c++: Fix ICE in cxx_printable_name_internal [PR123578]

c++: Fix ICE in cxx_printable_name_internal [PR123578]

On the following testcase, we end up with cxx_printable_name_internal
recursion, in particular
+../../gcc/cp/pt.cc:20736
+../../gcc/cp/error.cc:625
+template_args=<tree_vec 0x7fffe94b59b0>, flags=4) at ../../gcc/cp/error.cc:1876
The ICE is due to double free, that function is doing caching of up to 4
printed names, but if such a recursion happens, the inner call can change
ring_counter etc. and the caller will then store the result in a different
ring element from what was freed and so the freed one can be left
unmodified.

The patch fixes it by moving the lang_decl_name call earlier, after the:
  /* See if this print name is lying around.  */
  for (i = 0; i < PRINT_RING_SIZE; i++)
    if (uid_ring[i] == DECL_UID (decl) && translate == trans_ring[i])
      /* yes, so return it.  */
      return print_ring[i];
loop and repeating the loop again just for the theoretical case
that some recursion would add the same entry.

The ring_counter adjustment and decision which cache entry to reuse
for the cache is then done without the possibility of ring_counter
or the cache being changed in the middle.

2026-01-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123578
	* tree.cc (cxx_printable_name_internal): Call lang_decl_name before
	finding the slot to cache it in and repeat search in the cache
	after the call.

	* g++.dg/cpp2a/pr123578.C: New test.