[libcxx-commits] [libcxx] [libc++] Fix insufficiently guarded call to `copy_file_impl_fstream` (PR #214706)

[Anthony Latsis via libcxx-commits]

https://github.com/AnthonyLatsis updated https://github.com/llvm/llvm-project/pull/214706

>From 4283f490ca159b265fa8b6ed0607ec28c50cf414 Mon Sep 17 00:00:00 2001
From: Anthony Latsis <alatsis at apple.com>
Date: Fri, 7 Aug 2026 10:27:28 +0100
Subject: [PATCH] [libc++] Don't use `copy_file_impl_fstream` when not declared

`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"
```
---
 libcxx/src/filesystem/operations.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
index 3ff62c102d0ad..c4540176116f1 100644
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -360,7 +360,7 @@ bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_cod
   ec.clear();
   return true;
 }
-#elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM)
+#elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM) && defined(_LIBCPP_FILESYSTEM_NEED_FSTREAM)
 bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
   return copy_file_impl_fstream(read_fd, write_fd, ec);
 }