Open Radar

 
While trying to create generic functions for transforming Sequences or Generators in various ways [initially, attempting to port Python's itertools to swift]. I keep running into crashes, where the compiler segfaults, apparently while trying to infer or check types somewhere.

See https://github.com/abarnert/swift-itertools/blob/d2de3034ee268cbdba3b36215d0915b5c4e17724/teetest.swift for an example that reliably crashes the compiler. Commenting out the code inside Tee.next (return nil), inside tee (return []), and at the top level has no effect. Changing either or both structs to classes replaces the segfault with a compiler fatal error ("LLVM ERROR: unimplemented IRGen feature! non-fixed class layout"), but that doesn't help much, and I don't think either one should be a class anyway (although I could easily be wrong).

Earlier attempts at many of the functions in https://github.com/abarnert/swift-itertools produced identical crashes. In some cases there was clearly a type error in my code (e.g., declaring a parameter or member as a Sequence instead of an S) that I was able to find and fix; in others, I just kept rewriting things until it worked, or figured out why it was impossible…