We should also test that this behaves as an actual terminating call. Try this:

function foo(?string $s): void
{
    if ($s === null) {
        trigger_error('foo', E_USER_ERROR);
    }
    assertType('string', $s);
}

hmm interessting.. you are right, this case is failling atm.

I had the impression that phpstan would handle a NeverType returned from a DynamicFunctionReturnTypeExtension automatically as a terminating call. wouldn't that make sense, instead of adding such logic case by case?

You need return new NeverType(true);

already fixed :-). thx.