Don't report already assigned errors when setting or unsetting an offset on array access objects set as class properties.

src/Node/ClassPropertiesNode.php

@@ -2,6 +2,7 @@
 
 namespace PHPStan\Node;
 
+use ArrayAccess;
 use Override;
 use PhpParser\Node;
 use PhpParser\Node\Expr\Array_;
@@ -13,6 +14,8 @@
 use PhpParser\NodeAbstract;
 use PHPStan\Analyser\Scope;
 use PHPStan\Node\Expr\PropertyInitializationExpr;
+use PHPStan\Node\Expr\SetOffsetValueTypeExpr;
+use PHPStan\Node\Expr\UnsetOffsetExpr;
 use PHPStan\Node\Method\MethodCall;
 use PHPStan\Node\Property\PropertyAssign;
 use PHPStan\Node\Property\PropertyRead;
@@ -22,6 +25,7 @@
 use PHPStan\Rules\Properties\ReadWritePropertiesExtensionProvider;
 use PHPStan\TrinaryLogic;
 use PHPStan\Type\NeverType;
+use PHPStan\Type\ObjectType;
 use PHPStan\Type\TypeUtils;
 use function array_diff_key;
 use function array_key_exists;
@@ -211,6 +215,19 @@
 
 			if ($usage instanceof PropertyWrite) {
 				if (array_key_exists($propertyName, $initializedPropertiesMap)) {
+					$originalNode = $usage->getOriginalNode();
+
+					if ($originalNode instanceof PropertyAssignNode) {
+						$assignedExpr = $originalNode->getAssignedExpr();
+
+						if (
+							($assignedExpr instanceof SetOffsetValueTypeExpr || $assignedExpr instanceof UnsetOffsetExpr)
+							&& (new ObjectType(ArrayAccess::class))->isSuperTypeOf($scope->getType($assignedExpr->getVar()))->yes()
+						) {
+							continue;
+						}
+					}
+
 					$hasInitialization = $initializedPropertiesMap[$propertyName]->or($usageScope->hasExpressionType(new PropertyInitializationExpr($propertyName)));
 					if (
 						!$hasInitialization->no()

src/Node/ClassStatementsGatherer.php

@@ -150,6 +150,7 @@ private function gatherNodes(Node $node, Scope $scope): void
 					new PropertyFetch(new Expr\Variable('this'), new Identifier($node->getName())),
 					$scope,
 					true,
+					$node,
 				);
 			}
 			return;
@@ -194,7 +195,7 @@ private function gatherNodes(Node $node, Scope $scope): void
 			return;
 		}
 		if ($node instanceof PropertyAssignNode) {
-			$this->propertyUsages[] = new PropertyWrite($node->getPropertyFetch(), $scope, false);
+			$this->propertyUsages[] = new PropertyWrite($node->getPropertyFetch(), $scope, false, $node);
 			$this->propertyAssigns[] = new PropertyAssign($node, $scope);
 			return;
 		}
@@ -212,7 +213,7 @@ private function gatherNodes(Node $node, Scope $scope): void
 			}
 
 			$this->propertyUsages[] = new PropertyRead($node->expr, $scope);
-			$this->propertyUsages[] = new PropertyWrite($node->expr, $scope, false);
+			$this->propertyUsages[] = new PropertyWrite($node->expr, $scope, false, $node);
 			return;
 		}
 		if ($node instanceof FunctionCallableNode) {

src/Node/Property/PropertyWrite.php

@@ -2,17 +2,20 @@
 
 namespace PHPStan\Node\Property;
 
+use PhpParser\Node\Expr\AssignRef;
 use PhpParser\Node\Expr\PropertyFetch;
 use PhpParser\Node\Expr\StaticPropertyFetch;
 use PHPStan\Analyser\Scope;
+use PHPStan\Node\ClassPropertyNode;
+use PHPStan\Node\PropertyAssignNode;
 
 /**
  * @api
  */
 final class PropertyWrite
 {
 
-	public function __construct(private PropertyFetch|StaticPropertyFetch $fetch, private Scope $scope, private bool $promotedPropertyWrite)
+	public function __construct(private PropertyFetch|StaticPropertyFetch $fetch, private Scope $scope, private bool $promotedPropertyWrite, private ClassPropertyNode|PropertyAssignNode|AssignRef|null $originalNode = null)
 	{
 	}
 
@@ -34,4 +37,9 @@ public function isPromotedPropertyWrite(): bool
 		return $this->promotedPropertyWrite;
 	}
 
+	public function getOriginalNode(): ClassPropertyNode|PropertyAssignNode|AssignRef|null
+	{
+		return $this->originalNode;
+	}
+
 }

tests/PHPStan/Rules/Properties/MissingReadOnlyPropertyAssignRuleTest.php

@@ -46,4 +46,15 @@ public static function getAdditionalConfigFiles(): array
 		);
 	}
 
+	#[RequiresPhp('>= 8.1')]
+	public function testBug13856(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13856.php'], [
+			[
+				'Readonly property Bug13856\foo2::$store is already assigned.',
+				28,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Properties/data/bug-13856.php

@@ -0,0 +1,30 @@
+<?php // lint >= 8.1
+
+namespace Bug13856;
+
+use SplObjectStorage;
+
+class foo
+{
+	/** @var SplObjectStorage<object, mixed> */
+    private readonly SplObjectStorage $store;
+
+    public function __construct()
+    {
+        $this->store = new SplObjectStorage();
+        $this->store[(object) ['foo' => 'bar']] = true;
+        unset($this->store[(object) ['foo' => 'bar']]);
+    }
+}
+
+class foo2
+{
+    /** @var array<int, bool> */
+    private readonly array $store;
+
+    public function __construct()
+    {
+        $this->store[1] = true;
+        $this->store[2] = false;
+    }
+}