Fix strict inference on overriding methods and setters.

See the change to the spec: dart-lang/language#595

Change-Id: I5aeb20eb175f7018ecc99f398ea4d463b1e7874e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/118920
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/analyzer/lib/src/generated/resolver.dart

@@ -384,6 +384,18 @@ class BestPracticesVerifier extends RecursiveAstVisitor<void> {
   void visitMethodDeclaration(MethodDeclaration node) {
     bool wasInDeprecatedMember = _inDeprecatedMember;
     ExecutableElement element = node.declaredElement;
+    bool elementIsOverride() {
+      if (element is ClassMemberElement) {
+        Name name = new Name(_currentLibrary.source.uri, element.name);
+        Element enclosingElement = element.enclosingElement;
+        if (enclosingElement is ClassElement) {
+          InterfaceType classType = enclosingElement.thisType;
+          return _inheritanceManager.getOverridden(classType, name) != null;
+        }
+      }
+      return false;
+    }
+
     if (element != null && element.hasDeprecated) {
       _inDeprecatedMember = true;
     }
@@ -392,7 +404,9 @@ class BestPracticesVerifier extends RecursiveAstVisitor<void> {
       //checkForOverridingPrivateMember(node);
       _checkForMissingReturn(node.returnType, node.body, element, node);
       _checkForUnnecessaryNoSuchMethod(node);
-      _checkStrictInferenceReturnType(node.returnType, node, node.name.name);
+      if (_strictInference && !node.isSetter && !elementIsOverride()) {
+        _checkStrictInferenceReturnType(node.returnType, node, node.name.name);
+      }
       _checkStrictInferenceInParameters(node.parameters);
       super.visitMethodDeclaration(node);
     } finally {

pkg/analyzer/test/src/diagnostics/inference_failure_on_function_return_type_test.dart

@@ -60,6 +60,38 @@ class C {
 ''', [error(HintCode.INFERENCE_FAILURE_ON_FUNCTION_RETURN_TYPE, 12, 9)]);
   }
 
+  test_classInstanceMethod_overriding() async {
+    await assertNoErrorsInCode(r'''
+class C {
+  int f() => 7;
+}
+
+class D extends C {
+  f() => 9;
+}
+
+class E implements C {
+  f() => 9;
+}
+
+class F with C {
+  f() => 9;
+}
+
+mixin M on C {
+  f() => 9;
+}
+
+mixin N {
+  int g() => 7;
+}
+
+class G with N {
+  g() => 9;
+}
+''');
+  }
+
   test_classInstanceMethod_withReturnType() async {
     await assertNoErrorsInCode(r'''
 class C {
@@ -77,11 +109,11 @@ class C {
   }
 
   test_classInstanceSetter() async {
-    await assertErrorsInCode(r'''
+    await assertNoErrorsInCode(r'''
 class C {
   set f(int x) => print(x);
 }
-''', [error(HintCode.INFERENCE_FAILURE_ON_FUNCTION_RETURN_TYPE, 12, 25)]);
+''');
   }
 
   test_classStaticMethod() async {