https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Types.html#//apple_ref/swift/grammar/function-type

If a function type includes more than a single arrow (->), the function types are grouped from right to left.
For example, the function type (Int) -> (Int) -> Int is understood as (Int) -> ((Int) -> Int)
―that is, a function that takes an Int and returns another function that takes and returns an Int.

引数は必ず()で囲まないといけないから、もし ((Int) -> (Int)) -> Int なら前半の()が必須
そういう意味でも (Int) -> ((Int) -> Int) としか解釈できない