PHPStan impure function that modifies a parent property

Given the code below, PHPStan reports an error that the comparison on line 36 always evaluates to true, making the return true statement unreachable. But if I run the code, the output is 'true'.

How can I prevent this error, assuming that I cannot modify the ExternalService and the input argument?

PHPStan Playground: https://phpstan.org/r/17ee1a74-50c3-4fe2-a284-dd15ca86f211

<?php declare(strict_types = 1); class Child { public ParentModel $parent; public function __construct(ParentModel $parent) { $this->parent = $parent; } } class ParentModel { public int $property = 0; public Child $child; public function __construct() { $this->child = new Child($this); } } class MyService { public function operate(ParentModel $model): bool { if ($model->property !== 0) { return false; } $externalService = new ExternalService(); $externalService->operate($model->child); if ($model->property !== 1) { return false; } return true; } } class ExternalService { /** * @phpstan-impure */ public function operate(Child $child): void { $child->parent->property = 1; } } var_dump((new MyService)->operate(new ParentModel()));