Issue Description:

In the following code example, there are two functions with the same base name f and the same variadic parameter x. Both functions are candidates for all four function calls shown below. However, even though the function calls only differ in the number of arguments that are passed to the variadic parameter x, function overload resolution doesn't always pick the same function:

func f(x: Int..., y: Int = 0) { print(x, y) }
func f(x: Int...) { print(x) }

f()              // []
f(x: 1)              // [1]
f(x: 1, 2)       // [1, 2] 0
f(x: 1, 2, 3)        // [1, 2, 3]

As far as I can tell, the first function is selected when the number of arguments passed to the variadic parameter x matches the total number of parameters of the first function. Here's another example:

func f(x: Int..., y: Int = 0, z: Int = 0) { print(x, y, z) }
func f(x: Int...) { print(x) }

f()              // []
f(x: 1)              // [1]
f(x: 1, 2)       // [1, 2]
f(x: 1, 2, 3)        // [1, 2, 3] 0 0