UndefinedVariable "$__fake_offset_var__"

[PurpleVsGreen]

Thank you for making Psalm such an amazing analyzer.
I might have found a bug but I'm not sure.

https://psalm.dev/r/cccdb532d0
https://psalm.dev/r/c114e494ca

The error goes away when I use an anonymous function as the callable for the inner array_map.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/cccdb532d0

<?php
function bar(array $a): array {
	return $a;
}

/**
 * @param array[] $x
 *
 * @return array[]
 */
function foo(array $x): array {
	return array_map(
		"array_merge",
		array_map(
			"bar",
			$x
		),
		$x
	);
}

Psalm output (using commit 646ba98):

ERROR: UndefinedVariable - 18:3 - Cannot find referenced variable $__fake_offset_var__

https://psalm.dev/r/c114e494ca

<?php
class Test {

	/** @param array[] $x */
	public function foo(array $x): array {
		return array_map(
			"array_merge",
			array_map(
				[$this, "bar"],
				$x
			),
			$x
		);
	}

	public function bar(array $a): array {
		return $a;
	}

}

Psalm output (using commit 646ba98):

ERROR: UndefinedVariable - 12:4 - Cannot find referenced variable $__fake_offset_var__

[AndrolGenhald]

Best I can tell, this is due to the nested array_map. $__fake_offset_var__ is removed from the context at ArrayMapReturnTypeProvider.php:470, and at some point it tries to use $__fake_offset_var__ when only $x and $x['$__fake_offset_var__] are set (I don't really understand why $x['$__fake_offset_var__] is being set in the first place though).

Using a temporary variable is another workaround: https://psalm.dev/r/b582aba92e

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/b582aba92e

<?php
function bar(array $a): array {
	return $a;
}

/**
 * @param array[] $x
 *
 * @return array[]
 */
function foo(array $x): array {
    $tmp = array_map(
        "bar",
        $x
    );
	return array_map(
		"array_merge",
		$tmp,
		$x
	);
}

Psalm output (using commit 38a3efe):

No issues!

[AndrolGenhald]

Ok, I'm convinced the $x['$__fake_offset_var__] isn't intended to be set and is polluting the context, it should probably be removed somehow.