Do not check the body of an impure method against an inherited @pure-unless-* tag

[zonuexe]

Found while working on #6667. FunctionPurityCheck checked the body of a method for purity whenever it had a @pure-unless-callable-is-impure or @pure-unless-parameter-passed tag, even when the method itself is marked @phpstan-impure and only inherits the tag:

interface Replacer
{
	/**
	 * @param-out int $count
	 * @pure-unless-parameter-passed $count
	 */
	public function replace(string $subject, int &$count = 0): string;
}

final class ImpureReplacer implements Replacer
{
	/** @phpstan-impure */
	public function replace(string $subject, int &$count = 0): string
	{
		echo $subject; // Impure echo in pure method ImpureReplacer::replace().
		$count = 1;

		return $subject;
	}
}

The method declares itself impure, so PHPStan shouldn't call it a pure method or report its side effects. With bleeding edge, MethodSignatureRule reports the override itself as method.impureOverridePureUnlessParameterPassed or method.impureOverridePureUnlessCallable.

Changes

• FunctionPurityCheck::check() checks the body against the conditional purity only when the method is not marked impure. A method marked @phpstan-impure goes through the existing impure branch, so one without any side effects gets impureMethod.pure like any other.
• The test covers both tags, an impure override with and without side effects, and an override without a purity tag, whose body PHPStan still checks.

[VincentLanglet]

I don't think your example is valid because if I call

Replacer::replace() I'll expect that the call is pure, which is invalid with your implementation.