lint: simplify parameter reassignment check to avoid false negatives

R=helinx@google.com

Bug: https://github.com/dart-lang/sdk/issues/61175
Change-Id: I13e0953406c6f35f92dba9a1860f40323eda30d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/460240
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Auto-Submit: Nishtha Jain <jnishtha305@gmail.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

pkg/linter/lib/src/rules/parameter_assignments.dart

@@ -92,17 +92,22 @@ class _DeclarationVisitor extends RecursiveAstVisitor<void> {
   }
 
   @override
-  visitAssignmentExpression(AssignmentExpression node) {
+  void visitAssignmentExpression(AssignmentExpression node) {
+    if (!_isFormalParameterReassigned(parameter, node)) return;
+
     if (paramIsNotNullByDefault) {
-      if (_isFormalParameterReassigned(parameter, node)) {
-        reportLint(node);
-      }
-    } else if (paramDefaultsToNull) {
-      if (_isFormalParameterReassigned(parameter, node)) {
+      reportLint(node);
+      return;
+    }
+
+    if (paramDefaultsToNull) {
+      if (node.operator.type.lexeme == '??=') {
         if (hasBeenAssigned) {
           reportLint(node);
         }
         hasBeenAssigned = true;
+      } else {
+        reportLint(node);
       }
     }
 

pkg/linter/test/rules/parameter_assignments_test.dart

@@ -17,6 +17,21 @@ class ParameterAssignmentsTest extends LintRuleTest {
   @override
   String get lintRule => LintNames.parameter_assignments;
 
+  test_assignment_inIfElseBranches() async {
+    await assertDiagnostics(
+      r'''
+void foo({String? value}) {
+  if (1 == 1) {
+    value = ' $value';
+  } else {
+    value = ' $value';
+  }
+}
+''',
+      [lint(48, 17), lint(82, 17)],
+    );
+  }
+
   test_assignment_nullableParameter() async {
     await assertDiagnostics(
       r'''