c++: ambiguous member lookup for rewritten cands [PR113529]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk/13?

-- >8 --

Here we handle the operator expression u < v inconsistently: in a SFINAE
context (such as a requires-expression) we accept the it, and in a
non-SFINAE context we reject it with

  error: request for member 'operator<=>' is ambiguous

as per [class.member.lookup]/6.  This inconsistency is ultimately
because we neglect to propagate error_mark_node after recursing in
add_operator_candidates, fixed like so.

	PR c++/113529

gcc/cp/ChangeLog:

	* call.cc (add_operator_candidates): Propagate error_mark_node
	result after recursing to find rewritten candidates.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-sfinae3.C: New test.

gcc/cp/call.cc

@@ -6896,9 +6896,14 @@ add_operator_candidates (z_candidate **candidates,
 	{
 	  flags |= LOOKUP_REWRITTEN;
 	  if (rewrite_code != code)
-	    /* Add rewritten candidates in same order.  */
-	    add_operator_candidates (candidates, rewrite_code, ERROR_MARK,
-				     arglist, lookups, flags, complain);
+	    {
+	      /* Add rewritten candidates in same order.  */
+	      tree r = add_operator_candidates (candidates, rewrite_code,
+						ERROR_MARK, arglist, lookups,
+						flags, complain);
+	      if (r == error_mark_node)
+		return error_mark_node;
+	    }
 
 	  z_candidate *save_cand = *candidates;
 

gcc/testsuite/g++.dg/cpp2a/spaceship-sfinae3.C

@@ -0,0 +1,22 @@
+// PR c++/113529
+// { dg-do compile { target c++20 } }
+
+#include <compare>
+
+struct A {
+  auto operator<=>(const A&) const = default;
+  bool operator<(const A&) const;
+};
+struct B {
+  auto operator<=>(const B&) const = default;
+};
+struct C : A, B { };
+
+
+template<class T>
+void f(T u, T v) {
+  static_assert(!requires { u < v; });
+  u < v; // { dg-error "request for member 'operator<=>' is ambiguous" }
+}
+
+template void f(C, C);