Issue Description:

Tested on Xcode 10 beta 1/Swift 4.2

If you have two optional arrays and add their contents into a new result variable, you might lose the contents of the right-hand array if you don't properly wrap both arrays in parenthesis when nil coalescing.

It's unclear if this is a compiler bug, or defined behavior if you know what's going on, but the results of this are unexpected so I'm considering it a bug.

Observe the following reproducible code snippet:

let array1: [Int]? = [1, 2]
let array2: [Int]? = [3]

let result1 = array1 ?? [] + (array2 ?? [])
result1
// products: [1, 2] // array2's contents are dropped

let result2 = (array1 ?? []) + (array2 ?? [])
result2
// products: [1, 2, 3]

The first concatenation compiles, as you have to use at least one set of parenthesis, however this produces an unexpected result as it completely drops the contents in array2. The second concatenation compiles and produces the expected result by concatenating the contents of both arrays.