fix #25578, implement @covariant parameter overrides

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, 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.
 
 library analyzer.test.src.task.strong.checker_test;
 
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'strong_test_helper.dart';
 
@@ skipping 481 matching lines @@
   var toStringClosure2 = helper.toString;
   (/*info:DYNAMIC_INVOKE*/toStringClosure2());
   int hashCode = /*info:DYNAMIC_CAST*/helper.hashCode;
 
   baz().toString();
   baz().hashCode;
 }
 ''');
   }
 
+  void test_covariantOverride() {
+    addFile(r'''
+library meta;
+class _Covariant { const _Covariant(); }
+const Object covariant = const _Covariant();
+    ''', name: '/meta.dart');
+
+    checkFile(r'''
+import 'meta.dart';
+class C {
+  num f(num x) => x;
+}
+class D extends C {
+  int f(@covariant int x) => x;
+}
+class E extends D {
+  int f(Object x) => /*info:DOWN_CAST_IMPLICIT*/x;
+}
+class F extends E {
+  int f(@covariant int x) => x;
+}
+class G extends E implements D {}
+
+class D_error extends C {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(int x) => x;
+}
+class E_error extends D {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(@covariant double x) => 0;
+}
+class F_error extends E {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(@covariant double x) => 0;
+}
+class G_error extends E implements D {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(@covariant double x) => 0;
+}
+    ''');
+  }
+
+  void test_covariantOverride_leastUpperBound() {
+    addFile(r'''
+library meta;
+class _Covariant { const _Covariant(); }
+const Object covariant = const _Covariant();
+    ''', name: '/meta.dart');
+
+    checkFile(r'''
+import "meta.dart";
+abstract class Top {}
+abstract class Left implements Top {}
+abstract class Right implements Top {}
+abstract class Bottom implements Left, Right {}
+
+abstract class TakesLeft {
+  m(Left x);
+}
+abstract class TakesRight {
+  m(Right x);
+}
+abstract class TakesTop implements TakesLeft, TakesRight {
+  m(Top x); // works today
+}
+abstract class TakesBottom implements TakesLeft, TakesRight {
+  // LUB(Left, Right) == Top, so this is an implicit cast from Top to Bottom.
+  m(@covariant Bottom x);
+}
+    ''');
+  }
+
+  void test_covariantOverride_markerIsInherited() {
+    addFile(r'''
+library meta;
+class _Covariant { const _Covariant(); }
+const Object covariant = const _Covariant();
+    ''', name: '/meta.dart');
+
+    checkFile(r'''
+import 'meta.dart';
+class C {
+  num f(@covariant num x) => x;
+}
+class D extends C {
+  int f(int x) => x;
+}
+class E extends D {
+  int f(Object x) => /*info:DOWN_CAST_IMPLICIT*/x;
+}
+class F extends E {
+  int f(int x) => x;
+}
+class G extends E implements D {}
+
+class D_error extends C {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(String x) => 0;
+}
+class E_error extends D {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(double x) => 0;
+}
+class F_error extends E {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(double x) => 0;
+}
+class G_error extends E implements D {
+  /*error:INVALID_METHOD_OVERRIDE*/int f(double x) => 0;
+}
+    ''');
+  }
+
   void test_dynamicInvocation() {
     checkFile('''
 typedef dynamic A(dynamic x);
 class B {
   int call(int x) => x;
   double col(double x) => x;
 }
 void main() {
   {
     B f = new B();
@@ skipping 3215 matching lines @@
     // Regression test for https://github.com/dart-lang/sdk/issues/25069
     checkFile('''
 typedef int Foo();
 void foo() {}
 void main () {
   Foo x = /*error:INVALID_ASSIGNMENT,info:USE_OF_VOID_RESULT*/foo();
 }
 ''');
   }
 }