Add test for Nullable-By-Default (Null is subtype of all types).

tests/language/null_bottom_test.dart

+// Copyright (c) 2016, 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 testing the ternary operator.
+
+import "package:expect/expect.dart";
+
+// Test that `Null` acts like the bottom type - less than any other type.
+
+bool checkedMode = () { bool c = false; assert(c = true); return c; } ();

[floitsch, 2016/12/20 10:52:58]

isCheckedMode

[Lasse Reichstein Nielsen, 2017/01/02 12:48:50]

Done.

+
+typedef R Fun<A, R>(A argument);
+
+class C<T> {
+  const C();
+  T returns() => null;
+  void accepts(T x) {}
+}
+
+class NullBound<T extends num> {}
+
+main() {
+  testClassTypes();
+  testFunctionTypes();
+}
+
+void testClassTypes() {
+  var cn = new C<Null>();
+
+  Expect.isTrue(cn is C<Null>);
+  Expect.isTrue(cn is C<Object>);
+  Expect.isTrue(cn is C<int>);
+
+  Expect.isNotNull(cn as C<Null>);
+  Expect.isNotNull(cn as C<Object>);
+  Expect.isNotNull(cn as C<int>);
+
+  var ccn = new C<C<Null>>();
+
+  Expect.isTrue(ccn is C<C<Null>>);
+  Expect.isTrue(ccn is C<C<Object>>);
+  Expect.isTrue(ccn is C<C<int>>);
+
+  Expect.isNotNull(ccn as C<C<Null>>);
+  Expect.isNotNull(ccn as C<C<Object>>);
+  Expect.isNotNull(ccn as C<C<int>>);
+
+  var ci = new C<int>();
+  Expect.isFalse(ci is C<Null>);
+
+  var co = new C<Object>();
+  Expect.isFalse(co is C<Null>);
+
+  if (!checkedMode) return;
+
+  List<int> = const <Null>[];
+
+  C<Null> x1 = cn;
+  C<Object> x2 = cn;
+
+  Expect.identical(x1 , cn);
+  Expect.identical(x2 , cn);
+
+  const C<Null> cocn = const C<Null>();
+  const C<Object> coco = cocn;
+  const C<int> coci = cocn;
+
+  Expect.identical(cocn, coco);
+  Expect.identical(cocn, coci);
+
+  Expect.throws(() {
+    Null x = "string" as dynamic;
+    use(x);  // Avoid "x unused" warning.
+  });
+
+  Expect.throws(() {
+    Null x = new Object();
+    use(x);  // Avoid "x unused" warning.
+  });
+
+  NullBound<int> nb = new NullBound<Null>();  // Should not fail.
+  use(nb);  // Avoid "nb unused" warning.
+}
+
+void testFunctionTypes() {
+  T1 t1 = new T1();
+  T2 t2 = new T2();
+  T1 t = t2;
+
+  Fun<int, Null> f1 = t1.foo;
+  Fun<Null, int> f2 = t.bar;
+  f1 = t1.baz;
+  f2 = t.qux;
+  use(f1);
+  use(f2);
+
+  var l = new List<Fun<Null, Null>>();
+  Expect.isTrue(l is List<Fun<int, Null>>);

[floitsch, 2016/12/20 10:52:58]

Why is that?
a List of Null->Null should not be a List of int->Null.

[Lasse Reichstein Nielsen, 2017/01/02 12:48:50]

Done.

+  l = new List<Fun<int, int>>;
+  Expect.isTrue(l is List<Fun<Null, num>>);
+
+  Expect.isTrue(((int _) => null) is Fun<int, Null>);
+
+  Null fun(int x) => null;
+  Fun<Null, int> fun2 = fun;  // Safe assignment.
+  if (fun2 is Fun<int, Null>) {
+    // If int->Null is *subtype* of Null->int (which it should be),
+    // then type promotion succeeds.
+    // If type promotion succeeds, the static type is int->Null, otherwise
+    // it's Null->int.
+    fun2(42);  // Should not give a warning after type promotion.
+    fun2(null).abs();  /// 03: static type warning, runtime error
+  }
+}
+
+class T1 {
+  Null foo(int x) => null;
+  int bar(Null x) => null;
+  Null baz(int x) => null;
+  int qux(Null x) => null;
+}
+
+class T2 extends T1 {
+  Null foo(Null x) => null;  /// 01: static type warning
+  Null bar(Null x) => null;
+  int baz(int x) => x;  /// 02: static type warning
+  int qux(int x) => x;
+}
+
+// Avoid "variable not used" warnings.
+use(x) {
+  return identical(x, x);
+}