c++: Add testcase for PR c++/93207

gcc/testsuite/ChangeLog:

	PR c++/93207
	* g++.dg/concepts/variadic5.C: New test.

gcc/testsuite/ChangeLog

 2020-04-14  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/93207
+	* g++.dg/concepts/variadic5.C: New test.
+
 	PR c++/85278
 	* g++.dg/concepts/diagnostic9.C: New test.
 

gcc/testsuite/g++.dg/concepts/variadic5.C

+// PR c++/93207
+// { dg-do compile { target concepts } }
+
+template <typename... Args>
+concept C = true;
+
+struct S
+{
+    template <typename... Args>
+    void f()
+    requires C<Args...>;
+};
+
+template <typename... Args>
+void S::f()
+requires C<Args...>
+{
+}
+
+void foo()
+{
+  S s;
+  s.f<>();
+  s.f<int>();
+  s.f<int, char>();
+}