c++: Check abstract type only on object creation. [PR86252]

c++: Check abstract type only on object creation. [PR86252]

Abstract checking has been problematic for a while; when I implemented an
earlier issue resolution to do more checking it led to undesirable
instantiations, and so backed some of it out.  During the C++20 process we
decided with P0929R2 that we should go the other way, and only check
abstractness when we're actually creating an object, not when merely forming
an array or function type.  This means that we can remove the machinery for
checking whether a newly complete class makes some earlier declaration
ill-formed.  This change was moved as a DR, so I'm applying it to all
standard levels.  This could be reconsidered if it causes problems, but I
don't expect it to.

The change to the libstdc++ result_of test brings the expected behavior in
line with that for incomplete types, but as in PR97841 I think the libstdc++
handling of incomplete types in this and other type_traits is itself wrong,
so I expect these lines and others to change again before long.

gcc/cp/ChangeLog:

	* decl.c (cp_finish_decl): Only check abstractness on definition.
	(require_complete_types_for_parms): Check abstractness here.
	(create_array_type_for_decl): Not here.
	(grokdeclarator, grokparms, complete_vars): Not here.
	* pt.c (tsubst, tsubst_arg_types, tsubst_function_type): Not here.
	* typeck2.c (struct pending_abstract_type): Remove.
	(struct abstract_type_hasher): Remove.
	(abstract_pending_vars, complete_type_check_abstract): Remove.
	(abstract_virtuals_error_sfinae): Handle arrays.
	* call.c (conv_is_prvalue): Split out from...
	(conv_binds_ref_to_prvalue): ...here.
	(implicit_conversion_1): Rename from implicit_conversion.
	(implicit_conversion): An abstract prvalue is bad.
	(convert_like_internal): Don't complain if expr is already
	error_mark_node.

gcc/testsuite/ChangeLog:

	* g++.dg/other/abstract1.C: Adjust.
	* g++.dg/other/abstract2.C: Adjust.
	* g++.dg/other/abstract4.C: Adjust.
	* g++.dg/other/abstract5.C: Adjust.
	* g++.dg/other/abstract8.C: New test.
	* g++.dg/template/sfinae-dr657.C: Adjust.
	* g++.old-deja/g++.other/decl3.C: Adjust.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/result_of/sfinae_friendly_1.cc: Adjust.