Rubocop: Spurious useless assignment warning on reduce

[michaelmior]

$ rubocop -V
0.21.0 (using Parser 2.1.9, running on ruby 2.0.0 x86_64-linux-gnu)

The following code example produces the warning below. Of course in this case the assignment isn't useless as sum is carried on to the next call of the block.

x = [1, 4, 9].reduce(0) do |sum, n|
  sum += Math.sqrt n
end
puts x

Inspecting 1 file
W

Offenses:

/tmp/x.rb:2:3: W: Useless assignment to variable - sum. Use just operator +.
  sum += Math.sqrt n
  ^^^

1 file inspected, 1 offense detected

[bbatsov]

@yujinakayama, please have a look at this.

[bbatsov]

@yujinakayama Actually - don't. This is not a bug, of course.

@michaelmior The assignment is redundant indeed. This is functionally the same:

x = [1, 4, 9].reduce(0) do |sum, n|
  sum + Math.sqrt(n)
end

You don't actually need to modify sum, your only concern is the return value of the block.

[michaelmior]

Ack, you're right. Sorry about that. Clearly I didn't take the time to think this through. I thought something seemed fishy. Thanks!

[amerdidit]

Well, at least this issue has one use: explaining this error to people who are encountering it themselves, and also thought it was a bug at first.

[yakryder]

I also thought this was a bug, and I wasted a chunk of time troubleshooting it. The error message was very unclear to me.

I believe this might be very easily fixed by saying something like 'use just operator + instead of +='.