[gcc r13-5664] libstdc++: Use ENOSYS for unsupported filesystem ops on AVR

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:5c43f06c228d169c370e99fa009154344fa305b8

commit r13-5664-g5c43f06c228d169c370e99fa009154344fa305b8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 2 16:00:21 2023 +0000

    libstdc++: Use ENOSYS for unsupported filesystem ops on AVR
    
    Because avr-libc <errno.h> defines most error numbers with duplicate
    values it's not sufficient to check #ifdef ENOTSUP when deciding which
    std::errc constant to use for the filesystem library's __unsupported()
    helper. Add a special case for AVR to always use the ENOSYS value.
    
    libstdc++-v3/ChangeLog:
    
            * src/filesystem/ops-common.h [AVR] (__unsupported): Always use
            errc::function_not_supported instead of errc::not_supported.

Diff:
---
 libstdc++-v3/src/filesystem/ops-common.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/filesystem/ops-common.h b/libstdc++-v3/src/filesystem/ops-common.h
index 02c75be09d2..abbfca43e5c 100644
--- a/libstdc++-v3/src/filesystem/ops-common.h
+++ b/libstdc++-v3/src/filesystem/ops-common.h
@@ -84,7 +84,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   inline error_code
   __unsupported() noexcept
   {
-#if defined ENOTSUP
+#if defined __AVR__
+    // avr-libc defines ENOTSUP and EOPNOTSUPP but with nonsense values.
+    // ENOSYS is defined though, so use an error_code corresponding to that.
+    // This contradicts the comment above, but we don't have much choice.
+    return std::make_error_code(std::errc::function_not_supported);
+#elif defined ENOTSUP
     return std::make_error_code(std::errc::not_supported);
 #elif defined EOPNOTSUPP
     // This is supposed to be for socket operations