[libc++] Don't use copy_file_impl_fstream when not declared

[AnthonyLatsis]

copy_file_impl_fstream is declared and defined only under _LIBCPP_FILESYSTEM_NEED_FSTREAM (which additionally requires _LIBCPP_HAS_LOCALIZATION), and then used in a copy_file_impl variant under a disjoint set of conditions. This causes any build that disables localization and elects this variant of copy_file_impl, as is currently the case when targeting wasi, to fail with a generic compiler error:

error: use of undeclared identifier 'copy_file_impl_fstream'

Instead, since this particular copy_file_impl implementation merely forwards to copy_file_impl_fstream, tweak its gating condition such that the aforementioned configuration hits the dedicated fallback branch, which offers a somewhat clearer error message:

#  error "Unknown implementation for copy_file_impl"

[llvmorg-github-actions]

@llvm/pr-subscribers-libcxx

Author: Anthony Latsis (AnthonyLatsis)

Changes

copy_file_impl_fstream is only defined under
_LIBCPP_FILESYSTEM_NEED_FSTREAM, which additionally requires _LIBCPP_HAS_LOCALIZATION. The _LIBCPP_FILESYSTEM_USE_FSTREAM dispatch called it unconditionally, so a build with filesystem enabled and localization disabled failed on any platform that provides neither sendfile nor copyfile:

error: use of undeclared identifier 'copy_file_impl_fstream'

Report EINVAL instead, as the sendfile/copy_file_range dispatch above already does in the same situation.

Assisted-by: Claude Code

---

Full diff: https://github.com/llvm/llvm-project/pull/214706.diff

1 Files Affected:

• (modified) libcxx/src/filesystem/operations.cpp (+6)

diff --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
index 3ff62c102d0ad..7099cd2c75ab6 100644
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -362,7 +362,13 @@ bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_cod
 }
 #elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM)
 bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+#  if defined(_LIBCPP_FILESYSTEM_NEED_FSTREAM)
   return copy_file_impl_fstream(read_fd, write_fd, ec);
+#  else
+  // iostreams are unavailable in the no-locale build, just fail.
+  ec.assign(EINVAL, std::system_category());
+  return false;
+#  endif
 }
 #else
 #  error "Unknown implementation for copy_file_impl"

[AnthonyLatsis]

cc @hamishknight

[ldionne]

I find the #elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM) && defined(_LIBCPP_FILESYSTEM_NEED_FSTREAM) conditional a bit odd, but I think that's because of the name _LIBCPP_FILESYSTEM_NEED_FSTREAM.

I made some suggestions on the PR, let me know what you think about them. I can live with either way but I think the larger refactoring I suggest leaves us with something a bit clearer.

[ldionne]

Suggested change

	// sendfile and copy_file_range need to fall back
	// to the fstream implementation for special files
	#if _LIBCPP_HAS_LOCALIZATION
	# include <fstream>
	# define _LIBCPP_FILESYSTEM_HAS_FSTREAM_FALLBACK
	#endif

[ldionne]

Suggested change

	#if defined(_LIBCPP_FILESYSTEM_HAS_FSTREAM_FALLBACK)

[ldionne]

Suggested change

	# if defined(_LIBCPP_FILESYSTEM_HAS_FSTREAM_FALLBACK)

[ldionne]

Suggested change

	#elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM) && defined(_LIBCPP_FILESYSTEM_NEED_FSTREAM)
	#elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM)

[ldionne]

Suggested change

	return copy_file_impl_fstream(read_fd, write_fd, ec);
	# if !defined(_LIBCPP_FILESYSTEM_HAS_FSTREAM_FALLBACK)
	# error copy_file intends to use std::fstream as a fallback, but std::fstream is not available in this configuration
	# endif
	return copy_file_impl_fstream(read_fd, write_fd, ec);