--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp0x/pr57981.C	Fri Oct 27 22:46:09 2017 +0900
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wall -Wextra" }
+
+template<class T>
+void f(T t, void* = 0)  // { dg-warning "unused parameter" }
+{
+}
+
+template<class T>
+auto g(T t) -> decltype(f(t))
+{
+  f(t);
+}
+
+int main()
+{
+  g(0);
+}