init-puzzle.swift

struct S {
  let x: Int
  
  init(x: Int) { self.x = x }
  
  // This is allowed
  init(x2: Int) {
    self.init(x: x2)
  }
  
  // This is also allowed
  init(x3: Int) {
    self = S(x: x2)
  }
  
  // This is also allowed
  init(x4: Int) {
    self.init(x: x4)
    self = S(x: x4)
  }
  
  // This is also allowed
  init(x5: Int) {
    self = S(x: x5)
    self = S(x: x5)
  }
  
  // But this is not
  init(x6: Int) {
    self.init(x: x6)
    self.init(x: x6)
  }
}

// Either x4, x5 and x6 should all be banned, or x6 should work.

// The reason is that I noticed that this doesn't work:

init(y: Bool) {
  if y {
    self = S(x: 0)
  } else {
    self.init(x: 1)
  }
}

// The fix is quite simple; the SIL for 'self = ...' and 'self.init' is
// already identical, only the AST differs. However, treating them
// identically in definite initialization exposes the issue at the top
// of the gist. We need to figure out if we want to allow multiple
// initialization of 'self' or not, regardless of how it was initialized.