Pitch: Eliding commas from multiline expression lists

[jawbroken]

@griotspeak

I understand that this is a constructed example meant to illustrate a point and I take that point, I just think that it is difficult to construct a reasonable situation where you will bump up against the illustrated problem while writing code that would be acceptably clear without this change.

Here's a similar example from earlier in the thread that isn't unreasonable and is likely to confuse unless you have a strong understanding of how prefix and infix operators work in Swift.

@jawbroken

An example of some confusion this might cause:

let a = [
   1
  -1
  +1
]

looks like it could be splitting a single expression across multiple lines, but is actually declaring an array with 3 elements. The very similar

let a: [Int] = [
  1
  - 1	
  + 1
]

currently produces an array with a single element. I don't think this is technically ambiguous, because unary operators can't be separated from their operands, but it is very subtle.

Note: Similar confusion already exists to some extent

let b = 1
+ 1 // b = 2

let b = 1 // b = 1
+1 // Warning: Result of operator '+' is unused

but at least you get a warning.

@Douglas_Gregor made a great argument about the similarities between semicolon elision and comma elision, and everyone accepts that it's not technically ambiguous, but there are key differences in the possible warning behaviour that are being glossed over. How important you find good warnings in cases like these will affect how acceptable this pitch is to you.