[gcc r13-8255] c++: ambiguous member lookup for rewritten cands [PR113529]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:663d9e168bc1f2649721436f5188563eda9d04f0

commit r13-8255-g663d9e168bc1f2649721436f5188563eda9d04f0
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jan 24 17:11:09 2024 -0500

    c++: ambiguous member lookup for rewritten cands [PR113529]
    
    Here we handle the operator expression u < v inconsistently: in a SFINAE
    context we accept 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.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit fecb45a936d62ca24dd8b4985ea0555c28edefa8)

Diff:
---
 gcc/cp/call.cc                                 | 21 +++++++++++++++------
 gcc/testsuite/g++.dg/cpp2a/spaceship-sfinae3.C | 22 ++++++++++++++++++++++
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 85274b81d7e2..d012680f1857 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -6731,7 +6731,9 @@ op_is_ordered (tree_code code)
 
 /* Subroutine of build_new_op: Add to CANDIDATES all candidates for the
    operator indicated by CODE/CODE2.  This function calls itself recursively to
-   handle C++20 rewritten comparison operator candidates.
+   handle C++20 rewritten comparison operator candidates.  Returns NULL_TREE
+   upon success, and error_mark_node if something went wrong that prevented
+   us from performing overload resolution (e.g. ambiguous member name lookup).
 
    LOOKUPS, if non-NULL, is the set of pertinent namespace-scope operator
    overloads to consider.  This parameter is used when instantiating a
@@ -6908,11 +6910,16 @@ add_operator_candidates (z_candidate **candidates,
 
       if (rewrite_code)
 	{
+	  tree r;
 	  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.  */
+	      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;
 
@@ -6921,8 +6928,10 @@ add_operator_candidates (z_candidate **candidates,
 	  vec<tree,va_gc> *revlist = make_tree_vector ();
 	  revlist->quick_push ((*arglist)[1]);
 	  revlist->quick_push ((*arglist)[0]);
-	  add_operator_candidates (candidates, rewrite_code, ERROR_MARK,
-				   revlist, lookups, flags, complain);
+	  r = add_operator_candidates (candidates, rewrite_code, ERROR_MARK,
+				       revlist, lookups, flags, complain);
+	  if (r == error_mark_node)
+	    return error_mark_node;
 
 	  /* Release the vec if we didn't add a candidate that uses it.  */
 	  for (z_candidate *c = *candidates; c != save_cand; c = c->next)
diff --git a/gcc/testsuite/g++.dg/cpp2a/spaceship-sfinae3.C b/gcc/testsuite/g++.dg/cpp2a/spaceship-sfinae3.C
new file mode 100644
index 000000000000..ca159260ec79
--- /dev/null
+++ b/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);