[libcxx][CMake] `LIBCXX_ENABLE_FILESYSTEM=OFF` is not respected on Windows for freestanding runtimes build

[moleium]

When configuring a freestanding libcxx build on Windows using the runtimes build entrypoint, setting -DLIBCXX_ENABLE_FILESYSTEM=OFF does not prevent filesystem source files from being added to the build target. This results in a compilation failure, as these sources have dependencies (e.g, Unicode, wide characters) that are typically disabled in such a configuration.

This issue does not occur on Linux with an identical configruation.

Environment

• OS: Windows 10 22H2
• Compiler: Clang 21.1.1 (From MSYS2 clang64 toolchain)
• Build System: Ninja
• LLVM commit: main branch

Reproduction

1. From the root of llvm-project, run the following CMake command:

cmake -G Ninja -S runtimes -B build-freestanding 
-DLLVM_ENABLE_RUNTIMES=libcxx
-DCMAKE_BUILD_TYPE=Release
-DCMAKE_INSTALL_PREFIX=install
-DCMAKE_C_COMPILER=clang
-DCMAKE_CXX_COMPILER=clang++
-DLIBCXX_ENABLE_SHARED=OFF
-DLIBCXX_ENABLE_STATIC=ON
-DLIBCXX_CXX_ABI=none
-DLIBCXX_ENABLE_EXCEPTIONS=OFF
-DLIBCXX_ENABLE_RTTI=OFF
-DLIBCXX_ENABLE_FILESYSTEM=OFF
-DLIBCXX_ENABLE_LOCALIZATION=OFF
-DLIBCXX_ENABLE_UNICODE=OFF
-DLIBCXX_ENABLE_WIDE_CHARACTERS=OFF
-DLIBCXX_ENABLE_THREADS=OFF
-DLIBCXX_ENABLE_MONOTONIC_CLOCK=OFF
-DLIBCXX_ENABLE_TIME_ZONE_DATABASE=OFF
-DLIBCXX_ENABLE_RANDOM_DEVICE=OFF
-DLIBCXX_ENABLE_NEW_DELETE_DEFINITIONS=OFF
-DLIBCXX_INCLUDE_TESTS=OFF
-DLIBCXX_INCLUDE_BENCHMARKS=OFF
-DLIBCXX_USE_COMPILER_RT=ON

2. Go to the build directoryt and run ninja:

cd build-freestanding
ninja

Actual Behavior

The build fails with compilation errors in filesystem-related source files.

... llvm-project/build-freestanding/include/c++/v1/__filesystem/path.h:186:9: error: unknown type name 'wstring'; did you mean 'string'?
  186 | typedef wstring __path_string;
      |         ^~~~~~~
      |         string
... llvm-project/build-freestanding/include/c++/v1/__fwd/string.h:45:7: note: 'string' declared here
   45 | using string = basic_string<char>;
      |       ^

Expected Behavior

The build should succeed. Source files from the libcxx/src/filesystem directory should not be part of the build when LIBCXX_ENABLE_FILESYSTEM is OFF.

Speculation

Probable root cause is in libcxx/src/CMakeLists.txt. Filesystem source files (filesystem_*.cpp) are unconditionally added to the LIBCXX_SOURCES variable.

These files should be moved inside the guard for LIBCXX_ENABLE_FILESYSTEM block.

[philnik777]

Disabling wide characters on windows isn't really supported. What's even the point of that?
Re. whether it should be there in a non-filesystem build: yes. The parts of the filesystem API that don't actually depend on the filesystem shouldn't be removed just because the system we're running on doesn't have a filesystem. In other words, if a system can support a feature then we should provide it.

[moleium]

@philnik777

Disabling wide characters on windows isn't really supported. What's even the point of that?

The point is to target a baremetal environment specifically a Windows kernel driver. Full Win32 API support, including for wide characters or localization is unavailable

Re. whether it should be there in a non-filesystem build: yes. The parts of the filesystem API that don't actually depend on the filesystem shouldn't be removed...

That would be acceptable if those components were self contained. But they are not. They include dependencies on features explicitly disabled in a freestanding configuration.

As an update, I did try building with LIBCXX_ENABLE_WIDE_CHARACTERS=ON and LIBCXX_ENABLE_UNICODE=ON. The build still fails.
The error is missing __narrow_to_utf8 and __widen_from_utf8 templates. These are guarded by _LIBCPP_HAS_LOCALIZATION [1] which is correctly disabled for this build via LIBCXX_ENABLE_LOCALIZATION=OFF.

... /include/c++/v1/__filesystem/path.h:329:11: error: no template named '__narrow_to_utf8'
  329 |   typedef __narrow_to_utf8<sizeof(wchar_t) * __CHAR_BIT__> _Narrower;
      |           ^
... include/c++/v1/__filesystem/path.h:330:11: error: no template named '__widen_from_utf8'
  330 |   typedef __widen_from_utf8<sizeof(_ECharT) * __CHAR_BIT__> _Widener;
      |           ^
... include/c++/v1/__filesystem/path.h:335:16: error: called object type '_Narrower' (aka 'int') is not a function or function pointer
  335 |     _Narrower()(back_inserter(__utf8), __src.data(), __src.data() + __src.size());
      |     ~~~~~~~~~~~^
... include/c++/v1/__filesystem/path.h:336:15: error: called object type '_Widener' (aka 'int') is not a function or function pointer
  336 |     _Widener()(back_inserter(__dest), __utf8.data(), __utf8.data() + __utf8.size());
      |     ~~~~~~~~~~^

[1] libcxx/include/__filesystem/path.h L208

[godcorsec]

And you're sure you 100% read the documentation?