[cfe] Eliminate UnknownType in async type downcasts

Cherry Pick CL for #42615

Closes #43013.

Bug: #42615
Change-Id: Ieb289acee04b3868304d320d3c2231fd6eb94dd6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154322
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>

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

@@ -522,7 +522,8 @@ class _AsyncClosureContext implements ClosureContext {
  statement.expression = inferrer.ensureAssignable(
  futureValueType, expressionType, statement.expression,
  fileOffset: statement.expression.fileOffset,
- runtimeCheckedType: _returnContext,
+ runtimeCheckedType:
+ inferrer.computeGreatestClosure2(_returnContext),
  isVoidAllowed: false)
  ..parent = statement;
  }
@@ -559,7 +560,8 @@ class _AsyncClosureContext implements ClosureContext {
  statement.expression,
  fileOffset: statement.fileOffset,
  isVoidAllowed: true,
- runtimeCheckedType: _returnContext);
+ runtimeCheckedType:
+ inferrer.computeGreatestClosure(_returnContext));
  statement.expression = expression..parent = statement;
  }
 

pkg/front_end/testcases/general/issue42615.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2020, 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';
+
+class Class<T> {
+ Class({FutureOr<List<T>> Function() a});
+}
+
+dynamic method() => null;
+
+main() {
+ Class(a: () async => method());
+}

pkg/front_end/testcases/general/issue42615.dart.outline.expect

@@ -0,0 +1,24 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+import "dart:async";
+
+class Class<T extends core::Object* = dynamic> extends core::Object {
+ constructor •({() →* FutureOr<core::List<self::Class::T*>*>* a}) → self::Class<self::Class::T*>*
+ ;
+ abstract member-signature get _identityHashCode() → core::int*;
+ abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+ abstract member-signature operator ==(dynamic other) → core::bool*;
+ abstract member-signature get hashCode() → core::int*;
+ abstract member-signature method toString() → core::String*;
+ abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+ abstract member-signature get runtimeType() → core::Type*;
+}
+static method method() → dynamic
+ ;
+static method main() → dynamic
+ ;

pkg/front_end/testcases/general/issue42615.dart.strong.expect

@@ -0,0 +1,30 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+import "dart:async";
+
+class Class<T extends core::Object* = dynamic> extends core::Object {
+ constructor •({() →* FutureOr<core::List<self::Class::T*>*>* a = #C1}) → self::Class<self::Class::T*>*
+ : super core::Object::•()
+ ;
+ abstract member-signature get _identityHashCode() → core::int*;
+ abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+ abstract member-signature operator ==(dynamic other) → core::bool*;
+ abstract member-signature get hashCode() → core::int*;
+ abstract member-signature method toString() → core::String*;
+ abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+ abstract member-signature get runtimeType() → core::Type*;
+}
+static method method() → dynamic
+ return null;
+static method main() → dynamic {
+ new self::Class::•<dynamic>(a: () → FutureOr<core::List<dynamic>*>* async => self::method() as{TypeError} FutureOr<core::List<dynamic>*>*);
+}
+
+constants {
+ #C1 = null
+}

pkg/front_end/testcases/general/issue42615.dart.strong.transformed.expect

@@ -0,0 +1,57 @@
+library;
+import self as self;
+import "dart:core" as core;
+import "dart:async" as asy;
+
+import "dart:async";
+
+class Class<T extends core::Object* = dynamic> extends core::Object {
+ constructor •({() →* FutureOr<core::List<self::Class::T*>*>* a = #C1}) → self::Class<self::Class::T*>*
+ : super core::Object::•()
+ ;
+ abstract member-signature get _identityHashCode() → core::int*;
+ abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+ abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+ abstract member-signature operator ==(dynamic other) → core::bool*;
+ abstract member-signature get hashCode() → core::int*;
+ abstract member-signature method toString() → core::String*;
+ abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+ abstract member-signature get runtimeType() → core::Type*;
+}
+static method method() → dynamic
+ return null;
+static method main() → dynamic {
+ new self::Class::•<dynamic>(a: () → FutureOr<core::List<dynamic>*>* /* originally async */ {
+ final asy::_AsyncAwaitCompleter<core::List<dynamic>*>* :async_completer = new asy::_AsyncAwaitCompleter::•<core::List<dynamic>*>();
+ FutureOr<core::List<dynamic>*>* :return_value;
+ dynamic :async_stack_trace;
+ (dynamic) →* dynamic :async_op_then;
+ (core::Object*, core::StackTrace*) →* dynamic :async_op_error;
+ core::int* :await_jump_var = 0;
+ dynamic :await_ctx_var;
+ function :async_op([dynamic :result, dynamic :exception, dynamic :stack_trace]) → dynamic yielding 
+ try {
+ #L1:
+ {
+ :return_value = self::method() as{TypeError} FutureOr<core::List<dynamic>*>*;
+ break #L1;
+ }
+ asy::_completeOnAsyncReturn(:async_completer, :return_value);
+ return;
+ }
+ on dynamic catch(dynamic exception, core::StackTrace* stack_trace) {
+ :async_completer.{asy::Completer::completeError}(exception, stack_trace);
+ }
+ :async_stack_trace = asy::_asyncStackTraceHelper(:async_op);
+ :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+ :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+ :async_completer.{asy::_AsyncAwaitCompleter::start}(:async_op);
+ return :async_completer.{asy::Completer::future};
+ });
+}
+
+constants {
+ #C1 = null
+}