[SR-1466] Collection.Indices doesn't expose Comparable elements

[glessard]

Previous ID	SR-1466
Radar	None
Original Reporter	@glessard
Type	Bug
Status	Closed
Resolution	Done

Environment

Apple Swift version 3.0-dev (LLVM dffa09ffd8, Clang 9f0d189820, Swift 1c720b8)
(on OS X)

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	None
Priority	Medium

md5: f99df57548a484701b7b53e48286a839

Issue Description:

Comparable is a requirement of Collection.Index, yet the Sequence returned by Collection.Indices hides that its element type conforms to Comparable.

func f<C: Collection>(c: C) {
  let s = c.startIndex
  for i in c.indices {
    if s == i { break }
    // Argument type 'C.Indices.Iterator.Element' does not conform to expected type 'Comparable'
  }
}

I can restrict the generic parameter thusly:

func g<C: Collection where C.Indices.Iterator.Element == C.Index>(c: C) {
  let s = c.startIndex
  for i in c.indices {
    if s == i { break } // no problem
  }
}

It seems this should always be true.

[glessard]

Even less useful:

func function<C: Collection>(c: C) {
  for index in c.indices {
    _ = c[index] // Cannot subscript a value of type 'C' with an index of type 'C.Indices.Iterator.Element'
  }
}

[Dante-Broggi]

I think this would have been fixed in Swift 4.2, with SE-0143 (Conditional Conformances).

[glessard]

It was actually fixed as far back as Swift 4.0.2. Thanks for the reminder.

[glessard]

Fixed in Swift 4.0.2