Potential Issue with libc++ and C++ Modules and Precompiled Headers

This repository demonstrates a potential issue when using C++ modules and precompiled headers with LLVM's libc++ standard library implementation.

CMake Configuration Options

USE_PCH

• Type: ON/OFF
• Default: OFF
• Description: Enables precompiled headers configuration when set to ON.

USE_STD_FORMAT

• Type: ON/OFF
• Default: OFF
• Description: Enables string output using std::format for library standard testing.

Test Results

Standard Library Implementation	clang 19.1.7 (Fedora)	clang 21.0.0 (git)
libstdc++ 14.2.1 (Fedora)	OK	OK
libc++ 19.1.7 (Fedora)	Error	Error
libc++ 21.0.0 (git)	Error	Error

Build Configurations

Compiler Settings

clang 19.1.7 (Fedora 19.1.7-3.fc41)

COMPILER_CONFIG="-DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CXX_COMPILER_CLANG_SCAN_DEP=clang-scan-deps"

clang 21.0.0 (git 148111fdcf0e807fe74274b18fcf65c4cff45d63)

COMPILER_CONFIG="-DCMAKE_C_COMPILER=${LLVM_21}/bin/clang -DCMAKE_CXX_COMPILER=${LLVM_21}/bin/clang++ -DCMAKE_CXX_COMPILER_CLANG_SCAN_DEP=${LLVM_21}/bin/clang-scan-deps"

Standard Library Settings

libstdc++ 14.2.1 (Fedora 14.2.1-7.fc41)

STDLIB_CONFIG=""

libc++ 19.1.7 (Fedora 19.1.7-1.fc41)

STDLIB_CONFIG="-DCMAKE_EXE_LINKER_FLAGS_INIT='-stdlib=libc++' -DCMAKE_CXX_FLAGS='-stdlib++-isystem/usr/include/c++/v1'"

libc++ 21.0.0 (git 148111fdcf0e807fe74274b18fcf65c4cff45d63)

STDLIB_CONFIG="-DCMAKE_EXE_LINKER_FLAGS_INIT='-stdlib=libc++' \
               -DCMAKE_CXX_FLAGS='-stdlib++-isystem${LLVM_21}/include/c++/v1 -isystem${LLVM_21}/include/x86_64-unknown-linux-gnu/c++/v1' \
               -DCMAKE_CXX_STANDARD_LIBRARIES='-L${LLVM_21}/lib/x86_64-unknown-linux-gnu -lc++ -lc++abi'"

Building the Project

mkdir build
eval " cmake ${STDLIB_CONFIG} ${COMPILER_CONFIG} -DUSE_PCH=ON -G Ninja -B build"
ninja -C build -v

Additional Observations

The build succeeds in all configurations when precompiled headers are disabled (-DUSE_PCH=OFF).
The corresponding issue is open in the LLVM repository.