c++: overload sets and placeholder return type [PR64194]

In the testcase below, template argument deduction for the call
g(id<int>) goes wrong because the functions in the overload set id<int>
each have a yet-undeduced auto return type, and this undeduced return
type makes try_one_overload fail to match up any of the overloads with
g's parameter type, leading to g's template argument going undeduced and
to the overload set going unresolved.

This patch fixes this issue by performing return type deduction via
instantiation before doing try_one_overload, in a manner similar to what
resolve_address_of_overloaded_function does.

gcc/cp/ChangeLog:

	PR c++/64194
	* pt.c (resolve_overloaded_unification): If the function
	template specialization has a placeholder return type,
	then instantiate it before attempting unification.

gcc/testsuite/ChangeLog:

	PR c++/64194
	* g++.dg/cpp1y/auto-fn60.C: New test.

gcc/cp/pt.c

@@ -22120,7 +22120,16 @@ resolve_overloaded_unification (tree tparms,
 	  if (subargs != error_mark_node
 	      && !any_dependent_template_arguments_p (subargs))
 	    {
-	      elem = TREE_TYPE (instantiate_template (fn, subargs, tf_none));
+	      fn = instantiate_template (fn, subargs, tf_none);
+	      if (undeduced_auto_decl (fn))
+		{
+		  /* Instantiate the function to deduce its return type.  */
+		  ++function_depth;
+		  instantiate_decl (fn, /*defer*/false, /*class*/false);
+		  --function_depth;
+		}
+
+	      elem = TREE_TYPE (fn);
 	      if (try_one_overload (tparms, targs, tempargs, parm,
 				    elem, strict, sub_strict, addr_p, explain_p)
 		  && (!goodfn || !same_type_p (goodfn, elem)))

gcc/testsuite/g++.dg/cpp1y/auto-fn60.C

@@ -0,0 +1,11 @@
+// PR c++/64194
+// { dg-do compile { target c++14 } }
+
+template <typename T> void g(void (*)(T)) { }
+
+template <typename> auto id(int) { }
+template <typename> auto id(char) { return 0; }
+
+int main() {
+  g(id<int>);
+}