Can't check if Iterator is empty

[SimonFrings]

Bug report

Hey there, I'm working on ReactPHP and it seems we can't really check if an Iterator is empty or not. We could really use this for our Promise and Async component, happy about your insights on this.

Code snippet that reproduces the problem

https://phpstan.org/r/fac73ffc-131a-43d1-8cb7-b1a629374ac1

Expected output

I think PHPStan should detect that the Iterator is empty here.

Did PHPStan help you today? Did it make you happy in any way?

PHPStan always makes me happy

[ondrejmirtes]

The typesystem cannot express anything like non-empty-Iterator right now. You can do something like this to make your code sample pass but it's not very useful in practice: https://phpstan.org/r/d34a8454-8a5c-4971-9a02-f28508718f89