class_implements should return class-string type

[staabm]

https://psalm.dev/r/886622aa50

didn't expect

ERROR: ArgumentTypeCoercion - 21:4 - Argument 1 of p expects array<array-key, class-string>, parent type non-empty-array<string, string> provided

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/886622aa50

<?php declare(strict_types = 1);

class B {}
interface F {}

class HelloWorld extends B implements F
{

}

/** @param class-string[] $p */
function p($p) {
}

$p = class_parents(HelloWorld::class);
if ($p)
	p($p);

$p = class_implements(HelloWorld::class);
if ($p)
	p($p);

Psalm output (using commit 27d928b):

INFO: UnusedParam - 12:12 - Param $p is never referenced in this method

INFO: MissingReturnType - 12:10 - Method p does not have a return type, expecting void

ERROR: ArgumentTypeCoercion - 21:4 - Argument 1 of p expects array<array-key, class-string>, parent type non-empty-array<string, string> provided

[orklah]

This is easy to fix but I'm not sure what the long term vision is here.

There are two string pseudo-types: class-string and trait-string and there is a string pseudo-pseudo-type interface-string who is interpreted exactly as a class-string. This means, new class_implements(HelloWorld::class)[0]() would actually be valid code for Psalm.

I'm not sure why there isn't a full interface-string so I'm not sure this is a good idea.

[orklah]

I guess we could just return interface-strings. Please propose a PR if you want :)