Attachment: Download

Issue Description:

Paste the following into a playground.

struct PredicateSet<A> {
  let contains: (A) -> Bool

  func pullback<B>(_ f: @escaping (B) -> A) -> PredicateSet<B> {
    return PredicateSet<B>(contains: { b in self.contains(f(b)) })
  }
}


import Foundation

let p = PredicateSet<Data>(contains: { data in data.first == 0 })
let q: PredicateSet<URL> = p.pullback { url in try Data(contentsOf: url) } 

And get the following error on the last line:

Cannot convert value of type '_' to closure result type 'Data'

Force-try!-ing will let things compile, but it'd be nice to have better error messaging here. For example, in a plain ole function that isn't throwing, we get this nice error message:

Errors thrown from here are not handled