This pull request has been marked as ready for review.

@liamduckett I just realized that the fix of this PR produces a false-positive (see the newly added testcase).
I do not yet know how to fix the initial problem without also regressing the other example.

just to let you know, as #4153 has the same problem

@liamduckett I just realized that the fix of this PR produces a false-positive (see the newly added testcase). I do not yet know how to fix the initial problem without also regressing the other example.

just to let you know, as #4153 has the same problem

Thanks. I'm not sure I understand the new test case (probably due to my inexperience) - why would $bar being narrowed here, be a problem?

the initial types were wrongly narrowed:

/**
 * @param 'quux'|'qux' $constUnion
 */
function fooBar(object $bar, string $constUnion): void
{
	if (!method_exists($bar, $constUnion)) {
		throw new \Exception;
	}
 // $bar does not have both methods, it has only either 'quux' or 'qux' but not both
}

while in the initial loop check example it is correct that $bar will have both methods at once:

foreach (['qux', 'quux'] as $method) {
		assertType("'quux'|'qux'", $method);
		if (!method_exists($bar, $method)) {
			throw new \Exception;
		}
}