fix #27259, implement covariance checking for strong mode and DDC

tests/language_strong/covariant_subtyping_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.
+import 'package:expect/expect.dart';
+
+class Fields<T> {
+  T x;
+  T _y;
+  T _z;
+
+  m() {
+    _y = x;
+  }
+
+  n(Fields<T> c) {
+    c._z = x;
+  }
+}
+
+testField() {
+  Fields<Object> c = new Fields<int>();
+  Expect.throws(() {
+    c.x = 'hello';
+  });
+}
+
+testPrivateFields() {
+  Fields<Object> c = new Fields<int>()..x = 42;
+  c.m();
+  Expect.equals(c._y, 42);
+
+  Fields<Object> c2 = new Fields<String>()..x = 'hi';
+  c2.n(c2);
+  Expect.equals(c2._z, 'hi');
+  Expect.throws(() {
+    c.n(c2);
+  });
+  Expect.equals(c2._z, 'hi');
+}
+
+class NumBounds<T extends num> {
+  bool m(T t) => t.isNegative;
+}
+
+class MethodTakesNum extends NumBounds<int> {
+  bool m(num obj) => obj.isNegative; // does not need check
+}
+
+class MethodTakesInt extends NumBounds<int> {
+  bool m(int obj) => obj.isNegative; // needs a check
+}
+
+testClassBounds() {
+  NumBounds<num> d = new MethodTakesNum();
+  Expect.equals(d.m(-1.1), true);
+  d = new MethodTakesInt();
+  Expect.throws(() {
+    d.m(-1.1);
+  });
+}
+
+typedef void F<T>(T t);
+typedef G<T> = void Function<S extends T>(S s);
+
+class FnChecks<T> {
+  F<T> f;
+  G<T> g;
+  T _t;
+  T getT() => _t;
+  F<T> setterForT() {
+    return (T t) {
+      _t = t;
+    };
+  }
+}
+
+testReturnOfFunctionType() {
+  FnChecks<int> cInt = new FnChecks<int>();
+  FnChecks<Object> cObj = cInt;
+  cObj.setterForT();
+  Expect.throws(() {
+    cObj.setterForT()(42);
+  });
+  Expect.throws(() {
+    cObj.setterForT()('hi');
+  });
+  cInt.setterForT()(42);
+  Expect.equals(cObj.getT(), 42);
+  dynamic d = cObj.setterForT();
+  d(100);
+  Expect.equals(cInt.getT(), 100);
+  Expect.throws(() {
+    d('hi');
+  });
+}
+
+testFieldOfFunctionType() {
+  FnChecks<Object> c = new FnChecks<String>()..f = (String b) {};
+  Expect.throws(() {
+    F<Object> f = c.f;
+  });
+  Expect.throws(() {
+    var f = c.f;
+  });
+  Expect.throws(() {
+    c.f(42);
+  });
+  Expect.throws(() {
+    c.f('hi');
+  });
+  FnChecks<String> cStr = c;
+  cStr.f('hi');
+}
+
+testFieldOfGenericFunctionType() {
+  FnChecks<Object> c = new FnChecks<num>()
+    ..g = <S extends num>(S s) => s.isNegative;
+
+  Expect.throws(() {
+    G<Object> g = c.g;
+  });
+  Expect.throws(() {
+    var g = c.g;
+  });
+  Expect.throws(() {
+    c.g<String>('hi');
+  });
+  Expect.throws(() {
+    c.g<int>(42);
+  });
+  FnChecks<num> cNum = c;
+  cNum.g(42);
+}
+
+class Base {
+  int _t = 0;
+  add(int t) {
+    _t += t;
+  }
+}
+
+abstract class I<T> {
+  add(T t);
+}
+
+class ExtendsBase extends Base implements I<int> {}
+
+class MixinBase extends Object with Base implements I<int> {}
+
+class MixinBase2 = Object with Base implements I<int>;
+
+testMixinApplication() {
+  I<Object> i = new ExtendsBase();
+  I<Object> j = new MixinBase();
+  I<Object> k = new MixinBase2();
+  Expect.throws(() {
+    i.add('hi');
+  });
+  Expect.throws(() {
+    j.add('hi');
+  });
+  // TODO(jmesserly): this should also throw. It does not because DDC's
+  // technique for generating mixin aliases (mixin applications of the form
+  // `class X = Object with Y /* optional implements */;`) does not allow
+  // adding any methods in the class. The normal technique of generating
+  // a method that performs the check and then calls `super` will not work,
+  // because there is no superclass to call. We will need some sort of
+  // special case code to implement this, perhaps move the original
+  // method to a symbol, then generate a wrapper with the original method name,
+  // that checks and calls it.
+  k.add('hi');
+}
+
+abstract class GenericAdd<T> {
+  add<S extends T>(S t);
+}
+
+class GenericAdder implements GenericAdd<num> {
+  num _t = 0;
+  add<T extends num>(T t) {
+    _t = t;
+  }
+}
+
+testGenericMethodBounds() {
+  GenericAdd<Object> i = new GenericAdder();
+  Expect.throws(() {
+    i.add('hi');
+  });
+  Expect.throws(() {
+    i.add<String>(null);
+  });
+  i.add(null);
+  i.add(42);
+}
+
+class ClassF<T> {
+  T x;
+  void call(T t) {
+    x = t;
+  }
+}
+
+testCallMethod() {
+  ClassF<int> cc = new ClassF<int>();
+  ClassF<Object> ca = cc; // An upcast, per covariance.
+  F<Object> f = ca;
+  Expect.equals(f.runtimeType.toString(), 'ClassF<int>');
+  Expect.throws(() {
+    f(new Object());
+  });
+}
+
+main() {
+  testField();
+  testPrivateFields();
+  testClassBounds();
+  testReturnOfFunctionType();
+  testFieldOfFunctionType();
+  testFieldOfGenericFunctionType();
+  testMixinApplication();
+  testGenericMethodBounds();
+  testCallMethod();
+}