Trailing comma in a type

What the heck is the following type:

(Int, => Double) => String

Note the trailing comma after Int. Apparently it is not a syntactic loophole, but something different from

(Int => Double) => String

E.g. when using overloading:

trait Foo {
  def bar(x: (Int, => Double) => String): Unit
  def bar(x: (Int  => Double) => String): Unit
}