Attachment: Download

Issue Description:

Using attaswift/Deque, using Collection methods sometimes confuses the compiler. Generally it just doesn't understand that Deque<T>.Element is T. Sometimes it complains that the compiler can't generate a diagnostic. And finally I got it to crash (attached).

I can't easily reproduce this in a Playground. It generally seemed to work fine. But in my project, Deque breaks the compiler any time I try to use Collection methods on it.

The crash came when I added (_buffers is a Deque<PlayBuffer>):

            let buffer = self._buffers.first