[linter] Fixes unnecessary_this false negative inside if case

Fixes: #60514
Change-Id: I934f22461b395177eca0046b0bf8797067ccbe46
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/421961
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Auto-Submit: Felipe Morschel <[email protected]>

Author: FMorschel

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -2281,7 +2281,9 @@ abstract final class CaseClause implements AstNode {
   GuardedPattern get guardedPattern;
 }
 
-final class CaseClauseImpl extends AstNodeImpl implements CaseClause {
+final class CaseClauseImpl extends AstNodeImpl
+    with AstNodeWithNameScopeMixin
+    implements CaseClause {
   @override
   final Token caseKeyword;
 

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

@@ -5236,6 +5236,11 @@ class ScopeResolverVisitor extends UnifyingAstVisitor<void> {
     if (caseClause != null) {
       var guardedPattern = caseClause.guardedPattern;
       _withNameScope(() {
+        caseClause.nameScope = nameScope;
+        var variables = guardedPattern.variables;
+        for (var variable in variables.values) {
+          _define(variable);
+        }
         guardedPattern.accept(this);
         node.ifTrue.accept(this);
       });

pkg/linter/test/rules/unnecessary_this_test.dart

@@ -166,6 +166,18 @@ class A {
     );
   }
 
+  test_shadowInIfCaseClause() async {
+    await assertNoDiagnostics(r'''
+class A {
+  int? value;
+
+  void m(A a) {
+    if (a case A(:var value) when value != this.value) {}
+  }
+}
+''');
+  }
+
   test_shadowInMethodBody() async {
     await assertNoDiagnostics(r'''
 class C {