Swift bug with extending generic typealiases?

bug.swift

struct F<A, B> {
  let f: (A) -> B
}

typealias G<A> = F<A, Int>

extension G {
  func something(a: A) -> Int {
    return self.f(a)
    // ^--- error: cannot convert return expression of type 'B' to return type 'Int'
  }
}

extension F where B == Int {
  func somethingElse(a: A) -> Int {
    return self.f(a)
    // ^--- this works
  }
}

Update: this is not a swift compiler bug, just an unimplemented part of generic typealiases: https://twitter.com/dgregor79/status/847975206538813440