Truthy isset($arr[$k]) should narrow $k

[staabm]

closes phpstan/phpstan#11716
closes phpstan/phpstan#8559

see https://www.php.net/manual/en/language.types.array.php

[staabm]

I am puzzled by the test and its expectations:

phpstan-src/tests/PHPStan/Rules/Variables/data/bug-10577.php

Lines 9 to 27 in cb8f910

	private const MAP = [
	'10' => 'Test1',
	'20' => 'Test2',
	];
	public function validate(string $value): void
	{
	$value = trim($value);
	if ($value === '') {
	throw new \RuntimeException();
	}
	$value = self::MAP[$value] ?? $value;
	assertType("'Test1'|'Test2'", self::MAP[$value]);
	// ...

I feel the assertion in this test is wrong, see https://3v4l.org/WD7OG
after this PR we now get a *ERROR* which is not better either though ;)

[ondrejmirtes]

Doesn't have to be just a variable. Getting rid of instanceof like these leads to bugfixes:

• d3a2a92?w=1
• 00d2caf?w=1 (regression tests in a follow-up commit)

[phpstan-bot]

This pull request has been marked as ready for review.

[staabm]

in a followup I will look into implications for $k in array_key_exists

[ondrejmirtes]

I think this is solid enough to merge this :) One more thing: regression test for phpstan/phpstan#8559 please.

[ondrejmirtes]

Thank you!