--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/diagnostic/pr65923.C	Thu Oct 25 07:37:49 2018 +0900
@@ -0,0 +1,23 @@
+// { dg-do compile { target c++14 } }
+
+#include <chrono>
+
+using std::literals::chrono_literals::operator""s;
+
+struct X
+{
+  friend constexpr std::chrono::duration<long double> std::literals::chrono_literals::operator""s(long double);
+};
+
+struct X2
+{
+  friend constexpr X operator""foo(long double) {return {};} // { dg-warning "literal operator suffixes not preceded" }
+};
+
+namespace std
+{
+  template<> void swap(X&, X&)
+  {
+    constexpr std::chrono::duration<long double> operator""s(long double);
+  }
+}