FlattenSequence

mattrips · June 14, 2020, 3:35am

I'd like to examine the existing conformance of FlattenSequence to Collection. Where would I find it?

Context and Diligence:

I'm working with FlattenSequence. The documentation states that its only protocol conformance is Sequence. I'm trying to extend FlattenSequence to conditionally conform to Collection, but I'm getting an error stating that the Standard Library already conforms it to Collection. The error is as follows:

Conformance of 'FlattenSequence' to protocol 'Collection' conflicts with that stated in the type's module 'Swift' and will be ignored; there cannot be more than one conformance, even with different conditional bounds

Obviously, that error differs from the documentation.

When working with an instance of FlattenSequence, code completion reveals methods consistent with the type conforming to FlattenSequence.

I've examined the Flatten.swift source file in the Standard Library, and find no conformance to Collection.

jrose · June 14, 2020, 4:17am

It's done through the typealias FlattenCollection:

github.com

apple/swift/blob/16c9c007af69bce822e79b0a73d6d51ce9e1e729/stdlib/public/core/Flatten.swift#L212


      
            ///
            /// - Parameter hasher: The hasher to use when combining the components
            ///   of this instance.
            @inlinable
            public func hash(into hasher: inout Hasher) {
              hasher.combine(_outer)
              hasher.combine(_inner)
            }
          }
          
          
extension FlattenCollection: Collection {
            /// The position of the first element in a non-empty collection.
            ///
            /// In an empty collection, `startIndex == endIndex`.
            @inlinable // lazy-performance
            public var startIndex: Index {
              let end = _base.endIndex
              var outer = _base.startIndex
              while outer != end {
                let innerCollection = _base[outer]
                if !innerCollection.isEmpty {

Looks like a docs bug that it doesn't show up on developer.apple.com!

mattrips · June 14, 2020, 4:20am

Thank you! Should I file a bug report on the doc?

jrose · June 14, 2020, 4:27am

Yes please! Since Apple manages those docs, it's probably best to start with Feedback Assistant rather than bugs.swift.org. (It may turn out to be an indexing bug, but if it's not it would take longer to get to the correct team.)

mattrips · June 14, 2020, 4:57am

Done. I had jumped the gun, and first reported it at bugs.swift.org. I've duplicated the report with Feedback Assistant.