Fatal error: DateTimeInterface can't be implemented by user classes

[MircoBabin]

Description

The following code:

https://3v4l.org/AVYra#v8.5.9

<?php

// Why is this allowed ? 
// This pattern is used by Carbon - https://github.com/briannesbitt/Carbon
class MyDateTimeImmutable extends DateTimeImmutable implements DateTimeInterface
{
    public function format(string $format): string
    {
        return 'Why is this allowed?';
    }
}

// And why is this disallowed ?
// Fatal error: DateTimeInterface can't be implemented by user classes
class MyTime implements DateTimeInterface
{
}

Resulted in this output:

Fatal error: DateTimeInterface can't be implemented by user classes in /in/AVYra on line 14

But I expected this output instead:

Fatal error: Class MyTime contains ... abstract methods and must therefore be declared abstract or implement the remaining methods

PHP Version

PHP 8.5.9

Operating System

https://3v4l.org/

[Girgias]

DateTimeInterface is an internal interface that can only be implemented by the relevant data classes defined in the date extension.

If you extend one of those classes then the interface is already implemented and redeclaring it is allowed.

This is similar to how the Throwable interface works.