Don't promote this inside extension methods.

Bug: https://github.com/dart-lang/sdk/issues/44652
Change-Id: I05e5acfbff1d498fa3ad513d956510909ec7d24f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/178920
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/_fe_analyzer_shared/test/flow_analysis/type_promotion/data/this.dart

@@ -0,0 +1,24 @@
+// 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.
+
+// This test verifies that `this` cannot be promoted, even if it appears in an
+// extension method.
+
+class C {
+  void insideClass() {
+    if (this is D) {
+      this;
+    }
+  }
+}
+
+class D extends C {}
+
+extension on C {
+  void insideExtension() {
+    if (this is D) {
+      this;
+    }
+  }
+}

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

@@ -5,6 +5,7 @@
 import 'dart:core' hide MapEntry;
 
 import 'package:_fe_analyzer_shared/src/util/link.dart';
+import 'package:front_end/src/api_prototype/lowering_predicates.dart';
 import 'package:kernel/ast.dart';
 import 'package:kernel/src/legacy_erasure.dart';
 import 'package:kernel/type_algebra.dart' show Substitution;
@@ -6567,8 +6568,12 @@ class InferenceVisitor
         if (nonNullableType != variable.type) {
           promotedType = nonNullableType;
         }
-      } else if (!variable.isLocalFunction) {
+      } else if (variable.isLocalFunction) {
         // Don't promote local functions.
+      } else if (isExtensionThis(variable)) {
+        // Don't promote the synthetic variable `#this` that we use to represent
+        // `this` inside extension methods.
+      } else {
         promotedType = inferrer.flowAnalysis.variableRead(node, variable);
       }
     } else {

tests/language/extension_methods/static_extension_this_not_promoted_error_test.dart

@@ -0,0 +1,55 @@
+// 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.
+
+// This test verifies that attempts to promote the type of `this` inside an
+// extension method have no effect.
+
+void f(dynamic d) {}
+
+class C {
+  int? cProp;
+}
+
+extension on C? {
+  void testCQuestion() {
+    if (this != null) {
+      f(this.cProp);
+      //^^^^
+      // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
+      //     ^
+      // [cfe] Property 'cProp' cannot be accessed on 'C?' because it is potentially null.
+      f(cProp);
+      //^^^^^
+      // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
+      // [cfe] Property 'cProp' cannot be accessed on 'C?' because it is potentially null.
+    }
+  }
+}
+
+class D extends C {
+  int? dProp;
+}
+
+extension on C {
+  void testC() {
+    if (this is D) {
+      f(this.dProp);
+      //     ^^^^^
+      // [analyzer] COMPILE_TIME_ERROR.UNDEFINED_GETTER
+      // [cfe] The getter 'dProp' isn't defined for the class 'C'.
+      f(dProp);
+      //^^^^^
+      // [analyzer] COMPILE_TIME_ERROR.UNDEFINED_IDENTIFIER
+      // [cfe] The getter 'dProp' isn't defined for the class 'C'.
+    }
+  }
+}
+
+main() {
+  C().testCQuestion();
+  C().testC();
+  D().testCQuestion();
+  D().testC();
+  (null as C?).testCQuestion();
+}