[cfe] Handle for-in on type variables in TypeEnvironment.forInElement…

…Type

Closes #39565

Change-Id: I5b4c437879be0a97b306c5cc3579faac917d7e7c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127162
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/front_end/test/static_types/data/for_in.dart

@@ -159,3 +159,23 @@ customStreamIterator(StreamWithCustomIterator stream) async {
     e;
   }
 }
+
+void genericIterable<T extends Iterable<T>>(T x) {
+  /*current: T*/
+  for (var y in
+      /*cfe|dart2js.T*/
+      /*cfe:nnbd.T!*/
+      x) {
+    /*T*/ y;
+  }
+}
+
+void genericStream<T extends Stream<T>>(T x) async {
+  /*current: T*/
+  await for (var y in
+      /*cfe|dart2js.T*/
+      /*cfe:nnbd.T!*/
+      x) {
+    /*T*/ y;
+  }
+}

pkg/kernel/lib/type_environment.dart

@@ -99,6 +99,11 @@ abstract class TypeEnvironment extends SubtypeTester {
   DartType forInElementType(ForInStatement node, DartType iterableType) {
     // TODO(johnniwinther): Update this to use the type of
     //  `iterable.iterator.current` if inference is updated accordingly.
+    while (iterableType is TypeParameterType) {
+      TypeParameterType typeParameterType = iterableType;
+      iterableType =
+          typeParameterType.promotedBound ?? typeParameterType.parameter.bound;
+    }
     if (node.isAsync) {
       InterfaceType type =
           getTypeAsInstanceOf(iterableType, coreTypes.streamClass);