[gcc r11-5865] c++: Distinguish ambiguity from no valid candidate

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:a988a398d6daef3072cd2d07a21980911d8f93fc

commit r11-5865-ga988a398d6daef3072cd2d07a21980911d8f93fc
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Dec 7 17:21:47 2020 -0500

    c++: Distinguish ambiguity from no valid candidate
    
    Several recent C++ features are specified to try overload resolution, and if
    no viable candidate is found, do something else.  But our error return
    doesn't distinguish between that situation and finding multiple viable
    candidates that end up being ambiguous.  We're already trying to separately
    return the single function we found even if it ends up being ill-formed for
    some reason; for ambiguity let's pass back error_mark_node, to be
    distinguished from NULL_TREE meaning no viable candidate.
    
    gcc/cp/ChangeLog:
    
            * call.c (build_new_op_1): Set *overload for ambiguity.
            (build_new_method_call_1): Likewise.

Diff:
---
 gcc/cp/call.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index f1e0bcb796b..221e3de0c70 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -6357,6 +6357,8 @@ build_new_op_1 (const op_location_t &loc, enum tree_code code, int flags,
 	      print_z_candidates (loc, candidates);
 	    }
 	  result = error_mark_node;
+	  if (overload)
+	    *overload = error_mark_node;
 	}
       else if (TREE_CODE (cand->fn) == FUNCTION_DECL)
 	{
@@ -10438,6 +10440,8 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
 		free (pretty_name);
 	    }
 	  call = error_mark_node;
+	  if (fn_p)
+	    *fn_p = error_mark_node;
 	}
       else
 	{