False positive and bad autocorrect in Style/TrailingUnderscoreVariable

[mvz]

For complex destructuring assignments, Style/TrailingUnderscoreVariable gives false positives and performs the wrong autocorrect.

Expected behavior

Given the following code:

a = [1, 2, [3, 4, 5]]
_, b, (_, c,) = a
_, d, (_, e, _) = a

p a, b, c, d, e

I expect RuboCop to:

• report no offense on the second line
• report an offense on the third line, suggesting a change to _, d, (_, e,) = a
• autocorrect the third line to _, d, (_, e,) = a

Actual behavior

RuboCop reports:

foo.rb:2:7: C: Do not use trailing _s in parallel assignment. Prefer _, b, = a.
_, b, (_, c,) = a
      ^^^^^^^^
foo.rb:3:7: C: Do not use trailing _s in parallel assignment. Prefer _, d, = a.
_, d, (_, e, _) = a
      ^^^^^^^^^^

RuboCop autocorrects the code to:

a = [1, 2, [3, 4, 5]]
_, b, = a
_, d, = a

p a, b, c, d, e

Steps to reproduce the problem

Given a file foo.rb with:

a = [1, 2, [3, 4, 5]]
_, b, (_, c,) = a
_, d, (_, e, _) = a

p a, b, c, d, e

• run rubocop foo.rb to show the false positive
• run rubocop -a foo.rb to show the broken autocorrect

RuboCop version

Include the output of rubocop -V. Here's an example:

$ rubocop -V
0.50.0 (using Parser 2.4.0.0, running on ruby 2.4.1 x86_64-linux)