[gcc r16-2367] libstdc++: Export std::dextents from std.cc.in [PR121174]

[Jakub Jelinek]

https://gcc.gnu.org/g:7083aeaef5b6d8be1ae419b834e8e56d90026ace

commit r16-2367-g7083aeaef5b6d8be1ae419b834e8e56d90026ace
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sun Jul 20 08:12:57 2025 +0200

    libstdc++: Export std::dextents from std.cc.in [PR121174]
    
    r16-442 implemented both std::extents and std::dextents (and perhaps other
    stuff), but exported only std::extents.
    I went through https://eel.is/c++draft/mdspan.syn and I think std::dextents
    is the only one implemented but not exported.
    
    The following patch exports it, and additionally appends some further
    entities to the FIXME list, those all seems to be unimplemented yet.
    
    2025-07-20  Jakub Jelinek  <jakub@redhat.com>
    
            PR libstdc++/121174
            * src/c++23/std.cc.in (std::dextents): Export.  Add to FIXME comments
            other not yet implemented nor exported <mdspan> entities.

Diff:
---
 libstdc++-v3/src/c++23/std.cc.in | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
index 1bd4bd58f212..9301ed90c00f 100644
--- a/libstdc++-v3/src/c++23/std.cc.in
+++ b/libstdc++-v3/src/c++23/std.cc.in
@@ -1850,12 +1850,15 @@ export namespace std
 export namespace std
 {
   using std::extents;
+  using std::dextents;
   using std::layout_left;
   using std::layout_right;
   using std::layout_stride;
   using std::default_accessor;
   using std::mdspan;
-  // FIXME layout_left_padded, layout_right_padded, aligned_accessor, mdsubspan
+  // FIXME layout_left_padded, layout_right_padded, aligned_accessor,
+  // strided_slice, submdspan_mapping_result, full_extent_t, full_extent,
+  // submdspan_extents, mdsubspan
 }
 #endif