Generic types broken into separate lines

BookExample.swift

func anyCommonElements <T, U where T: Sequence, U: Sequence, T.GeneratorType.Element: Equatable, T.GeneratorType.Element == U.GeneratorType.Element> (lhs: T, rhs: U) -> Bool {
    for lhsItem in lhs {
        for rhsItem in rhs {
            if lhsItem == rhsItem {
                return true
            }
        }
    }
    return false
}

anyCommonElements([1, 2, 3], [3])

ProposedConvention.swift

func anyCommonElements
    <T, U where T: Sequence, U: Sequence, T.GeneratorType.Element: Equatable, T.GeneratorType.Element == U.GeneratorType.Element>
    (lhs: T, rhs: U) -> Bool 
    {
    for lhsItem in lhs {
        for rhsItem in rhs {
            if lhsItem == rhsItem {
                return true
            }
        }
    }
    return false
}

anyCommonElements([1, 2, 3], [3])

If you wanted to go buck wild you could decompose it even further to roughly one constraint per line (assuming the whitespace doesn't make swift sad)

func anyCommonElements
    <T, U where
       T: Sequence, U: Sequence, 
       T.GeneratorType.Element: Equatable, 
       T.GeneratorType.Element == U.GeneratorType.Element>
    (lhs: T, rhs: U) -> Bool 
    {
    for lhsItem in lhs {
        for rhsItem in rhs {
            if lhsItem == rhsItem {
                return true
            }
        }
    }
    return false
}

@angusiguess Good call, Looks like this is what apple does internally https://github.com/apple/swift/blob/master/stdlib/public/core/Arrays.swift.gyb#L690-L693