Implement least upper bound.

tests/language/least_upper_bound_test.dart

+// Copyright (c) 2013, 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.
+
+// Test least upper bound through type checking of conditionals.
+
+class A {
+  var a;
+}
+class B {
+  var b;
+}
+class C extends B {
+  var c;
+}
+class D extends B {
+  var d;
+}
+
+class E<T> {
+  T e;
+
+  E(this.e);
+}
+class F<T> extends E<T> {
+  T f;
+
+  F(T f) : this.f = f, super(f);
+}
+
+void main() {
+  testAB(new A(), new B());
+  testBC(new C(), new C());
+  testCD(new C(), new D());
+  testEE(new F<C>(new C()), new F<C>(new C()));
+  testEF(new F<C>(new C()), new F<C>(new C()));
+}
+
+void testAB(A a, B b) {
+  A r1 = true ? a : b; /// 01: ok
+  B r2 = false ? a : b; /// 02: ok
+  (true ? a : b).a = 0; /// 03: static type warning
+  (false ? a : b).b = 0; /// 04: static type warning
+  var c = new C();
+  (true ? a : c).a = 0; /// 05: ok
+  (false ? c : b).b = 0; /// 06: ok
+}
+
+void testBC(B b, C c) {
+  B r1 = true ? b : c; /// 07: ok
+  C r2 = false ? b : c; /// 08: ok
+  (true ? b : c).b = 0; /// 09: ok
+  (false ? b : c).c = 0; /// 10: static type warning
+  var a = null;
+  (true ? b : a).b = 0; /// 11: ok
+  (false ? a : b).c = 0; /// 12: ok
+  (true ? c : a).b = 0; /// 13: ok
+  (false ? a : c).c = 0; /// 14: ok
+}
+
+void testCD(C c, D d) {
+  C r1 = true ? c : d; /// 15: ok
+  D r2 = false ? c : d; /// 16: ok
+  (true ? c : d).b = 0; /// 17: ok
+  (false ? c : d).b = 0; /// 18: ok
+  (true ? c : d).c = 0; /// 19: static type warning
+  (false ? c : d).d = 0; /// 20: static type warning
+}
+
+void testEE(E<B> e, E<C> f) {
+  // The least upper bound of E<B> and E<C> is Object since the supertypes are
+  //     {E<B>, Object} for E<B> and
+  //     {E<C>, Object} for E<C> and
+  // Object is the most specific type in the intersection of the supertypes.
+  E<B> r1 = true ? e : f; /// 21: ok
+  F<C> r2 = false ? e : f; /// 22: ok
+  try {
+    A r3 = true ? e : f; /// 23: ok
+    B r4 = false ? e : f; /// 24: ok
+  } catch (e) {
+    // Type error in checked mode.
+  }
+  (true ? e : f).e = null; /// 25: static type warning
+  (false ? e : f).e = null; /// 26: static type warning
+}
+
+void testEF(E<B> e, F<C> f) {
+  // The least upper bound of E<B> and F<C> is Object since the supertypes are
+  //     {E<B>, Object} for E<B> and
+  //     {F<C>, E<C>, Object} for F<C> and
+  // Object is the most specific type in the intersection of the supertypes.
+  E<B> r1 = true ? e : f; /// 27: ok
+  F<C> r2 = false ? e : f; /// 28: ok
+  try {
+    A r3 = true ? e : f; /// 29: ok
+    B r4 = false ? e : f; /// 30: ok
+  } catch (e) {
+    // Type error in checked mode.
+  }
+  var r5;
+  r5 = (true ? e : f).e; /// 31: static type warning
+  r5 = (false ? e : f).f; /// 32: static type warning
+}