Most traversable are not rewindable. Can't this just create new kinds of issues, thus not improving anything in the end? Just wondering.

That's very edge case. Users can't seriously expect there is a way to check if non-rewindable traversable contains an element without modifying its internal iterator. That's unfixable.

ExpressionLanguage is good fit for custom domain DSLs, I think we don't need to expect non rewindable traversables there.

I'm 👎 here because it breaks using the traversable a second time (like for a second "in" check)
That'd be WTF to the end user. Better ask the dev that provides the variables to do this iterator_to_array() call.

You say most traversables are non-rewindable. Do you have some of such Class in mind for which this issue applies? I can't find any. Most common object this would help with are doctrine collections, which don't have such issue. I'm really curious, because I've never seen \Traversable for which I couldn't use in iterator_to_array twice in a row.

Eg generators yes.

I mean actual class in the wild (e.g. class used in Symfony) I can test it with. This obviously works https://3v4l.org/VMQnX

A generator is the return value of calling $generator(). This is an example of non rewindable iterator.

https://3v4l.org/FtNnQ

This is just one example of non rewindable iterator.
Definitely no go on my side for this PR. The code should not create more WTFs than it pretends to solve.
Sorry about that.

Closing for the reasons explained by @nicolas-grekas. Changing this behavior would be a BC break anyway.

Ok. I'll make sure to remind this each time someone introduces \Traversable support in his PR