// Example 1

// * Builds in Swift 4.2
// * Does NOT build in Swift 5
// * SectionModel defined _after_ Model
// * SectionModel contains RowID associatedtype referencing the
//   RowIDConstraint typealias which is Hashable 
protocol Example1TableViewModels {
    typealias SectionConstraint = Example1TableViewSectionModels
    associatedtype Section: SectionConstraint

    var sections: [Section] { get }
}

extension Example1TableViewModels {
    func cellIndexPath(
        forRowIdentifiedBy rowIdentifier: Section.RowID
    )
        -> (Int,Int)?
    {
        for indexAndSection in zip(0...,sections) {
            if let rowIDIndex = indexAndSection.1.rowIDs.firstIndex(
                   of: rowIdentifier // Argument type 'Self.Section.RowID' does not conform to expected type 'Equatable'
               )
            {
                return (indexAndSection.0,rowIDIndex)
            }
        }
        return nil
    }
}
protocol Example1TableViewSectionModels: Equatable {
    typealias RowIDConstraint = Hashable
    associatedtype RowID : RowIDConstraint

    var rowIDs: [RowID] { get }
}

// Example 2

// * Builds in Swift 4.2
// * Builds in Swift 5
// * SectionModel defined _after_ Model
// * SectionModel contains RowID associatedtype constrained to Hashable directly
protocol Example2TableViewModels {
    typealias SectionConstraint = Example2TableViewSectionModels
    associatedtype Section: SectionConstraint

    var sections: [Section] { get }
}
extension Example2TableViewModels {
    func cellIndexPath(
        forRowIdentifiedBy rowIdentifier: Section.RowID
    )
        -> (Int,Int)?
    {
        for indexAndSection in zip(0...,sections) {
            if let rowIDIndex = indexAndSection.1.rowIDs.firstIndex(
                   of: rowIdentifier
               )
            {
                return (indexAndSection.0,rowIDIndex)
            }
        }
        return nil
    }
}
protocol Example2TableViewSectionModels: Equatable {
    associatedtype RowID : Hashable

    var rowIDs: [RowID] { get }
}

// Example 3

// * Builds in Swift 4.2
// * Builds in Swift 5
// * SectionModel defined _before_ Model
// * SectionModel contains RowID associatedtype referencing the
//   RowIDConstraint typealias which is Hashable
protocol Example3TableViewSectionModels: Equatable {
    typealias RowIDConstraint = Hashable
    associatedtype RowID : RowIDConstraint

    var rowIDs: [RowID] { get }
}
protocol Example3TableViewModels {
    typealias SectionConstraint = Example3TableViewSectionModels
    associatedtype Section: SectionConstraint

    var sections: [Section] { get }
}
extension Example3TableViewModels {
    func cellIndexPath(
        forRowIdentifiedBy rowIdentifier: Section.RowID
    )
        -> (Int,Int)?
    {
        for indexAndSection in zip(0...,sections) {
            if let rowIDIndex = indexAndSection.1.rowIDs.firstIndex(
                   of: rowIdentifier
               )
            {
                return (indexAndSection.0,rowIDIndex)
            }
        }
        return nil
    }
}