c++: Fix defaulted <=> fallback to < and == [PR96299]

I thought I had implemented P1186R3, but apparently I didn't read it closely
enough to understand the point of the paper, namely that for a defaulted
operator<=>, if a member type doesn't have a viable operator<=>, we will use
its operator< and operator== if the defaulted operator has an specific
comparison category as its return type; the compiler can't guess if it
should be strong_ordering or something else, but the user can make that
choice explicit.

The libstdc++ test change was necessary because of the change in
genericize_spaceship from op0 > op1 to op1 < op0; this should be equivalent,
but isn't because of PR88173.

gcc/cp/ChangeLog:

	PR c++/96299
	* cp-tree.h (build_new_op): Add overload that omits some parms.
	(genericize_spaceship): Add location_t parm.
	* constexpr.c (cxx_eval_binary_expression): Pass it.
	* cp-gimplify.c (genericize_spaceship): Pass it.
	* method.c (genericize_spaceship): Handle class-type arguments.
	(build_comparison_op): Fall back to op</== when appropriate.

gcc/testsuite/ChangeLog:

	PR c++/96299
	* g++.dg/cpp2a/spaceship-synth-neg2.C: Move error.
	* g++.dg/cpp2a/spaceship-p1186.C: New test.

libstdc++-v3/ChangeLog:

	PR c++/96299
	* testsuite/18_support/comparisons/algorithms/partial_order.cc:
	One more line needs to use VERIFY instead of static_assert.

1 files changed, 8 insertions, 1 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 66ad114..6270fad 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6335,6 +6335,13 @@ extern tree build_new_op			(const op_location_t &,
 						 enum tree_code,
 						 int, tree, tree, tree, tree *,
 						 tsubst_flags_t);
+/* Wrapper that leaves out the usually-null op3 and overload parms.  */
+inline tree build_new_op (const op_location_t &loc, enum tree_code code,
+			  int flags, tree arg1, tree arg2,
+			  tsubst_flags_t complain)
+{
+  return build_new_op (loc, code, flags, arg1, arg2, NULL_TREE, NULL, complain);
+}
 extern tree build_op_call			(tree, vec<tree, va_gc> **,
 						 tsubst_flags_t);
 extern bool aligned_allocation_fn_p		(tree);
@@ -7807,7 +7814,7 @@ extern tree merge_types				(tree, tree);
 extern tree strip_array_domain			(tree);
 extern tree check_return_expr			(tree, bool *);
 extern tree spaceship_type			(tree, tsubst_flags_t = tf_warning_or_error);
-extern tree genericize_spaceship		(tree, tree, tree);
+extern tree genericize_spaceship		(location_t, tree, tree, tree);
 extern tree cp_build_binary_op                  (const op_location_t &,
 						 enum tree_code, tree, tree,
 						 tsubst_flags_t);