Fix #6777: Bogus write-only error when appending to an array-like object in private class property (#5102)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

src/Node/ClassStatementsGatherer.php

@@ -13,6 +13,8 @@
 use PhpParser\Node\Identifier;
 use PHPStan\Analyser\Scope;
 use PHPStan\Node\Constant\ClassConstantFetch;
+use PHPStan\Node\Expr\SetExistingOffsetValueTypeExpr;
+use PHPStan\Node\Expr\SetOffsetValueTypeExpr;
 use PHPStan\Node\Property\PropertyAssign;
 use PHPStan\Node\Property\PropertyRead;
 use PHPStan\Node\Property\PropertyWrite;
@@ -200,7 +202,15 @@ private function gatherNodes(Node $node, Scope $scope): void
 			return;
 		}
 		if ($node instanceof PropertyAssignNode) {
-			$this->propertyUsages[] = new PropertyWrite($node->getPropertyFetch(), $scope, false);
+			$propertyFetch = $node->getPropertyFetch();
+			$assignedExpr = $node->getAssignedExpr();
+			if ($assignedExpr instanceof SetOffsetValueTypeExpr || $assignedExpr instanceof SetExistingOffsetValueTypeExpr) {
+				$propertyType = $scope->getType($propertyFetch);
+				if (!$propertyType->isObject()->no()) {
+					$this->propertyUsages[] = new PropertyRead($propertyFetch, $scope);
+				}
+			}
+			$this->propertyUsages[] = new PropertyWrite($propertyFetch, $scope, false);
 			$this->propertyAssigns[] = new PropertyAssign($node, $scope);
 			return;
 		}

tests/PHPStan/Rules/DeadCode/UnusedPrivatePropertyRuleTest.php

@@ -130,16 +130,6 @@ public function testRule(): void
 				191,
 				$tip,
 			],
-			[
-				'Property UnusedPrivateProperty\WriteToCollection::$collection1 is never read, only written.',
-				221,
-				$tip,
-			],
-			[
-				'Property UnusedPrivateProperty\WriteToCollection::$collection2 is never read, only written.',
-				224,
-				$tip,
-			],
 		]);
 		$this->analyse([__DIR__ . '/data/TestExtension.php'], [
 			[
@@ -410,4 +400,13 @@ public function testBug9213(): void
 		$this->analyse([__DIR__ . '/data/bug-9213.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.0')]
+	public function testBug6777(): void
+	{
+		$this->alwaysWrittenTags = [];
+		$this->alwaysReadTags = [];
+
+		$this->analyse([__DIR__ . '/data/bug-6777.php'], []);
+	}
+
 }

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

@@ -0,0 +1,74 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug6777;
+
+class HelloWorld
+{
+	/** @param \ArrayObject<int, string> $array */
+	public function __construct(private \ArrayObject $array){}
+
+	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 WithAssignByReference
+{
+	/** @param \ArrayObject<int, string> $array */
+	public function __construct(private \ArrayObject $array){}
+
+	public function send(string &$s) : void{
+		$this->array[] =& $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;
+	}
+}