c: fix checking for a tag for variably modified tagged types [PR119612]

The checking assertion added for PR118765 did not take into account
that add_decl_expr can change TYPE_NAME to a TYPE_DECL with no name
for certain cases of variably modified types.  This also implies that we
might sometimes not reliably detect the absence of a tag when only
considering TYPE_NAME.  This patch introduces a new helper function
c_type_tag to reliable compute the tag for a tagged types and uses it
for code where the switch to C23 may cause regressions.

	PR c/119612

gcc/c/ChangeLog:
	* c-tree.h (c_type_tag): Add prototype.
	* c-typeck.cc (c_type_tag): New function.
	(tagged_types_tu_compatible_p, composite_type_internal): Use
	c_type_tag.
	* c-decl.cc (c_struct_hasher::hash, previous_tag): Use c_type_tag.

gcc/testsuite/ChangeLog:
	* gcc.dg/gnu23-tag-6.c: New test.
	* gcc.dg/pr119612.c: New test.

0 files changed, 0 insertions, 0 deletions