[cfe] Make the context for await expressions consistent with analyzer.

Fixes https://github.com/dart-lang/language/issues/3649.
Fixes https://github.com/dart-lang/sdk/issues/55418.

Bug: https://github.com/dart-lang/language/issues/3649
Change-Id: Ifb2fe47bb343a357e2338843775f140c01bd8a88
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/361302
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

CHANGELOG.md

@@ -1,5 +1,14 @@
 ## 3.5.0
 
+### Language
+
+- **Breaking Change** [#55418][]: The context used by the compiler to perform
+  type inference on the operand of an `await` expression has been changed to
+  match the behavior of the analyzer. This change is not expected to make any
+  difference in practice.
+
+[#55418]: https://github.com/dart-lang/sdk/issues/55418
+
 ### Dart Runtime
 
 - The Dart VM only executes sound null safe code, running of unsound null

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

@@ -857,9 +857,10 @@ class InferenceVisitorImpl extends InferenceVisitorBase
   @override
   ExpressionInferenceResult visitAwaitExpression(
       AwaitExpression node, DartType typeContext) {
-    if (!typeSchemaEnvironment.isEmptyContext(typeContext)) {
-      typeContext = wrapFutureOrType(typeContext);
+    if (typeContext is DynamicType) {
+      typeContext = const UnknownType();
     }
+    typeContext = wrapFutureOrType(typeContext);
     ExpressionInferenceResult operandResult = inferExpression(
         node.operand, typeContext,
         isVoidAllowed: !isNonNullableByDefault);

tests/language/await/await_with_dynamic_context_test.dart

@@ -0,0 +1,47 @@
+// Copyright (c) 2024, 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.
+
+// Verify that if an `await` expression is analyzed in a `dynamic` context, it
+// supplies a context of `FutureOr<_>` to its operand, rather than `dynamic`
+// (which was the front end behavior prior to fixing
+// https://github.com/dart-lang/language/issues/3649).
+
+// Contexts of `FutureOr<_>` and `dynamic` are difficult to distinguish. This
+// test distinguishes them by awaiting the expression `id(((null as Future<B>?)
+// ??  (Future.value(C())..expectStaticType<Exactly<Future<C>>>())))`, where `C`
+// is a class that extends `B`, and `id` is a generic identity function having
+// type `T Function<T>(T)`.
+//
+// This works because, if it is analyzed in context `Future<_>`, then:
+// - The call to `id` is analyzed in context `Future<_>`.
+// - Therefore the `??` expression is analyzed in context `Future<_>`.
+// - Therefore the RHS of `??` is analyzed in context `Future<_>`.
+// - Therefore downwards inference doesn't impose any constraint on the type
+//   parameter of `Future.value(C())`.
+// - Therefore `Future.value(C())` gets its type parameter from upwards
+//   inference.
+// - So it receives a type parameter of `C`.
+// - So the static type of `Future.value(C())` is `Future<C>`.
+//
+// Whereas if it is analyzed in context `dynamic`, then:
+// - The call to `id` is analyzed in context `dynamic`.
+// - Therefore the `??` expression is analyzed in context `_`.
+// - Therefore the static type of the LHS of `??` is used as the context for the
+//   RHS. That is, the RHS is analyzed in context `Future<B>?`.
+// - Therefore downwards inference constraints the type parameter of
+//   `Future.value(C())` to `B`.
+// - So the static type of `Future.value(C())` is `Future<B>`.
+
+import '../static_type_helper.dart';
+
+class B {}
+
+class C extends B {}
+
+T id<T>(T t) => t;
+
+main() async {
+  dynamic x = await id(((null as Future<B>?) ??
+      (Future.value(C())..expectStaticType<Exactly<Future<C>>>())));
+}