c++: Implement C++20 'using enum'. [PR91367]

c++: Implement C++20 'using enum'.  [PR91367]

This feature allows the programmer to import enumerator names into the
current scope so later mentions don't need to use the fully-qualified name.
These usings are not subject to the usual restrictions on using-decls: in
particular, they can move between class and non-class scopes, and between
classes that are not related by inheritance.  This last caused difficulty
for our normal approach to using-decls within a class hierarchy, as we
assume that the class where we looked up a used declaration is derived from
the class where it was first declared.  So to simplify things, in that case
we make a clone of the CONST_DECL in the using class.

Thanks to Nathan for the start of this work: in particular, the
lookup_using_decl rewrite.

The changes to dwarf2out revealed an existing issue with the D front-end: we
were doing the wrong thing for importing a D CONST_DECL, because
dwarf2out_imported_module_or_decl_1 was looking through it to its type,
expecting it to be an enumerator, but in one case in thread.d, the constant
had type int.  Adding the ability to import a C++ enumerator also fixed
that, but that led to a crash in force_decl_die, which didn't know what to
do with a CONST_DECL.  So now it does.

Co-authored-by: Nathan Sidwell <nathan@acm.org>
gcc/cp/ChangeLog:

	* cp-tree.h (USING_DECL_UNRELATED_P): New.
	(CONST_DECL_USING_P): New.
	* class.c (handle_using_decl): If USING_DECL_UNRELATED_P,
	clone the CONST_DECL.
	* name-lookup.c (supplement_binding_1): A clone hides its
	using-declaration.
	(lookup_using_decl): Rewrite to separate lookup and validation.
	(do_class_using_decl): Adjust.
	(finish_nonmember_using_decl): Adjust.
	* parser.c (make_location): Add cp_token overload.
	(finish_using_decl): Split out from...
	(cp_parser_using_declaration): ...here.  Don't look through enums.
	(cp_parser_using_enum): New.
	(cp_parser_block_declaration): Call it.
	(cp_parser_member_declaration): Call it.
	* semantics.c (finish_id_expression_1): Handle enumerator
	used from class scope.

gcc/ChangeLog:

	* dwarf2out.c (gen_enumeration_type_die): Call
	equate_decl_number_to_die for enumerators.
	(gen_member_die): Don't move enumerators to their
	enclosing class.
	(dwarf2out_imported_module_or_decl_1): Allow importing
	individual enumerators.
	(force_decl_die): Handle CONST_DECL.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/inh-ctor28.C: Adjust expected diagnostic.
	* g++.dg/cpp0x/inh-ctor33.C: Likewise.
	* g++.dg/cpp0x/using-enum-1.C: Add comment.
	* g++.dg/cpp0x/using-enum-2.C: Allowed in C++20.
	* g++.dg/cpp0x/using-enum-3.C: Likewise.
	* g++.dg/cpp1z/class-deduction69.C: Adjust diagnostic.
	* g++.dg/inherit/using5.C: Likewise.
	* g++.dg/cpp2a/using-enum-1.C: New test.
	* g++.dg/cpp2a/using-enum-2.C: New test.
	* g++.dg/cpp2a/using-enum-3.C: New test.
	* g++.dg/cpp2a/using-enum-4.C: New test.
	* g++.dg/cpp2a/using-enum-5.C: New test.
	* g++.dg/cpp2a/using-enum-6.C: New test.
	* g++.dg/debug/dwarf2/using-enum.C: New test.