Issue Description:

When using shorthand initializers (leading dot syntax) with a generic function to deduce the initializer parameter, the compiler can not deduce the type of the generic function.

Here's a reproducing Playground

import UIKit

func switchOnCategory<T>(_ categoryToValue: [Int: T]) -> T {
    return categoryToValue.first!.value
}

let label = UILabel()
label.font = .systemFont(ofSize: switchOnCategory([0: 15.5, 1: 20.5]))

Getting a `Generic parameter 'T' could not be inferred` compiler error on the last line. This works fine with Xcode 9 and Swift 4.1.

The issue can be worked around in two ways.

1) Explicitly state the init type:

label.font = UIFont.systemFont(...)

2) Explicitly specialize the parameter type:

label.font = .systemFont(ofSize: switchOnCategory([0: 15.5, 1: 20.5]) as CGFloat)