A quest for Collection's Index definition

Using Swift
1

Hi:
From the source code of Index definition, it's simple.
But when I use the Cmd+Click to see the Index definition from xcode, it's totally different:

associatedtype Index : Comparable where Self.Index == Self.Indices.Element, Self.Indices.Element == Self.Indices.Index, Self.Indices.Index == Self.SubSequence.Index, Self.SubSequence.Index == Self.Indices.Indices.Element, Self.Indices.Indices.Element == Self.Indices.Indices.Index, Self.Indices.Indices.Index == Self.SubSequence.Indices.Element, Self.SubSequence.Indices.Element == Self.SubSequence.Indices.Index, Self.SubSequence.Indices.Index == Self.SubSequence.Indices.Indices.Element, Self.SubSequence.Indices.Indices.Element == Self.SubSequence.Indices.Indices.Index

I guess that the compiler should does something, but I can't find any clue from source code of Swift.

Can someone give me a hint？

Thanks！！！

2

It depends on where you cmd+click from.
Many of the generic contexts add further constraints to associated type, it’ll likely end up like looking that for very specialized protocol that Xcode infer to.

3

On further investigation, it seems that it is indeed normal Collection.Index.

To paint a more complete picture, you might want to include other associated types:

protocol Collection {
  associatedtype Index: Comparable
  associatedtype SubSequence: Collection = Slice<Self>
    where SubSequence.Index == Index,
      Element == SubSequence.Element,
      SubSequence.SubSequence == SubSequence
  associatedtype Indices : Collection = DefaultIndices<Self>
    where Indices.Element == Index, 
      Indices.Index == Index,
      Indices.SubSequence == Indices
}

If you gather all constraints regarding Index, it'd be

SubSequence.Index == Index
Indices.Element == Index
Indices.Index == Index
Indices.SubSequence == Indices

Indices.Indices.Index == Indices.Index // Indices being Collection
and so on...

Which should be able to infer all of the constraints that Xcode found.

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