Broaden write-only property check to cover union types and non-ArrayAccess objects

- Simplify condition to `!$propertyType->isObject()->no()` so that any type
  that *might* be an object triggers a PropertyRead when appending via `[]`
- Add test cases for Threaded (non-ArrayAccess object), ArrayObject|array
  union, and ArrayObject|string union

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Node/ClassStatementsGatherer.php

@@ -206,10 +206,7 @@ private function gatherNodes(Node $node, Scope $scope): void
 			$assignedExpr = $node->getAssignedExpr();
 			if ($assignedExpr instanceof SetOffsetValueTypeExpr || $assignedExpr instanceof SetExistingOffsetValueTypeExpr) {
 				$propertyType = $scope->getType($propertyFetch);
-				if (
-					!$propertyType->isArray()->yes()
-					&& $propertyType->isObject()->yes()
-				) {
+				if (!$propertyType->isObject()->no()) {
 					$this->propertyUsages[] = new PropertyRead($propertyFetch, $scope);
 				}
 			}

tests/PHPStan/Rules/DeadCode/data/bug-6777.php

@@ -13,3 +13,52 @@ public function send(string $s) : void{
 		$this->array[] = $s;
 	}
 }
+
+class WithThreaded
+{
+	private \Threaded $collection;
+
+	public function __construct()
+	{
+		$this->collection = new \Threaded();
+	}
+
+	public function add(string $s): void
+	{
+		$this->collection[] = $s;
+	}
+}
+
+class WithUnionObjectArray
+{
+	/** @var \ArrayObject<int, string>|array<int, string> */
+	private \ArrayObject|array $collection;
+
+	/** @param \ArrayObject<int, string>|array<int, string> $collection */
+	public function __construct(\ArrayObject|array $collection)
+	{
+		$this->collection = $collection;
+	}
+
+	public function add(string $s): void
+	{
+		$this->collection[] = $s;
+	}
+}
+
+class WithUnionObjectString
+{
+	/** @var \ArrayObject<int, string>|string */
+	private \ArrayObject|string $collection;
+
+	/** @param \ArrayObject<int, string>|string $collection */
+	public function __construct(\ArrayObject|string $collection)
+	{
+		$this->collection = $collection;
+	}
+
+	public function add(string $s): void
+	{
+		$this->collection[] = $s;
+	}
+}