c++: Fix constrained conversion op.

We don't want to promote a conversion from viable == 0 to viable == -1.
Found in ranges-v3.

gcc/cp/ChangeLog
2020-02-28  Jason Merrill  <jason@redhat.com>

	* call.c (build_user_type_conversion_1): Don't look at the second
	conversion of a non-viable candidate.

gcc/cp/ChangeLog

+2020-02-28  Jason Merrill  <jason@redhat.com>
+
+	* call.c (build_user_type_conversion_1): Don't look at the second
+	conversion of a non-viable candidate.
+
 2020-02-28  Jakub Jelinek  <jakub@redhat.com>
 
 	P1937R2 - Fixing inconsistencies between const{expr,eval} functions

gcc/cp/call.c

@@ -4083,6 +4083,10 @@ build_user_type_conversion_1 (tree totype, tree expr, int flags,
 
       for (cand = candidates; cand != old_candidates; cand = cand->next)
 	{
+	  if (cand->viable == 0)
+	    /* Already rejected, don't change to -1.  */
+	    continue;
+
 	  tree rettype = TREE_TYPE (TREE_TYPE (cand->fn));
 	  conversion *ics
 	    = implicit_conversion (totype,

gcc/testsuite/g++.dg/cpp2a/concepts-conv1.C

+// { dg-do compile { target concepts } }
+
+template <class T> concept False = false;
+
+template <class T>
+struct A
+{
+  explicit operator bool ();
+  explicit operator bool () requires False<T>;
+};
+
+int main()
+{
+  int i { A<int>() };		// { dg-error "" }
+}