Allow write access to offsets of readonly ArrayAccess properties

In case a readonly property is an object which implements ArrayAccess,
offet accesses to said property are turned into method calls on PHP's
side and are allowed even on read-only properties.

this fixes phpstan/phpstan#8929

Author: pilif

src/Rules/Properties/ReadOnlyPropertyAssignRule.php

@@ -2,6 +2,7 @@
 
 namespace PHPStan\Rules\Properties;
 
+use ArrayAccess;
 use PhpParser\Node;
 use PHPStan\Analyser\Scope;
 use PHPStan\Node\PropertyAssignNode;
@@ -10,6 +11,7 @@
 use PHPStan\Rules\Rule;
 use PHPStan\Rules\RuleErrorBuilder;
 use PHPStan\ShouldNotHappenException;
+use PHPStan\Type\ObjectType;
 use PHPStan\Type\TypeUtils;
 use function in_array;
 use function sprintf;
@@ -89,6 +91,10 @@ public function processNode(Node $node, Scope $scope): array
 				continue;
 			}
 
+			if ((new ObjectType(ArrayAccess::class))->isSuperTypeOf($propertyReflection->getNativeType())->yes()) {
+				continue;
+			}
+
 			$errors[] = RuleErrorBuilder::message(sprintf('Readonly property %s::$%s is assigned outside of the constructor.', $declaringClass->getDisplayName(), $propertyReflection->getName()))
 				->identifier('property.readOnlyAssignNotInConstructor')
 				->build();

tests/PHPStan/Rules/Properties/ReadOnlyPropertyAssignRuleTest.php

@@ -168,4 +168,13 @@ public function testBug6773(): void
 		]);
 	}
 
+	public function testBug8929(): void
+	{
+		if (PHP_VERSION_ID < 80100) {
+			self::markTestSkipped('Test requires PHP 8.1');
+		}
+
+		$this->analyse([__DIR__ . '/data/bug-8929.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Properties/data/bug-8929.php

@@ -0,0 +1,19 @@
+<?php declare(strict_types = 1); // lint >= 8.1
+
+namespace Bug8929;
+
+class Test
+{
+    /** @var \WeakMap<object, mixed> */
+    protected readonly \WeakMap $cache;
+
+    public function __construct()
+    {
+        $this->cache = new \WeakMap();
+    }
+
+    public function add(object $key, mixed $value): void
+    {
+        $this->cache[$key] = $value;
+    }
+}