--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr80471.C	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,23 @@
+// PR c++/80471
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fconcepts" }
+
+template <class, class>
+constexpr bool is_same = false;
+template <class T>
+constexpr bool is_same<T, T> = true;
+
+template<class T>
+decltype(auto) f(T&& a){return a;}
+
+decltype(auto) g(auto&& a){return a;}
+
+auto z = [](auto&& a) -> decltype(auto) { return a; };
+
+int main()
+{
+  int i;
+  static_assert(is_same< decltype(f(i)), int& >, "");
+  static_assert(is_same< decltype(g(i)), int& >, "");
+  static_assert(is_same< decltype(z(i)), int& >, "");
+}