[Fix #9980] Fix a false positive for Style/IdenticalConditionalBranches

[koic]

Fixes #9980.

This PR fixes a false positive for Style/IdenticalConditionalBranches when assigning to a variable used in a condition.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[Drowze]

One case that I had issues with autocorrection for this particular cop on our codebase is specifically as follows:

if shipment.assignments.find_by(employee_id: employee_id)
  shipment.assignments.destroy_all
else
  shipment.assignments.destroy_all

  # do other stuff
end

When running this cop's autocorrection, that excerpt was refactored to:

shipment.assignments.destroy_all

if shipment.assignments.find_by(employee_id: employee_id)  
else
  # do other stuff
end

After rubocop's autocorrection, the code always falls to the else block. I think this PR wouldn't solve that particular case, right? I'm also not sure if there's a way for this cop to be safe on such cases 🤔

[vlad-pisanov]

I believe this autocorrect is inherently unsafe. Consider:

if method_that_modifies_global_state
  method_that_relies_on_global_state
  foo
else
  method_that_relies_on_global_state
  goo
end

There's no way to safely factor out method_that_relies_on_global_state before method_that_modifies_global_state is executed. method_that_modifies_global_state could modify all sorts of things like global vars, mocks, DB records, etc. that method_that_relies_on_global_state needs to function properly.

[koic]

@vlad-pisanov Ah... you're right! This cop can try to reduce false positives, but I understand that it cannot be safe auto-correction because a side effect of method invocation.

[koic]

I opened a separated PR #9985 because I think this should be marked as unsafe for auto-correction.
https://docs.rubocop.org/rubocop/1.18/usage/auto_correct.html#safe-auto-correct

[bbatsov]

Thanks for tackling this!