c++: defaulted <=> with bitfields [PR102490]

c++: defaulted <=> with bitfields [PR102490]

The testcases in the patch are either miscompiled or ICE with checking,
because the defaulted operator== is synthesized too early (but only if
constexpr), when the corresponding class type is still incomplete type.  The
problem is that at that point the bitfield FIELD_DECLs still have as
TREE_TYPE their underlying type rather than integral type with their
precision and when layout_class_type is called for the class soon after
that, it changes those types but the COMPONENT_REFs type stay the way that
they were during the operator== synthesize_method type and the middle-end is
then upset by the mismatch of types.  As what exact type will be given isn't
just a one liner but quite long code especially for over-sized bitfields, I
think it is best to just not synthesize the comparison operators so early
and call defaulted_late_check for them once again as soon as the class is
complete.

This is also a problem for virtual operator<=>, where we need to compare the
noexcept-specifier to validate the override before the class is complete.
Rather than try to defer that comparison, maybe_instantiate_noexcept now
calls maybe_synthesize_method, which calls build_comparison_op in
non-defining mode if the class isn't complete yet.  In that situation we
also might not have base fields yet, so we look in the binfo for the bases.

Co-authored-by: Jason Merrill <jason@redhat.com>
2021-10-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98712
	PR c++/102490
	* cp-tree.h (maybe_synthesize_method): Declare.
	* method.c (genericize_spaceship): Use
	LOOKUP_NORMAL | LOOKUP_NONVIRTUAL | LOOKUP_DEFAULTED instead of
	LOOKUP_NORMAL for flags.
	(comp_info): Remove defining member.  Add complain, code, retcat.
	(comp_info::comp_info): Adjust.
	(do_one_comp): Split out from build_comparison_op.   Use
	LOOKUP_NORMAL | LOOKUP_NONVIRTUAL | LOOKUP_DEFAULTED instead of
	LOOKUP_NORMAL for flags.
	(build_comparison_op): Add defining argument. Adjust comp_info
	construction.  Use defining instead of info.defining.  Assert that
	if defining, ctype is a complete type.  Walk base binfos.
	(synthesize_method, maybe_explain_implicit_delete,
	explain_implicit_non_constexpr): Adjust build_comparison_op callers.
	(maybe_synthesize_method): New function.
	* class.c (check_bases_and_members): Don't call defaulted_late_check
	for sfk_comparison.
	(finish_struct_1): Call it here instead after class has been
	completed.
	* pt.c (maybe_instantiate_noexcept): Call maybe_synthesize_method
	instead of synthesize_method.

	* g++.dg/cpp2a/spaceship-synth8.C (std::strong_ordering): Provide
	more complete definition.
	(std::strong_ordering::less, std::strong_ordering::equal,
	std::strong_ordering::greater): Define.
	* g++.dg/cpp2a/spaceship-synth12.C: New test.
	* g++.dg/cpp2a/spaceship-synth13.C: New test.
	* g++.dg/cpp2a/spaceship-synth14.C: New test.
	* g++.dg/cpp2a/spaceship-eq11.C: New test.
	* g++.dg/cpp2a/spaceship-eq12.C: New test.
	* g++.dg/cpp2a/spaceship-eq13.C: New test.