How can I let phpstan know that a string contains the name of a class of a given type?

[mind-bending-forks]

I've got some code that uses the scope resolution operator (Paamayim Nekudotayim, ::), to access a method on a variable that contains the name of a class that is guaranteed to implement an interface that defines that method.

For example, here is an interface

interface ITransformer
{
	public static function Transform( string $theInput, bool &$theErrorEncountered ) : string;
}

and a class implementing it

class Transformer implements ITransformer
{
	public static function Transform( string $theInput, bool &$theErrorEncountered ) : string
	{
		if ( $theInput === 'invalid' )
		{
			$theErrorEncountered = true;
			return '';
		}
		return strtoupper( trim($theInput) );
	}
}

The name of a class implementing ITransformer is in a variable, and this is used to access the Transform method:

$transformer = 'Transformer';
$input = ' asdasda asdasd ';
$error = false;
// At this point, I know that $transformer is the name of a class implementing `ITransformer`
// How can I tell phpstan that?
$output = $transformer::Transform( $input, $error );

At present, phpstan is not understanding that $transformer::Transform looks like ITransformer::Transform and this is provoking follow on errors.

See, for example: https://phpstan.org/r/ada88746-fd60-4187-abfc-046ed28bf750 PHPStan thinks that $error is certainly false, whereas, because the second argument of the Transform method is passed by reference, it might not be, and the test for false following it is valid.

How can I tell phpstan that $transformer is a string containing the name of a class that implements ITransformer?

Thanks.

[ondrejmirtes]

Hi,
this works: https://phpstan.org/r/bdd8200a-b3cb-49c6-8c09-675aca255411

Which means that for your original example to work, a slight change needs to be done in PHPStan internals, around this place: https://github.com/phpstan/phpstan-src/blob/18f8473fa5c8e957f5e24d25961160ac89afd506/src/Analyser/NodeScopeResolver.php#L2070 (same logic needs to be executed in the $expr->name instanceof Expr branch as here).

Feel free to tackle this problem, or open a bug report.

[mind-bending-forks]

Wow, that was quick. Thanks. I'll file a bug report.

[mind-bending-forks]

One thing though. Your proposed solution would address the simple case example above, for which phpstan would be able to resolve the variable $transformer to the string 'Transformer' and proceed from there. However, imagine that phpstan would not be able to determine the value of the variable $transformer in the more complex actual code from which I extracted this example. In such cases, I think it would be necessary to inform phpstan what the string contains by means of some code comment, somehow.

[ondrejmirtes]

You can always mark it with /** @var class-string<Transformer> $transformer */. Works in @param and @return too.

See the docs:

• https://phpstan.org/writing-php-code/phpdocs-basics
• https://phpstan.org/writing-php-code/phpdoc-types

[mind-bending-forks]

Brilliant. That looks exactly like what I need. (I can't believe I missed that in the docs, because I looked, and even saw callable-string and decided that wasn't what I wanted, but somehow missed class-string!) I can't get it to work though. Irrespective of whether I add such a comment, phpstan still behaves as if the method doesn't alter values of the method arguments, whereas in this case, it can.

See https://phpstan.org/r/1d07eda2-72d4-4bc5-8a22-21acf1b007b3 vs https://phpstan.org/r/b1aeaf45-a9b8-4eb5-87da-030249420670

I presume that's because of this issue (#5020) and so I just need to be patient, or is that a separate issue?

[ondrejmirtes]

Yes, the internals still need to be modified in order for this to work.