c++: Fix up recent cp_parser_template_id regression [PR123186]

c++: Fix up recent cp_parser_template_id regression [PR123186]

On Fri, Dec 12, 2025 at 09:39:18AM -0500, Patrick Palka wrote:
> +      TREE_TYPE (templ)
> +     = build_typename_type (TYPE_CONTEXT (TREE_TYPE (templ)),
> +                            TYPE_NAME (TREE_TYPE (templ)),
> +                            fullname,
> +                            get_typename_type_tag (TREE_TYPE (templ)));
>        template_id = templ;
>      }
>    else

This change causes ICE e.g. on the following testcase.

The problem is that build_typename_type expects IDENTIFIER_NODE as the
second argument, e.g. it uses it as
      tree d = build_decl (input_location, TYPE_DECL, name, t);
argument.  But TYPE_NAME doesn't have to be an IDENTIFIER_NODE, it can
be a TYPE_DECL too and when we build a TYPE_DECL with TYPE_DECL as
DECL_NAME, it breaks all kinds of assumptions everywhere in the FE as well
as middle-end.

Fixed by using TYPE_IDENTIFIER instead.

2025-12-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123186
	* parser.cc (cp_parser_template_id): Use TYPE_IDENTIFIER instead of
	TYPE_NAME in second build_typename_type argument.

	* g++.dg/template/crash133.C: New test.