class People extends TypedCollection
{
-     protected static $allowedTypes = [Person::class];
+     protected static array $allowedTypes = [Person::class];
}

+ /**
+  * @extends TypedCollection<array-key, int|string|Person>
+  */
class MixedItemCollection extends TypedCollection
{
    protected static array $allowedTypes = ['int', 'string', Person::class];
}