Compiler panic evaluating complex associated types meant to [emulate generic associated types](http://lukaskalbertodt.github.io/2018/08/03/solving-the-generalized-streaming-iterator-problem-without-gats.html). [Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=69b21f8d609b6c3a57b939e241e289dd)