On 12/03/2017 16:58, Don Giovanni via swift-users wrote:
I'm trying to adopt the RandomAccessCollection protocol from a class. I
deliberately leave out the func index(before:) function because there
is already a default implementation of that function in
RandomAccessCollection. I do understand that that function's
implementation is required because of BiDirectionalCollection from which
RandomAccessCollection inherits.
My question is why would the compiler complain that func index(before:)
implementation is required when there is already a default
implementation in RandomAccessCollection that i can use?
The default implementation for index(before:) is in an extension with
these constraints:
extension RandomAccessCollection where Index : Strideable,
Index.Stride == IndexDistance,
Indices == CountableRange<Index> {
...
public func index(before i: Self.Index) -> Self.Index
...
}
Your collection type doesn't satify these constraints because
Container2.Indices is DefaultRandomAccessIndices. If you replace the
typealias with:
typealias Indices = CountableRange<Int>
it will work.
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