[flow] Part 3 Issue 3658 Avoid writing promotion information for postfix inc/dec expressions.

Postfix increment and decrement expressions should not be saving any promotion information.

An example of where saving the promotion information after the write is unsafe is:
```
class A {
  A operator +(int i) {
    return new B();
  }
}

class B extends A {}

main() {
  A x = A();
  if ((x++) is B) {
    // x should not be B
  }
}
```

This change is only for the analyzer because the CFE does something different (converts the postfix increment/decrement into a let expression).

Bug: dart-lang/language#3658
Change-Id: Ic22f69bf79da66965908ade80bdf70399f0bcaa3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391494
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Kallen Tu <[email protected]>

Author: kallentu

pkg/_fe_analyzer_shared/lib/src/flow_analysis/flow_analysis.dart

@@ -822,6 +822,10 @@ abstract class FlowAnalysis<Node extends Object, Statement extends Node,
   /// state that was saved by [pushSubpattern].
   void popSubpattern();
 
+  /// Call this method when writing to the [variable] with type [writtenType] in
+  /// a postfix increment or decrement operation.
+  void postIncDec(Node node, Variable variable, Type writtenType);
+
   /// Retrieves the type that a property named [propertyName] is promoted to, if
   /// the property is currently promoted.  Otherwise returns `null`.
   ///
@@ -1818,6 +1822,12 @@ class FlowAnalysisDebug<Node extends Object, Statement extends Node,
     _wrap('popSubpattern()', () => _wrapped.popSubpattern());
   }
 
+  @override
+  void postIncDec(Node node, Variable variable, Type writtenType) {
+    _wrap(
+        'postIncDec()', () => _wrapped.postIncDec(node, variable, writtenType));
+  }
+
   @override
   Type? promotedPropertyType(PropertyTarget<Expression> target,
       String propertyName, Object? propertyMember, Type unpromotedType) {
@@ -5296,6 +5306,11 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
     assert(context is _PatternContext<Type>);
   }
 
+  @override
+  void postIncDec(Node node, Variable variable, Type writtenType) {
+    _write(node, variable, writtenType, null, isPostfixIncDec: true);
+  }
+
   @override
   Type? promotedPropertyType(PropertyTarget<Expression> target,
       String propertyName, Object? propertyMember, Type unpromotedType) {
@@ -6274,8 +6289,12 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
 
   /// Common logic for handling writes to variables, whether they occur as part
   /// of an ordinary assignment or a pattern assignment.
+  ///
+  /// If [isPostfixIncDec] is `true`, the [node] is a postfix expression and we
+  /// won't store information about [variable].
   void _write(Node node, Variable variable, Type writtenType,
-      ExpressionInfo<Type>? expressionInfo) {
+      ExpressionInfo<Type>? expressionInfo,
+      {bool isPostfixIncDec = false}) {
     Type unpromotedType = operations.variableType(variable);
     int variableKey = promotionKeyStore.keyForVariable(variable);
     SsaNode<Type> newSsaNode = new SsaNode<Type>(
@@ -6292,7 +6311,7 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
         unpromotedType: unpromotedType);
 
     // Update the type of the variable for looking up the write expression.
-    if (inferenceUpdate4Enabled && node is Expression) {
+    if (inferenceUpdate4Enabled && node is Expression && !isPostfixIncDec) {
       _Reference<Type> reference = _variableReference(
         variableKey,
         unpromotedType,
@@ -6891,6 +6910,9 @@ class _LegacyTypePromotion<Node extends Object, Statement extends Node,
   @override
   void popSubpattern() {}
 
+  @override
+  void postIncDec(Node node, Variable variable, Type writtenType) {}
+
   @override
   Type? promotedPropertyType(PropertyTarget<Expression> target,
           String propertyName, Object? propertyMember, Type unpromotedType) =>

pkg/analyzer/lib/src/dart/resolver/postfix_expression_resolver.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:_fe_analyzer_shared/src/types/shared_type.dart';
+import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ -176,8 +177,17 @@ class PostfixExpressionResolver {
       if (operand is SimpleIdentifier) {
         var element = operand.staticElement;
         if (element is PromotableElement) {
-          _resolver.flowAnalysis.flow
-              ?.write(node, element, SharedTypeView(operatorReturnType), null);
+          if (_resolver.definingLibrary.featureSet
+              .isEnabled(Feature.inference_update_4)) {
+            _resolver.flowAnalysis.flow?.postIncDec(
+              node,
+              element,
+              SharedTypeView(operatorReturnType),
+            );
+          } else {
+            _resolver.flowAnalysis.flow?.write(
+                node, element, SharedTypeView(operatorReturnType), null);
+          }
         }
       }
       node.recordStaticType(receiverType, resolver: _resolver);

pkg/analyzer/test/src/dart/resolution/postfix_expression_test.dart

@@ -2,6 +2,7 @@
 // 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 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/dart/error/syntactic_errors.dart';
 import 'package:analyzer/src/error/codes.dart';
@@ -11,10 +12,42 @@ import 'context_collection_resolution.dart';
 
 main() {
   defineReflectiveSuite(() {
+    defineReflectiveTests(InferenceUpdate4Test);
     defineReflectiveTests(PostfixExpressionResolutionTest);
   });
 }
 
+@reflectiveTest
+class InferenceUpdate4Test extends PubPackageResolutionTest {
+  @override
+  List<String> get experiments {
+    return [
+      ...super.experiments,
+      Feature.inference_update_4.enableString,
+    ];
+  }
+
+  test_isExpression_notPromoted() async {
+    await assertNoErrorsInCode('''
+f() {
+  num x = 2;
+  if ((x++) is int) {
+    x;
+  }
+}
+''');
+    var nonPromotedIdentifier = findNode.simple('x;');
+    assertResolvedNodeText(nonPromotedIdentifier, r'''
+SimpleIdentifier
+  token: x
+  staticElement: x@12
+  element: x@12
+  staticType: num
+''');
+    assertType(nonPromotedIdentifier, 'num');
+  }
+}
+
 @reflectiveTest
 class PostfixExpressionResolutionTest extends PubPackageResolutionTest {
   test_dec_simpleIdentifier_parameter_int() async {