the minimum
# required when HPX_WITH_CXX_MODULES is enabled.
export(
  TARGETS ${HPX_EXPORT_INTERNAL_TARGETS}
  NAMESPACE HPXInternal::
  FILE "${CMAKE_CURRENT_BINARY_DIR}/lib/cmake/${HPX_PACKAGE_NAME}/HPXInternalTargets.cmake"
       CXX_MODULES_DIRECTORY cxx-modules
)

# Export HPXInternalTargets in the install directory
install(
  EXPORT HPXInternalTargets

[arpittkhandelwal]

The failing ARM cross-compiling CIs point towards a missing CMake version check (that CI currently uses CMake V3.22.1). We need to update that docker image (unrelated to this PR), but should also make sure we see a proper error messages if CMake doesn't support C++ modules yet and those are enabled. For this PR here we should make sure that C++ module CMake constructs are being used only if HPX_WITH_CXX_MODULES=ON.

I've updated both the logic and the comments to correctly enforce and state CMake 3.28+ as the minimum. I also guarded the CXX_MODULES_DIRECTORY construct in export() so it is exclusively used when HPX_WITH_CXX_MODULES=ON.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.