Flow analysis: Fix "why not promoted" functionality for implicit this

Bug: #44898
Change-Id: Ief6014c34aef329e5f408916f5d8ffe14a62effd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/187940
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

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

@@ -935,6 +935,8 @@ abstract class FlowAnalysis<Node extends Object, Statement extends Node,
   /// promotion, to retrieve information about why [target] was not promoted.
   /// This call must be made right after visiting [target].
   ///
+  /// If [target] is `null` it is assumed to be an implicit reference to `this`.
+  ///
   /// The returned value is a map whose keys are types that the user might have
   /// been expecting the target to be promoted to, and whose values are reasons
   /// why the corresponding promotion did not occur.  The caller is expected to
@@ -943,7 +945,7 @@ abstract class FlowAnalysis<Node extends Object, Statement extends Node,
   /// occurs due to the target having a nullable type, the caller should report
   /// a non-promotion reason associated with non-promotion to a non-nullable
   /// type).
-  Map<Type, NonPromotionReason> whyNotPromoted(Expression target);
+  Map<Type, NonPromotionReason> whyNotPromoted(Expression? target);
 
   /// Register write of the given [variable] in the current state.
   /// [writtenType] should be the type of the value that was written.
@@ -1460,7 +1462,7 @@ class FlowAnalysisDebug<Node extends Object, Statement extends Node,
   }
 
   @override
-  Map<Type, NonPromotionReason> whyNotPromoted(Expression target) {
+  Map<Type, NonPromotionReason> whyNotPromoted(Expression? target) {
     return _wrap(
         'whyNotPromoted($target)', () => _wrapped.whyNotPromoted(target),
         isQuery: true);
@@ -4257,13 +4259,16 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
   }
 
   @override
-  Map<Type, NonPromotionReason> whyNotPromoted(Expression target) {
-    if (identical(target, _expressionWithReference)) {
-      Reference<Variable, Type>? reference = _expressionReference;
-      if (reference != null) {
-        return reference.getNonPromotionReasons(
-            _current.variableInfo, typeOperations);
-      }
+  Map<Type, NonPromotionReason> whyNotPromoted(Expression? target) {
+    Reference<Variable, Type>? reference;
+    if (target == null) {
+      reference = new _ThisReference<Variable, Type>();
+    } else if (identical(target, _expressionWithReference)) {
+      reference = _expressionReference;
+    }
+    if (reference != null) {
+      return reference.getNonPromotionReasons(
+          _current.variableInfo, typeOperations);
     }
     return {};
   }
@@ -4850,7 +4855,7 @@ class _LegacyTypePromotion<Node extends Object, Statement extends Node,
   void whileStatement_end() {}
 
   @override
-  Map<Type, NonPromotionReason> whyNotPromoted(Expression target) {
+  Map<Type, NonPromotionReason> whyNotPromoted(Expression? target) {
     return {};
   }
 

pkg/_fe_analyzer_shared/test/flow_analysis/flow_analysis_mini_ast.dart

@@ -144,6 +144,10 @@ Statement if_(Expression condition, List<Statement> ifTrue,
         [List<Statement>? ifFalse]) =>
     new _If(condition, ifTrue, ifFalse);
 
+Statement implicitThis_whyNotPromoted(
+        void Function(Map<Type, NonPromotionReason>) callback) =>
+    new _WhyNotPromoted_ImplicitThis(callback);
+
 Statement labeled(Statement body) => new _LabeledStatement(body);
 
 Statement localFunction(List<Statement> body) => _LocalFunction(body);
@@ -1760,6 +1764,30 @@ class _WhyNotPromoted extends Expression {
   }
 }
 
+class _WhyNotPromoted_ImplicitThis extends Statement {
+  final void Function(Map<Type, NonPromotionReason>) callback;
+
+  _WhyNotPromoted_ImplicitThis(this.callback) : super._();
+
+  @override
+  String toString() => 'implicit this (whyNotPromoted)';
+
+  @override
+  void _preVisit(AssignedVariables<Node, Var> assignedVariables) {}
+
+  @override
+  void _visit(
+      Harness h, FlowAnalysis<Node, Statement, Expression, Var, Type> flow) {
+    assert(!Type._allowComparisons);
+    Type._allowComparisons = true;
+    try {
+      callback(flow.whyNotPromoted(null));
+    } finally {
+      Type._allowComparisons = false;
+    }
+  }
+}
+
 class _WrappedExpression extends Expression {
   final Statement? before;
   final Expression expr;

pkg/_fe_analyzer_shared/test/flow_analysis/flow_analysis_test.dart

@@ -5683,6 +5683,40 @@ main() {
         ]);
       });
     });
+
+    group('because this', () {
+      test('explicit', () {
+        var h = Harness()
+          ..addSubtype('D', 'Object?', true)
+          ..addFactor('Object?', 'D', 'Object?');
+        h.run([
+          if_(this_('C').isNot('D'), [
+            return_(),
+          ]),
+          this_('C').whyNotPromoted((reasons) {
+            expect(reasons.keys, unorderedEquals([Type('D')]));
+            var nonPromotionReason = reasons.values.single;
+            expect(nonPromotionReason, TypeMatcher<ThisNotPromoted>());
+          }).stmt,
+        ]);
+      });
+
+      test('implicit', () {
+        var h = Harness()
+          ..addSubtype('D', 'Object?', true)
+          ..addFactor('Object?', 'D', 'Object?');
+        h.run([
+          if_(this_('C').isNot('D'), [
+            return_(),
+          ]),
+          implicitThis_whyNotPromoted((reasons) {
+            expect(reasons.keys, unorderedEquals([Type('D')]));
+            var nonPromotionReason = reasons.values.single;
+            expect(nonPromotionReason, TypeMatcher<ThisNotPromoted>());
+          }),
+        ]);
+      });
+    });
   });
 }
 

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

@@ -0,0 +1,22 @@
+// 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 validates integration of "why not promoted" when the user tries to
+// promote `this`.
+
+// TODO(paulberry): once we support adding "why not promoted" information to
+// errors that aren't related to null safety, test references to `this` in
+// classes and mixins.
+
+extension on int? {
+  extension_explicit_this() {
+    if (this == null) return;
+    this. /*notPromoted(thisNotPromoted)*/ isEven;
+  }
+
+  extension_implicit_this() {
+    if (this == null) return;
+    /*notPromoted(thisNotPromoted)*/ isEven;
+  }
+}

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

@@ -516,29 +516,27 @@ class ResolverVisitor extends ScopedVisitor {
   List<DiagnosticMessage> computeWhyNotPromotedMessages(
       Expression? receiver, SyntacticEntity errorEntity) {
     List<DiagnosticMessage> messages = [];
-    if (receiver != null) {
-      var whyNotPromoted = flowAnalysis?.flow?.whyNotPromoted(receiver);
-      if (whyNotPromoted != null) {
-        for (var entry in whyNotPromoted.entries) {
-          var whyNotPromotedVisitor = _WhyNotPromotedVisitor(
-              source, receiver, flowAnalysis!.dataForTesting);
-          if (typeSystem.isPotentiallyNullable(entry.key)) continue;
-          var message = entry.value.accept(whyNotPromotedVisitor);
-          if (message != null) {
-            if (flowAnalysis!.dataForTesting != null) {
-              var nonPromotionReasonText = entry.value.shortName;
-              if (whyNotPromotedVisitor.propertyReference != null) {
-                var id =
-                    computeMemberId(whyNotPromotedVisitor.propertyReference!);
-                nonPromotionReasonText += '($id)';
-              }
-              flowAnalysis!.dataForTesting!.nonPromotionReasons[errorEntity] =
-                  nonPromotionReasonText;
+    var whyNotPromoted = flowAnalysis?.flow?.whyNotPromoted(receiver);
+    if (whyNotPromoted != null) {
+      for (var entry in whyNotPromoted.entries) {
+        var whyNotPromotedVisitor = _WhyNotPromotedVisitor(
+            source, receiver, errorEntity, flowAnalysis!.dataForTesting);
+        if (typeSystem.isPotentiallyNullable(entry.key)) continue;
+        var message = entry.value.accept(whyNotPromotedVisitor);
+        if (message != null) {
+          if (flowAnalysis!.dataForTesting != null) {
+            var nonPromotionReasonText = entry.value.shortName;
+            if (whyNotPromotedVisitor.propertyReference != null) {
+              var id =
+                  computeMemberId(whyNotPromotedVisitor.propertyReference!);
+              nonPromotionReasonText += '($id)';
             }
-            messages = [message];
+            flowAnalysis!.dataForTesting!.nonPromotionReasons[errorEntity] =
+                nonPromotionReasonText;
           }
-          break;
+          messages = [message];
         }
+        break;
       }
     }
     return messages;
@@ -3343,13 +3341,16 @@ class _WhyNotPromotedVisitor
             PromotableElement> {
   final Source source;
 
-  final Expression _receiver;
+  final Expression? _receiver;
+
+  final SyntacticEntity _errorEntity;
 
   final FlowAnalysisDataForTesting? _dataForTesting;
 
   PropertyAccessorElement? propertyReference;
 
-  _WhyNotPromotedVisitor(this.source, this._receiver, this._dataForTesting);
+  _WhyNotPromotedVisitor(
+      this.source, this._receiver, this._errorEntity, this._dataForTesting);
 
   @override
   DiagnosticMessage? visitDemoteViaExplicitWrite(
@@ -3420,8 +3421,8 @@ class _WhyNotPromotedVisitor
     return DiagnosticMessageImpl(
         filePath: source.fullName,
         message: "'this' can't be promoted.",
-        offset: _receiver.offset,
-        length: _receiver.length);
+        offset: _errorEntity.offset,
+        length: _errorEntity.length);
   }
 
   DiagnosticMessageImpl _contextMessageForProperty(