[regression, libcxx, C++26] __cpp_lib_filesystem is not available in libc++ until <fstream> is included

[vedranmiletic]

Consider the following code:

#ifdef __cpp_lib_filesystem
#include <filesystem>
#endif

#include <fstream>

int main() {
#ifdef __cpp_lib_filesystem
  auto path = std::filesystem::current_path().string();
#endif
  return 0;
}

Compiling in C++23 mode works both with Clang 19 and 20 (version details below):

clang++ -std=c++23 -stdlib=libc++ fs.cpp

Compiling in C++26 mode fails with the following error:

$ clang++ -std=c++26 -stdlib=libc++ fs.cpp
fs.cpp:9:30: error: no member named 'current_path' in namespace 'std::filesystem'
    9 | auto path = std::filesystem::current_path().string();
      |             ~~~~~~~~~~~~~~~~~^
1 error generated.

only when using Clang 20 on Arch Linux:

$ clang++ --version
clang version 20.1.8
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

but compiles fine when using Clang 19 on FreeBSD:

% clang++ --version
FreeBSD clang version 19.1.7 (https://github.com/llvm/llvm-project.git llvmorg-19.1.7-0-gcd708029e0b2)
Target: x86_64-unknown-freebsd14.3
Thread model: posix
InstalledDir: /usr/bin

[philnik777]

You conclusion is incorrect. The problem is that <filesystem> is never explicitly included in your code. <fstream> just happens to provide it in older standards. You have to first #include <version> before testing any library FTMs. You can test this easily by adding an #error directive to your #include <filesystem> and note that in no version will the compiler diagnose anything. You can observe the same behaviour by adding -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES to your C++20/23 compilation.

[vedranmiletic]

Fair point. Thanks for the quick response!