Add testcase of PR c++/92536, already fixed.

	PR c++/92536
	* g++.dg/cpp1z/pr92536.C: New.

gcc/testsuite/ChangeLog

+2020-01-20  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/92536
+	* g++.dg/cpp1z/pr92536.C: New.
+
 2020-01-20  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/93094

gcc/testsuite/g++.dg/cpp1z/pr92536.C

+// { dg-do compile { target c++17 } }
+
+namespace std
+{
+  struct stop_token { };
+
+  template<typename Callback>
+    struct stop_callback
+    {
+      template<typename C>
+      stop_callback(stop_token, C&&) { }
+    };
+
+template<typename _Callback>
+    stop_callback(stop_token, _Callback) -> stop_callback<_Callback>;
+}
+
+int main()
+{
+  std::stop_token tok;
+  std::function<void()> f([](){});  // { dg-error "not a member|not declared" }
+  std::stop_callback cb(tok, f);
+}