Why does `RandomAccessCollection` demand `index(after:)` and `index(before:)`, but not `index(_:offsetBy:)`?

Using Swift
1

i am implementing a custom RandomAccessCollection with an Index type of UInt32.

struct SubSequence:RandomAccessCollection 
{
    typealias Index = Element.Offset 
    typealias SubSequence = Self 

    let indices:Range<Element.Offset>
    private 
    let storage:[Element]
    
    var startIndex:Element.Offset
    {
        self.indices.lowerBound
    }
    var endIndex:Element.Offset
    {
        self.indices.upperBound
    }
    subscript(offset:Element.Offset) -> Element 
    {
        _read 
        {
            yield  self.storage[.init(offset - self.startIndex)]
        }
    }
    subscript(range:Range<Element.Offset>) -> Self
    {
        .init(storage: self.storage, indices: range)
    }

    func index(before base:Element.Offset) -> Element.Offset
    {
        base - 1
    }
    func index(after base:Element.Offset) -> Element.Offset
    {
        base + 1
    }
    // func index(_ base:Element.Offset, offsetBy distance:Int) -> Element.Offset
    // {
    //     Element.Offset.init(Int.init(base) + distance)
    // }

somehow i am allowed to inherit the index(_:offsetBy:) implementation, but not the index(before:) or index(after:) requirements.

error: type 'Branch.Buffer<Element>.SubSequence' does not conform to protocol 'Collection'
    struct SubSequence:RandomAccessCollection 
           ^
Swift.RandomAccessCollection:3:28: note: candidate would match if 'DefaultIndices<Branch.Buffer<Element>.SubSequence>' was the same type as 'Range<Branch.Buffer<Element>.SubSequence.Index>' (aka 'Range<Element.Offset>')
    @inlinable public func index(after i: Self.Index) -> Self.Index
                           ^
Swift.RandomAccessCollection:11:19: note: protocol requires function 'index(before:)' with type '(Branch.Buffer<Element>.SubSequence.Index) -> Branch.Buffer<Element>.SubSequence.Index' (aka '(Element.Offset) -> Element.Offset')
    override func index(before i: Self.Index) -> Self.Index
                  ^
Swift.RandomAccessCollection:13:19: note: protocol requires function 'index(after:)' with type '(Branch.Buffer<Element>.SubSequence.Index) -> Branch.Buffer<Element>.SubSequence.Index' (aka '(Element.Offset) -> Element.Offset')
    override func index(after i: Self.Index) -> Self.Index
                  ^
Swift.Collection:27:10: note: protocol requires function 'index(after:)' with type '(Branch.Buffer<Element>.SubSequence.Index) -> Branch.Buffer<Element>.SubSequence.Index' (aka '(Element.Offset) -> Element.Offset')
    func index(after i: Self.Index) -> Self.Index

why? i would have assumed index(_:offsetBy:) is more important than index(before:) or index(after:).