Actually we should forbid extending Traversable. This should only only only be used by internal classes (Iterator, iteratorAggregate or classes providing the C get_iterator hooks)

@johannes: How could you then hint that a class must be Traversable in some way (either Iterator or Aggregate)? Or shouldn't that be possible at all?

You mean for a type hint which says "traversable and having that function"?  That might indeed be a valid reason I didn't think about ..

Yes, exactly. That's what I was trying to do when I encountered the issue.

The order dependency of interface names is a minor drawback, but same thing happens when one tries to put an abstract class between the interface and specific implementation. And this is a big problem:

<?php
interface IFoo extends \Traversable
{}

abstract class AFoo implements IFoo
{}
?>

Throws:
Class AFoo must implement interface Traversable as part of either Iterator or IteratorAggregate in Unknown on line 0

I don't care as much about the abstract issue (it's filed separately: https://bugs.php.net/bug.php?id=62609) but I'd really like to see the order restriction be relaxed on interfaces.

Fatal: https://3v4l.org/ogZk4
Fatal: https://3v4l.org/9VqNt
Fine: https://3v4l.org/JhECB

Expected: Every example should execute fine.

Looks like this got fixed in 7.4. Test added in https://github.com/php/php-src/commit/8c8f8c419378822f336aab13f301519cf2b95feb.