Rubocop: False positive for Lint/UnneededSplatExpansion

[aried3r]

While playing around I found this false positive. Admittedly this probably isn't code you would find in the wild a lot, but I was just playing around in the Ruby console when I found this.

Expected behavior

RuboCop should correctly recognize this use of the splat operator.

Actual behavior

RuboCop detects a Lint/UnneededSplatExpansion violation which, if followed, results in code that doesn't work as expected.

Steps to reproduce the problem

# splat_rubocop.rb

status = :halt

case status
when %i[halt stop]
  pp 'Not working'
end

case status
when *%i[halt stop]
  pp 'Working'
end

$ ruby splat_rubocop.rb
"Working"

$ rubocop splat_rubocop.rb
Inspecting 1 file
W

Offenses:

splat_rubocop.rb:9:6: W: Lint/UnneededSplatExpansion: Unnecessary splat expansion.
when *%i[halt stop]
     ^^^^^^^^^^^^^^

1 file inspected, 1 offense detected



md5-250807613494f80a6970f555ec646f2a



$ rubocop -V
0.58.2 (using Parser 2.5.1.2, running on ruby 2.5.1 x86_64-darwin16)

[koic]

@aried3r Thanks for the feedback.
This looks like it behaves as expected. The code that auto-corrected behaves as follows.

% rubocop -V
0.58.2 (using Parser 2.5.1.2, running on ruby 2.5.1 x86_64-darwin17)

% rubocop splat_rubocop.rb --only Lint/UnneededSplatExpansion -a
Inspecting 1 file
W

Offenses:

splat_rubocop.rb:9:6: W: [Corrected] Lint/UnneededSplatExpansion: Unnecessary splat expansion.
when *%i[halt stop]
     ^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

% cat splat_rubocop.rb
status = :halt

case status
when %i[halt stop]
  pp 'Not working'
end

case status
when :halt, :stop
  pp 'Working'
end

% ruby splat_rubocop.rb
"Working"

The difference due to auto-correct is as follows.

 status = :halt

 case status
 when %i[halt stop]
   pp 'Not working'
 end

 case status
-when *%i[halt stop]
+when :halt, :stop
   pp 'Working'
 end

I think there is not any problem. How is this?

[aried3r]

I think there is not any problem. How is this?

You're right, I didn't use the auto correct feature and misinterpreted what the cop was trying to tell me. I thought the cop meant to just remove the * instead of comma-separating the values myself.

Indeed there's nothing wrong, thanks!

[mikegee]

@aried3r I agree that the message isn't clear. What would have been more clear in your opinion?

[aried3r]

What would have been more clear in your opinion?

Hmm, I'm not sure what cases this cop covers, but in my case maybe instead of just _"Unnecessary splat expansion."_ something with the necessary action as well _"Unnecessary splat expansion. Use comma separated values instead."_.

What do you think?

[mikegee]

That sounds good to me. I opened a new issue if you or anybody else wants to help out with that.