Fix overly strict invalid return checking.

In changing the proposed spec to address comments in a way that was intended to be
non-breaking, I accidentally introduced some new (unintended) restrictions.  This
fixes the spec, rolls back the change in analyzer, and adjusts the language tests
appropriately.

Change-Id: I487b0eaacbfa1447d4ee909c0a56e435c2088990
Reviewed-on: https://dart-review.googlesource.com/70462
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>

docs/language/informal/invalid_returns.md

@@ -67,7 +67,7 @@ The rules for an asynchronous non-generator function with declared return type
   * and `flatten(S)` is `void`, `dynamic` or `Null`
 
 * `return exp;` where `exp` has static type `S` is a valid return if:
-  * `flatten(T)` is not `void`
+  * `T` is not `void`
   * and `flatten(S)` is not `void`
   * and `Future<flatten(S)>` is assignable to `T`
 
@@ -114,7 +114,7 @@ The rules for an asynchronous non-generator function with declared return type
 
 * `return;` is an error if `flatten(T)` is not `void`, `dynamic`, or `Null`
 
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
+* `return exp;` where `exp` has static type `S` is an error if `T` is
   `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
 
 * `return exp;` where `exp` has static type `S` is an error if `flatten(S)` is

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

@@ -5269,7 +5269,8 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
       if (toType.isDynamic || toType.isDartCoreNull || toType.isBottom) {
         return;
       }
-    } else {
+    }
+    if (!expectedType.isVoid && !fromType.isVoid) {
       var checkWithType = (!_inAsync)
           ? fromType
           : _typeProvider.futureType.instantiate(<DartType>[fromType]);

tests/language_2/invalid_returns/async_invalid_return_06_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-int v = null;
-Future<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_07_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-int v = null;
-FutureOr<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_09_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-Object v = null;
-Future<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_10_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-Object v = null;
-FutureOr<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_12_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-Future<int> v = null;
-Future<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_13_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-Future<int> v = null;
-FutureOr<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_15_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-FutureOr<int> v = null;
-Future<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_16_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-FutureOr<int> v = null;
-FutureOr<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_18_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-Future<Object> v = null;
-Future<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_19_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-Future<Object> v = null;
-FutureOr<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_21_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-FutureOr<Object> v = null;
-Future<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_invalid_return_22_test.dart

@@ -1,18 +0,0 @@
-// Copyright (c) 2018, 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 'dart:async';
-
-/*
-* `return exp;` where `exp` has static type `S` is an error if `flatten(T)` is
-  `void` and `flatten(S)` is not `void`, `dynamic`, or `Null`
-*/
-FutureOr<Object> v = null;
-FutureOr<void> test() async {
-  return /*@compile-error=unspecified*/ v;
-}
-
-void main() {
-  test();
-}

tests/language_2/invalid_returns/async_valid_returns.dart

@@ -319,7 +319,7 @@ FutureOr<void> async_Null_to_FutureOr_void_() async {
 
 /*
 * `return exp;` where `exp` has static type `S` is a valid return if:
-  * `flatten(T)` is not `void`
+  * `T` is not `void`
   * and `flatten(S)` is not `void`
   * and `Future<flatten(S)>` is assignable to `T`
 */
@@ -343,6 +343,46 @@ Future<int> async_Future_int__to_Future_int_() async {
   return fvi;
 }
 
+Future<void> async_int_to_Future_void__e() async => vi;
+Future<void> async_int_to_Future_void_() async {
+  return vi;
+}
+
+Future<void> async_Object_to_Future_void__e() async => vo;
+Future<void> async_Object_to_Future_void_() async {
+  return vo;
+}
+
+Future<void> async_FutureOr_int__to_Future_void__e() async => fovi;
+Future<void> async_FutureOr_int__to_Future_void_() async {
+  return fovi;
+}
+
+Future<void> async_Future_int__to_Future_void__e() async => fvi;
+Future<void> async_Future_int__to_Future_void_() async {
+  return fvi;
+}
+
+FutureOr<void> async_int_to_FutureOr_void__e() async => vi;
+FutureOr<void> async_int_to_FutureOr_void_() async {
+  return vi;
+}
+
+FutureOr<void> async_Object_to_FutureOr_void__e() async => vo;
+FutureOr<void> async_Object_to_FutureOr_void_() async {
+  return vo;
+}
+
+FutureOr<void> async_FutureOr_int__to_FutureOr_void__e() async => fovi;
+FutureOr<void> async_FutureOr_int__to_FutureOr_void_() async {
+  return fovi;
+}
+
+FutureOr<void> async_Future_int__to_FutureOr_void__e() async => fvi;
+FutureOr<void> async_Future_int__to_FutureOr_void_() async {
+  return fvi;
+}
+
 void main() {
   async_int_to_void_e();
   async_int_to_Future_void__e();
@@ -462,4 +502,20 @@ void main() {
   async_FutureOr_int__to_Future_int_();
   async_Future_int__to_Future_int__e();
   async_Future_int__to_Future_int_();
+  async_int_to_Future_void__e();
+  async_int_to_Future_void_();
+  async_Object_to_Future_void__e();
+  async_Object_to_Future_void_();
+  async_FutureOr_int__to_Future_void__e();
+  async_FutureOr_int__to_Future_void_();
+  async_Future_int__to_Future_void__e();
+  async_Future_int__to_Future_void_();
+  async_int_to_FutureOr_void__e();
+  async_int_to_FutureOr_void_();
+  async_Object_to_FutureOr_void__e();
+  async_Object_to_FutureOr_void_();
+  async_FutureOr_int__to_FutureOr_void__e();
+  async_FutureOr_int__to_FutureOr_void_();
+  async_Future_int__to_FutureOr_void__e();
+  async_Future_int__to_FutureOr_void_();
 }