c++: Disallow this specifier except for parameter declarations [PR113788]

c++: Disallow this specifier except for parameter declarations [PR113788]

The deducing this patchset added parsing of this specifier to
cp_parser_decl_specifier_seq unconditionally, but in the C++ grammar
this[opt] only appears in the parameter-declaration non-terminal, so
rather than checking in all the callers of cp_parser_decl_specifier_seq
except for cp_parser_parameter_declaration that this specifier didn't
appear I think it is far easier and closer to what the standard says
to only parse this specifier when called from
cp_parser_parameter_declaration.

2024-02-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/113788
	* parser.cc (CP_PARSER_FLAGS_PARAMETER): New enumerator.
	(cp_parser_decl_specifier_seq): Parse RID_THIS only if
	CP_PARSER_FLAGS_PARAMETER is set in flags.
	(cp_parser_parameter_declaration): Or in CP_PARSER_FLAGS_PARAMETER
	when calling cp_parser_decl_specifier_seq.

	* g++.dg/parse/pr113788.C: New test.