--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-using1.C	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,17 @@
+// PR c++/79591
+// { dg-do compile { target c++2a } }
+
+template <class> concept True = true;
+
+// Fine.
+namespace X {
+  void f(auto) {}
+  void f(True auto) {}
+}
+
+void f(auto) {}
+namespace Y {
+  void f(True auto) {}
+  using ::f;
+  // error: 'template<class auto:3> void f(auto:3)' conflicts with a previous declaration
+}