[swift-dev] SR-122 / CollectionsMoveIndices.swift Prototype

[Shawn Erickson]

I see a handful of issues like the following coming about because of
changes to `Range`. In the past `Range` conformed to `Collection` so
`count` came from `Collection` which returned a type of
`ForwardIndex.Distance` (e.g. `_SignedInteger`). It now comes from
`Strideable`’s `distance(to:)` which returns `Strideable.Stride` (e.g.
`SignedNumber`).

error: value of type 'C.Index.Stride' has no member 'toIntMax'
  let len = range.count.toIntMax()
            ~~~~~~^~~~~ ~~~~~~~~

Additionally when trying to provide `Strideable` optimized `Collection`
default implementations I hit issues like the following (note
`Collection.IndexDistance` minimally conforms to SignedInteger). I picked
`Strideable` as the optimization point since `RandomAccessCollection.Index`
minimally conforms to `Strideable` and `Strideable` provides the first
level of possible optimizations after `Comparable`.

extension Collection where Index : Strideable {
  public func next(i: Index) -> Index {
    return i.advanced(by: 1) <— this is ok
  }
  public func advance(i: Index, by n: IndexDistance) -> Index {
    return i.advanced(by: n) <— this errors
  }
…
}

error: cannot invoke 'advanced' with an argument list of type '(by:
Self.IndexDistance)'
    return i.advanced(by: n)
             ^
note: expected an argument list of type '(by: Self.Index.Stride)'
    return i.advanced(by: n)

Looking at things (see below ASCII art) I see that `_SignedInteger`
conforms to `SignedNumber `(and `SignedInteger` conforms to
`_SignedInteger`). `SignedNumber` doesn’t provide toIntMax nor can it act
as an `SignedInteger`. The use of `SignedNumber` in `Strideable` is hence
problematic. I guess the reason for `SignedNumber` is to let distances in
Strideable support non-integer types?

I am not sure the best path forward at the moment. Do I attempt to scope
things to `Strideable` where `Stride : SignedInteger` since
`Collection.IndexDistance` – as currently defined – doesn’t allow for
non-integer types? I am not sure the allowable scoping for Range in that
situation? ...or do we change `Strideable.Stride` to minimally conform to
`SignedInteger`? ...or?

Pardon my naïvety in this area of things,
-Shawn

————————————————————————————————————————
Equatable
  ^
Comparable
  ^
Strideable (associatedtype Stride : SignedNumber) ->
advanced(by:Self.Stride) & distance(to: Self)
  ^
  ^              SignedNumber
  ^                 ^
Integer & _SignedInteger —> func toIntMax()
  ^
SignedInteger —> func toIntMax()
————————————————————————————————————————
Indexable (associatedtype Index : Comparable)
  ^
Collection (associatedtype IndexDistance : SignedInteger)
  ^
BidrectionalCollection
  ^
RandomAccessCollection (associatedtype Index : Strideable)
————————————————————————————————————————
MutableIndexable (associatedtype Index : Comparable)
  ^
  ^    Collection
  ^       ^
MutableCollection (associatedtype IndexDistance : SignedInteger)
————————————————————————————————————————
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