Open Radar

 
Summary:
The stdlib’s declaration of Collection is:

protocol Collection : Sequence {
    subscript (i: Self.IndexType) -> Self.GeneratorType.Element { get }
}

This makes it appear like any type which conforms to Sequence and implements that subscript method can conform to Collection. However, this is insufficient; it also needs to conform to another protocol, _Collection, whose definition is unavailable but which requires startIndex/endIndex properties.


Steps to Reproduce:
1. Make a type, e.g. Set, from here: https://gist.github.com/robrix/e9e6c6693382a579c62a
2. Declare that the type conforms to Collection.
3. Provide an implementation of subscript per the declaration above.


Expected Results:
I expected that the type would be fully conformed to Collection and I could proceed.


Actual Results:
The compiler gives the errors shown in Notes, below.


Regression:
N/A


Notes:
These are the compiler errors:

/Users/rob/Developer/Projects/Swift/Set/Set/Set.swift:27:1: error: type 'Set<Element>' does not conform to protocol '_Collection'
extension Set : Collection {
^
Swift._Collection:3:7: note: protocol requires property 'startIndex' with type 'Set<Element>.IndexType'
  var startIndex: IndexType { get }
      ^
Swift._Collection:4:7: note: protocol requires property 'endIndex' with type 'Set<Element>.IndexType'
  var endIndex: IndexType { get }
      ^
Swift.Collection:1:10: note: type 'Set<Element>' does not conform to inherited protocol '_Collection'
protocol Collection : _Collection, Sequence