Do assignability checks on return/yield using the greatest closure of the context.

Without this fix, there is a risk that the unknown type ("?") will
leak into kernel code via the "as" expression inserted for an implicit
downcast.

Change-Id: Iea7276c8489eff80fe32f33e53efd42571c94a89
Reviewed-on: https://dart-review.googlesource.com/26861
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

@@ -213,7 +213,13 @@ class ClosureContext {
       var expectedType = isYieldStar
           ? _wrapAsyncOrGenerator(inferrer, returnContext)
           : returnContext;
-      inferrer.checkAssignability(expectedType, type, expression, fileOffset);
+      if (expectedType != null) {
+        inferrer.checkAssignability(
+            greatestClosure(inferrer.coreTypes, expectedType),
+            type,
+            expression,
+            fileOffset);
+      }
     }
   }
 
@@ -393,6 +399,7 @@ abstract class TypeInferrerImpl extends TypeInferrer {
   /// an implicit downcast if appropriate.
   Expression checkAssignability(DartType expectedType, DartType actualType,
       Expression expression, int fileOffset) {
+    assert(expectedType == null || isKnown(expectedType));
     // We don't need to insert assignability checks when doing top level type
     // inference since top level type inference only cares about the type that
     // is inferred (the kernel code is discarded).

pkg/front_end/testcases/return_with_unknown_type_in_context.dart

@@ -0,0 +1,14 @@
+// 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.
+
+bool f(List x) {
+  return x.expand((y) {
+    // Since y has type dynamic, y.split(',') has type dynamic, so an implicit
+    // downcast is needed.  The return context is Iterable<?>.  We should
+    // generate an implicit downcast to Iterable<dynamic>.
+    return y.split(',');
+  }).any((y) => y == 'z');
+}
+
+main() {}

pkg/front_end/testcases/return_with_unknown_type_in_context.dart.direct.expect

@@ -0,0 +1,10 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method f(core::List<dynamic> x) → core::bool {
+  return x.expand((dynamic y) → dynamic {
+    return y.split(",");
+  }).any((dynamic y) → dynamic => y.==("z"));
+}
+static method main() → dynamic {}

pkg/front_end/testcases/return_with_unknown_type_in_context.dart.outline.expect

@@ -0,0 +1,8 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method f(core::List<dynamic> x) → core::bool
+  ;
+static method main() → dynamic
+  ;

pkg/front_end/testcases/return_with_unknown_type_in_context.dart.strong.expect

@@ -0,0 +1,10 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method f(core::List<dynamic> x) → core::bool {
+  return x.{core::Iterable::expand}<dynamic>((dynamic y) → core::Iterable<dynamic> {
+    return y.split(",") as{TypeError} core::Iterable<dynamic>;
+  }).{core::Iterable::any}((dynamic y) → core::bool => y.{core::Object::==}("z"));
+}
+static method main() → dynamic {}

tests/language_2/language_2_kernel.status

@@ -1244,6 +1244,7 @@ async_star_test/03: CompileTimeError # Issue 31402 (Invocation arguments)
 async_star_test/04: CompileTimeError # Issue 31402 (Invocation arguments)
 async_star_test/05: CompileTimeError # Issue 31402 (Invocation arguments)
 async_star_test/none: CompileTimeError # Issue 31402 (Invocation arguments)
+await_test: CompileTimeError # Issue 31541
 bad_named_parameters2_test/01: MissingCompileTimeError
 bad_named_parameters_test/01: MissingCompileTimeError
 bad_named_parameters_test/02: MissingCompileTimeError