Add testcase of PR c++/90448, already fixed in trunk.

	PR c++/90448
	* g++.dg/cpp1y/lambda-generic-variadic20.C: New.

gcc/testsuite/ChangeLog

+2020-04-22  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/90448
+	* g++.dg/cpp1y/lambda-generic-variadic20.C: New.
+
 2020-04-22  Martin Sebor  <msebor@redhat.com>
 
 	PR middle-end/94647

gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic20.C

+// PR c++/90448
+// { dg-do compile { target c++14 } }
+
+template<class ... Ts> int fooV(Ts ... ts) {
+  auto L = [](auto ... a) {
+    auto M = [](decltype(a) ... b) -> void {
+    };
+    return M;
+  };
+
+  decltype(L(L, ts...)) (*fp)(decltype(L), decltype(ts) ...) = L;
+
+  return 0;
+}
+
+int run2 = fooV("BC", 3, 2.77, 'A', float{}, short{}, unsigned{});