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

[Patrick Palka]

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                                | 11 +++++++---
 .../g++.dg/cpp2a/spaceship-sfinae3.C          | 22 +++++++++++++++++++
 2 files changed, 30 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/spaceship-sfinae3.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 1f5ff417c81..183bb8c1348 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -7007,9 +7007,14 @@ 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;
 
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 00000000000..ca159260ec7
--- /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);
-- 
2.43.0.386.ge02ecfcc53