c++: constrained surrogate call functions [PR110535]

We weren't checking constraints on pointer/reference-to-function conversion
functions during overload resolution, which caused us to ICE on the first
testcase and reject the second testcase.

	PR c++/110535

gcc/cp/ChangeLog:

	* call.cc (add_conv_candidate): Check constraints.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-surrogate1.C: New test.
	* g++.dg/cpp2a/concepts-surrogate2.C: New test.

gcc/cp/call.cc

@@ -2588,6 +2588,14 @@ add_conv_candidate (struct z_candidate **candidates, tree fn, tree obj,
   if (*candidates && (*candidates)->fn == totype)
     return NULL;
 
+  if (!constraints_satisfied_p (fn))
+    {
+      reason = constraint_failure ();
+      viable = 0;
+      return add_candidate (candidates, fn, obj, arglist, len, convs,
+			    access_path, conversion_path, viable, reason, flags);
+    }
+
   for (i = 0; i < len; ++i)
     {
       tree arg, argtype, convert_type = NULL_TREE;

gcc/testsuite/g++.dg/cpp2a/concepts-surrogate1.C

@@ -0,0 +1,12 @@
+// PR c++/110535
+// { dg-do compile { target c++20 } }
+
+using F = int(int);
+
+template<bool B>
+struct A {
+ operator F*() requires B;
+};
+
+int i = A<true>{}(0);  // OK
+int j = A<false>{}(0); // { dg-error "no match" }

gcc/testsuite/g++.dg/cpp2a/concepts-surrogate2.C

@@ -0,0 +1,14 @@
+// PR c++/110535
+// { dg-do compile { target c++20 } }
+
+using F = int(int);
+using G = long(int);
+
+template<bool B>
+struct A {
+ operator F&() requires B;
+ operator G&() requires (!B);
+};
+
+int i = A<true>{}(0);  // { dg-bogus "ambiguous" }
+int j = A<false>{}(0); // { dg-bogus "ambiguous" }