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.

1 files changed, 15 insertions, 1 deletions

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 4452b9fa..0e8436e 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -22193,6 +22193,9 @@ gen_enumeration_type_die (tree type, dw_die_ref context_die)
 	  dw_die_ref enum_die = new_die (DW_TAG_enumerator, type_die, link);
 	  tree value = TREE_VALUE (link);
 
+	  if (DECL_P (value))
+	    equate_decl_number_to_die (value, enum_die);
+
 	  gcc_assert (!ENUM_IS_OPAQUE (type));
 	  add_name_attribute (enum_die,
 			      IDENTIFIER_POINTER (TREE_PURPOSE (link)));
@@ -25247,6 +25250,10 @@ gen_member_die (tree type, dw_die_ref context_die)
 		  splice = false;
 		}
 	    }
+	  else if (child->die_tag == DW_TAG_enumerator)
+	    /* Enumerators remain under their enumeration even if
+	       their names are introduced in the enclosing scope.  */
+	    splice = false;
 
 	  if (splice)
 	    splice_child_die (context_die, child);
@@ -26158,6 +26165,13 @@ force_decl_die (tree decl)
 	    decl_die = comp_unit_die ();
 	  break;
 
+	case CONST_DECL:
+	  /* Enumerators shouldn't need force_decl_die.  */
+	  gcc_assert (DECL_CONTEXT (decl) == NULL_TREE
+		      || TREE_CODE (DECL_CONTEXT (decl)) != ENUMERAL_TYPE);
+	  gen_decl_die (decl, NULL, NULL, context_die);
+	  break;
+
 	case TRANSLATION_UNIT_DECL:
 	  decl_die = comp_unit_die ();
 	  break;
@@ -26743,7 +26757,7 @@ dwarf2out_imported_module_or_decl_1 (tree decl,
   else
     xloc = expand_location (input_location);
 
-  if (TREE_CODE (decl) == TYPE_DECL || TREE_CODE (decl) == CONST_DECL)
+  if (TREE_CODE (decl) == TYPE_DECL)
     {
       at_import_die = force_type_die (TREE_TYPE (decl));
       /* For namespace N { typedef void T; } using N::T; base_type_die