Swift Error: 'Sequence' requires the types 'T' and 'ArraySlice' be equivalent

Question

I\'m trying to update a math library to be compatible with Swift 3, but I\'m running into an error:

\'Sequence\' requires the types \'T\' and \'ArraySlice<

Answer 1

Your code compiles fine as Swift 3.1 (Xcode 8.3.3). The error

'Sequence' requires the types 'T' and 'ArraySlice<T>' be equivalent

occurs when compiling as Swift 4 (Xcode 9, currently beta), because then the Sequence protocol already defines the

associatedtype Element where Self.Element == Self.Iterator.Element

which conflicts with your definition. You can either choose a different name for your type alias, or just remove it (and use T instead):

public struct Matrix<T> where T: FloatingPoint, T: ExpressibleByFloatLiteral {

    let rows: Int
    let columns: Int
    var grid: [T]

    public init(rows: Int, columns: Int, repeatedValue: T) {
        self.rows = rows
        self.columns = columns

        self.grid = [T](repeating: repeatedValue, count: rows * columns)
    }
}

extension Matrix: Sequence {
    public func makeIterator() -> AnyIterator<ArraySlice<T>> {
        let endIndex = rows * columns
        var nextRowStartIndex = 0

        return AnyIterator {
            if nextRowStartIndex == endIndex {
                return nil
            }

            let currentRowStartIndex = nextRowStartIndex
            nextRowStartIndex += self.columns

            return self.grid[currentRowStartIndex..<nextRowStartIndex]
        }
    }
}

This compiles and runs with both Swift 3 and 4.