c++: avoid -Wredundant-tags on a first declaration in use [PR 93824]

c++: avoid -Wredundant-tags on a first declaration in use [PR 93824]

-Wredundant-tags doesn't consider type declarations that are also
the first uses of the type, such as in 'void f (struct S);' and
issues false positives for those.  According to the reported that's
making it harder to use the warning to clean up LibreOffice.

The attached patch extends -Wredundant-tags to avoid these false
positives by relying on the same class_decl_loc_t::class2loc mapping
as -Wmismatched-tags.  The patch also improves the detection
of both issues in template declarations.

gcc/cp/ChangeLog
2020-03-27  Martin Sebor  <msebor@redhat.com>

	PR c++/94078
	PR c++/93824
	PR c++/93810
	* cp-tree.h (most_specialized_partial_spec): Declare.
	* parser.c (cp_parser_elaborated_type_specifier): Distinguish alias
	from declarations.
	(specialization_of): New function.
	(cp_parser_check_class_key): Move code...
	(class_decl_loc_t::add): ...to here.  Add parameters.  Avoid issuing
	-Wredundant-tags on first-time declarations in other declarators.
	Correct handling of template specializations.
	(class_decl_loc_t::diag_mismatched_tags): Also expect to be called
	when -Wredundant-tags is enabled.  Use primary template or partial
	specialization as the guide for uses of implicit instantiations.
	* pt.c (most_specialized_partial_spec): Declare extern.

gcc/testsuite/ChangeLog
2020-03-27  Martin Sebor  <msebor@redhat.com>

	PR c++/94078
	PR c++/93824
	PR c++/93810
	* g++.dg/warn/Wmismatched-tags-3.C: New test.
	* g++.dg/warn/Wmismatched-tags-4.C: New test.
	* g++.dg/warn/Wmismatched-tags-5.C: New test.
	* g++.dg/warn/Wmismatched-tags-6.C: New test.
	* g++.dg/warn/Wredundant-tags-3.C: Remove xfails.
	* g++.dg/warn/Wredundant-tags-6.C: New test.
	* g++.dg/warn/Wredundant-tags-7.C: New test.