ReflectionFunction return type is null on built in functions

Using the following reflection on built in functions, makes the return type always be null.

$reflectionFunction = new \ReflectionFunction('strtoupper');

// $reflectionFunction->getReturnType() returns null

While getDocComment does not work, how to get the return type of built in functions. I know this is expected, as the documentation describes. It is a question about how to proceed from here, to calculate the return type.