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]

Not up to standards ⛔

🔴 Issues 2 high

Alerts:
⚠ 2 issues (≤ 0 issues of at least minor severity)

Results:
2 new issues

Category	Results
Security	2 high

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🟢 Metrics 0 complexity · 0 duplication

Metric	Results
Complexity	0
Duplication	0

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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.

[hkaiser]

Can this be removed as well?

[arpittkhandelwal]

Yes, absolutely! Since the manual compiler flags that were below it have been removed, that return() check became a dangling zombie.

I've just pushed a commit that completely removes the redundant if(MSVC) checks from both the producer and consumer blocks.

[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

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

Changes:

• Remove manual BMI output/prebuilt-module-path flag injection and simplify module producer/consumer wiring.
• Re-enable tests/unit/build external build tests with modules enabled, with adjustments for pkg-config based builds.
• Improve robustness of module consumer setup by supporting both build-tree and installed (HPXInternal::) module interface targets.

Reviewed changes

Copilot reviewed 18 out of 18 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; adjusts pkg-config compile flags to disable module imports for that test path.
libs/CMakeLists.txt	Updates module producer configuration call site to match simplified signature.
examples/hello_world_component/CMakeLists.txt	Adds module interface target linking and enables module scanning via hpx_setup_target(... SCAN_FOR_MODULES ON) in macro setup.
cmake/HPX_SetupTarget.cmake	Makes module consumer setup work with both non-namespaced and HPXInternal:: imported module interface targets; errors if missing.
cmake/HPX_CXXModules.cmake	Simplifies module producer/consumer macros to rely on CMake-native module scanning/installation behavior (removes manual flag injection).
.cmake-format.py	Updates formatting config for the changed hpx_configure_module_producer signature.