method.impureOverridePureUnlessCallable

Code example

<?php declare(strict_types = 1);

interface PureUnlessParent
{

	/**
	 * @pure-unless-callable-is-impure $cb
	 */
	public function run(callable $cb): int;

}

class ImpureChild implements PureUnlessParent
{

	/**
	 * @phpstan-impure
	 */
	public function run(callable $cb): int // ERROR: Impure method ImpureChild::run() overrides method PureUnlessParent::run() marked @pure-unless-callable-is-impure.
	{
		echo 'side effect';

		return $cb(1);
	}

}

Why is it reported?

The @pure-unless-callable-is-impure tag declares that the parent method is pure except when the given callable parameter is impure. Its purity is conditional on the callable passed in, but the method's own body is expected to have no other side effects.

An overriding method marked @phpstan-impure breaks this contract: it introduces side effects regardless of the callable. Code that relies on the parent's conditional purity — for example when the parent is called with a pure callable — would then be reasoning about a method that is always impure. The override is therefore incompatible with the inherited purity guarantee.

How to fix it

Remove @phpstan-impure from the child and keep its body free of side effects other than invoking the flagged callable, so it honors the inherited @pure-unless-callable-is-impure contract:

 class ImpureChild implements PureUnlessParent
 {

-	/**
-	 * @phpstan-impure
-	 */
 	public function run(callable $cb): int
 	{
-		echo 'side effect';
-
 		return $cb(1);
 	}

 }

If the child genuinely needs side effects, the parent's purity contract cannot hold across all implementations. Remove @pure-unless-callable-is-impure from the parent method:

 interface PureUnlessParent
 {

-	/**
-	 * @pure-unless-callable-is-impure $cb
-	 */
 	public function run(callable $cb): int;

 }