Incorrect autocorrection by Style/CollectionCompact

[wendelscardua]

The new Style/CollectionCompact cop may change some code behavior. It assumes that reject can be replaced with compact even when it shouldn't. Probably it is applying to an Array#reject a correction that would only be valid for a Hash#reject. See below for an example.

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Expected behavior

When a reject method is called for an array with a block with two arguments, and this block only checks if the second argument is nil, this reject shouldn't be replaced with a compact.

Actual behavior

reject gets replaced with compact in that scenario.

Steps to reproduce the problem

• Create this ruby file (lets call it test.rb):

x = [[1, nil], [2, 3], [4, nil]]
x = x.reject { |_a, b| b.nil? }
puts x.size

• Run it

$ ruby test.rb
1

• Autocorrect this file with rubocop, selecting only the Style/CollectionCompact cop:

$ bundle exec rubocop --only Style/CollectionCompact test.rb -a
(some lines about new cops not enabled omited)
Inspecting 1 file
Scanning /home/wendel/projects/voltera-api/test.rb
C

Offenses:

test.rb:2:7: C: [Corrected] Style/CollectionCompact: Use compact instead of reject { |_x, y| y.nil? }.
x = x.reject { |_x, y| y.nil? }
      ^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

• File content is now:

x = [[1, 2], [3, nil], [4, nil]]
x = x.compact
puts x.size

• Run the modified script:

$ ruby test.rb
3

• The output is now 3 instead of 1

RuboCop version

$ bundle exec rubocop -V
1.2.0 (using Parser 2.7.2.0, rubocop-ast 1.1.1, running on ruby 2.7.1 x86_64-linux)
  - rubocop-performance 1.8.1
  - rubocop-rails 2.8.1
  - rubocop-rake 0.5.1

[koic]

Thank you for detailed feedback. I think this is difficult to detect without false positives and I opened a PR #9024.