Description:
------------
Errors are generated when attempting to:

Diff a datetimeimmutable and a datetime
Diff a datetime and a datetimeimmutable
Diff a datetimeimmutable and another datetimeimmutable

The documentation implies that this should work correctly, and that diff() accepts DateTimeInterface

Test script:
---------------
$di1 = new DateTimeImmutable('now');
$di2 = new DateTimeImmutable('+1 hour');

$d = new DateTime('-1 hour');

$di1->diff($d); // DateInterval +1H
$di1->diff($di2); // DateInterval -1H
$d->diff($di1); // DateInterval +1H

Expected result:
----------------
DateInterval objects in all three cases

Actual result:
--------------
Produced via `php -a`

$di1->diff($d);
// PHP Fatal error:  DateTime::diff() must be derived from DateTimeImmutable::diff in Unknown on line 0


$di1->diff($di2); // DateInterval -1H
// PHP Fatal error:  DateTime::diff() must be derived from DateTimeImmutable::diff in Unknown on line 0

$d->diff($di1); // DateInterval +1H
// PHP Warning:  DateTime::diff() expects parameter 1 to be DateTime, object given in php shell code on line 1
// bool(false)