Implicit retain cycle

Evolution Discussion
13

I believe they'll look different to the compiler, but there's another case to consider here where bar is already a function property that might (or might not) strongly retain self.

class S {
  let bar: () -> Void
  let handler: () -> Void

  func assign() {
    self.handler = bar // not necessarily a retain cycle, but it _might_ be
  }
}

Additionally, I feel this is similar in spirit to Request explicit capture of self instead of defaulting to a strong capture of self and might be related.