Tests for `void`.

tests/language/void_type3_test.dart

+// Copyright (c) 2017, 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 test for type checks involving the void type.
+
+import 'package:expect/expect.dart';
+
+typedef F = void Function(Object);
+typedef F2 = void Function([Object]);
+typedef F3 = void Function({Object x});
+
+typedef G = void Function(void);
+typedef G2 = void Function([void]);
+typedef G3 = void Function({void x});
+
+typedef H = int Function(void);
+typedef H2 = int Function([void]);
+typedef H3 = int Function({void x});
+
+void f(Object x) {}
+void f2([Object x]) {}
+void f3({Object x}) {}
+
+void g(void x) {}
+void g2([void x]) {}
+void g3({void x}) {}
+
+int h(void x) => 499;
+int h2([void x]) => 499;
+int h3({void x}) => 499;
+
+void expectsF(F f) {}
+void expectsG(G g) {}
+void expectsH(H h) {}
+
+void expectsF2(F2 f) {}
+void expectsG2(G2 g) {}
+void expectsH2(H2 h) {}
+
+void expectsF3(F3 f) {}
+void expectsG3(G3 g) {}
+void expectsH3(H3 h) {}
+
+main() {
+  Expect.isTrue(f is F);
+  Expect.isTrue(f is G);
+  Expect.isFalse(f is H);
+  expectsF(f);
+  expectsG(f);
+  expectsH(f);  /// 00: error
+
+  Expect.isTrue(f2 is F2);
+  Expect.isTrue(f2 is G2);
+  Expect.isFalse(f2 is H2);
+  expectsF2(f2);
+  expectsG2(f2);
+  expectsH2(f2);  /// 01: error
+
+  Expect.isTrue(f3 is F3);
+  Expect.isTrue(f3 is G3);
+  Expect.isFalse(f3 is H3);
+  expectsF3(f3);
+  expectsG3(f3);
+  expectsH3(f3);  /// 03: error
+
+  Expect.isTrue(g is F);
+  Expect.isTrue(g is G);
+  Expect.isFalse(g is H);
+  expectsF(g);  /// 04: error

[eernst, 2017/07/10 16:05:34]

I cannot see how this could be an error. It would also be surprising if `g is F`
returns true, and still the checked-mode check on the argument of `expectsF`
fails.

The crucial point is probably that `void` as a return type is given special
treatment in Dart 1.x (so a `void Function()` is _not_ an `Object Function()`,
only vice versa), but `void` as a type annotation and as an actual type argument
is not given special treatment: It behaves the same as `Object`.

+  expectsG(g);
+  expectsH(g);  /// 05: error
+
+  Expect.isTrue(g2 is F2);
+  Expect.isTrue(g2 is G2);
+  Expect.isFalse(g2 is H2);
+  expectsF2(g2);  /// 06: error

[eernst, 2017/07/10 16:05:34]

Same as line 70.

+  expectsG2(g2);
+  expectsH2(g2);  /// 07: error
+
+  Expect.isTrue(g3 is F3);
+  Expect.isTrue(g3 is G3);
+  Expect.isFalse(g3 is H3);
+  expectsF3(g3);  /// 08: error

[eernst, 2017/07/10 16:05:34]

Same as line 70.

+  expectsG3(g3);
+  expectsH3(g3);  /// 09: error
+
+  Expect.isTrue(h is F);
+  Expect.isTrue(h is G);
+  Expect.isTrue(h is H);
+  expectsF(h);
+  expectsG(h);
+  expectsH(h);
+
+  Expect.isTrue(h2 is H2);
+  Expect.isTrue(h2 is G2);
+  Expect.isTrue(h2 is H2);
+  expectsF(h2);
+  expectsG(h2);
+  expectsH(h2);
+
+  Expect.isTrue(h3 is H3);
+  Expect.isTrue(h3 is G3);
+  Expect.isTrue(h3 is H3);
+  expectsF(h3);
+  expectsG(h3);
+  expectsH(h3);
+}