[PATCH] Export std::dextents from std.cc.in [PR121174]

[Jakub Jelinek]

Hi!

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.

Tested on x86_64-linux, ok for trunk?

2025-07-19  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.

--- libstdc++-v3/src/c++23/std.cc.in.jj	2025-07-19 14:39:32.617034910 +0200
+++ libstdc++-v3/src/c++23/std.cc.in	2025-07-19 14:41:32.641471785 +0200
@@ -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
 

	Jakub