Bug report

On PHPStan level 9

When using isset($data['foo']) with $data being mixed, PHPStan complains about "Cannot access offset 'foo' on mixed.".

Also, when checking for value type, like is_string($json['Foo']['Bar']), the value type is not narrowed.

Code snippet that reproduces the problem

        public function sayHello(mixed $json): string
	{
		if (!isset($json['Foo']['Bar']) || !is_string($json['Foo']['Bar'])) {
			throw new \Exception('Bad JSON');
		}

		return $json['Foo']['Bar'];
	}

https://phpstan.org/r/9b18a65f-f28d-4374-8167-1e94703d9707

Expected output

PHPStan should not report an issue when trying to access offset inside isset, and after validating offset with isset, we should be able to assert the type

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