[tests] Add nested function subtype test

Change-Id: I04368d00010e81148c608ba0d8f0371a516862e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213966
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Erik Ernst <eernst@google.com>

tests/language/function_subtype/nested_function_type_test.dart

@@ -0,0 +1,44 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+// Two function types that are identical except the argument type of the
+// nested function is a type variable from the outer function.
+typedef Fn = void Function<S>(S val) Function<T>(T val);
+typedef Gn = void Function<S>(T val) Function<T>(T val);
+
+void Function<S>(S) fn<T>(T val) => <R>(R val) {};
+void Function<S>(T) gn<T>(T val) => <R>(T val) {};
+
+// The same pattern here except with bounds on the type arguments.
+typedef Xn = void Function<S extends num>(S val) Function<T extends num>(T val);
+typedef Yn = void Function<S extends num>(T val) Function<T extends num>(T val);
+
+void Function<S extends num>(S) xn<T extends num>(T val) =>
+    <R extends num>(R val) {};
+void Function<S extends num>(T) yn<T extends num>(T val) =>
+    <R extends num>(T val) {};
+
+// The nested function here uses concrete type in the argument position so it
+// should satisfy either of the previous typedefs.
+void Function<S extends num>(num) zn<T extends num>(T val) =>
+    <R extends num>(num val) {};
+
+void main() {
+  Expect.isTrue(fn is Fn);
+  Expect.isFalse(fn is Gn);
+
+  Expect.isTrue(gn is Gn);
+  Expect.isFalse(gn is Fn);
+
+  Expect.isTrue(xn is Xn);
+  Expect.isFalse(xn is Yn);
+
+  Expect.isTrue(yn is Yn);
+  Expect.isFalse(yn is Xn);
+
+  Expect.isTrue(zn is Xn);
+  Expect.isTrue(zn is Yn);
+}

tests/language_2/function_subtype/nested_function_type_test.dart

@@ -0,0 +1,46 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// @dart = 2.9
+
+import "package:expect/expect.dart";
+
+// Two function types that are identical except the argument type of the
+// nested function is a type variable from the outer function.
+typedef Fn = void Function<S>(S val) Function<T>(T val);
+typedef Gn = void Function<S>(T val) Function<T>(T val);
+
+void Function<S>(S) fn<T>(T val) => <R>(R val) {};
+void Function<S>(T) gn<T>(T val) => <R>(T val) {};
+
+// The same pattern here except with bounds on the type arguments.
+typedef Xn = void Function<S extends num>(S val) Function<T extends num>(T val);
+typedef Yn = void Function<S extends num>(T val) Function<T extends num>(T val);
+
+void Function<S extends num>(S) xn<T extends num>(T val) =>
+    <R extends num>(R val) {};
+void Function<S extends num>(T) yn<T extends num>(T val) =>
+    <R extends num>(T val) {};
+
+// The nested function here uses concrete type in the argument position so it
+// should satisfy either of the previous typedefs.
+void Function<S extends num>(num) zn<T extends num>(T val) =>
+    <R extends num>(num val) {};
+
+void main() {
+  Expect.isTrue(fn is Fn);
+  Expect.isFalse(fn is Gn);
+
+  Expect.isTrue(gn is Gn);
+  Expect.isFalse(gn is Fn);
+
+  Expect.isTrue(xn is Xn);
+  Expect.isFalse(xn is Yn);
+
+  Expect.isTrue(yn is Yn);
+  Expect.isFalse(yn is Xn);
+
+  Expect.isTrue(zn is Xn);
+  Expect.isTrue(zn is Yn);
+}