Avoid crash in Style/ConditionalAssignment cop with masgn

[pocke]

Problem

Style/ConditionalAssignment cop crashes with multiple assignment when EnforcedStyle is assign_to_condition.

e.g.

# .rubocop.yml
Style/ConditionalAssignment:
  EnforcedStyle: assign_inside_condition
  Enabled: true

# test.rb
x, y = a

$ rubocop -d
An error occurred while Style/ConditionalAssignment cop was inspecting /tmp/tmp.xqGCvflgnB/test.rb.
An error occurred while Style/ConditionalAssignment cop was inspecting /tmp/tmp.xqGCvflgnB/test.rb.
For /tmp/tmp.xqGCvflgnB: configuration from /tmp/tmp.xqGCvflgnB/.rubocop.yml
Default configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/default.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/enabled.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/disabled.yml
Inspecting 1 file
Scanning /tmp/tmp.xqGCvflgnB/test.rb
undefined method `begin_type?' for :x:Symbol
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/conditional_assignment.rb:289:in `assignment_node'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/conditional_assignment.rb:236:in `check_assignment_to_condition'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/conditional_assignment.rb:222:in `block (2 levels) in <class:ConditionalAssignment>'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:41:in `block (2 levels) in on_lvasgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:96:in `with_cop_error_handling'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:40:in `block in on_lvasgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `on_lvasgn'
(eval):2:in `block in on_mlhs'
(eval):2:in `each'
(eval):2:in `on_mlhs'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:45:in `on_mlhs'
(eval):2:in `block in on_masgn'
(eval):2:in `each'
(eval):2:in `on_masgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:45:in `on_masgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/ast_node/traversal.rb:12:in `walk'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:58:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:120:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:108:in `offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:51:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:243:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:190:in `block in do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:222:in `block in iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:186:in `do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:101:in `block in file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:111:in `file_offense_cache'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:99:in `file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:90:in `process_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:68:in `block in each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `reduce'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:57:in `inspect_files'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:36:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:72:in `execute_runner'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:27:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:13:in `block in <top (required)>'
/usr/lib/ruby/2.3.0/benchmark.rb:308:in `realtime'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:12:in `<top (required)>'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:22:in `load'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:22:in `<main>'
undefined method `begin_type?' for :y:Symbol
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/conditional_assignment.rb:289:in `assignment_node'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/conditional_assignment.rb:236:in `check_assignment_to_condition'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/conditional_assignment.rb:222:in `block (2 levels) in <class:ConditionalAssignment>'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:41:in `block (2 levels) in on_lvasgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:96:in `with_cop_error_handling'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:40:in `block in on_lvasgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `on_lvasgn'
(eval):2:in `block in on_mlhs'
(eval):2:in `each'
(eval):2:in `on_mlhs'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:45:in `on_mlhs'
(eval):2:in `block in on_masgn'
(eval):2:in `each'
(eval):2:in `on_masgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:45:in `on_masgn'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/ast_node/traversal.rb:12:in `walk'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:58:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:120:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:108:in `offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:51:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:243:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:190:in `block in do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:222:in `block in iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:186:in `do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:101:in `block in file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:111:in `file_offense_cache'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:99:in `file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:90:in `process_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:68:in `block in each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `reduce'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:57:in `inspect_files'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:36:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:72:in `execute_runner'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:27:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:13:in `block in <top (required)>'
/usr/lib/ruby/2.3.0/benchmark.rb:308:in `realtime'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:12:in `<top (required)>'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:22:in `load'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:22:in `<main>'
W

Offenses:

test.rb:2:1: W: Lint/UselessAssignment: Useless assignment to variable - x. Use _ or _x as a variable name to indicate that it won't be used.

2 errors occurred:
An error occurred while Style/ConditionalAssignment cop was inspecting /tmp/tmp.xqGCvflgnB/test.rb.
An error occurred while Style/ConditionalAssignment cop was inspecting /tmp/tmp.xqGCvflgnB/test.rb.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.46.0 (using Parser 2.3.3.1, running on ruby 2.3.2 x86_64-linux)
x, y = a
^
test.rb:2:4: W: Lint/UselessAssignment: Useless assignment to variable - y. Use _ or _y as a variable name to indicate that it won't be used.
x, y = a
   ^

1 file inspected, 2 offenses detected
Finished in 0.1687188179930672 seconds

Cause

lvasgn(and other asgn nodes) don't have assignment in masgn node.

# normal assignment
x = 1
# multiple assignment
y, z = something

# AST for the above code
# s(:begin,
#   s(:lvasgn, :x,
#     s(:int, 1)),
#   s(:masgn,
#     s(:mlhs,
#       s(:lvasgn, :y),
#       s(:lvasgn, :z)),
#     s(:send, nil, :something)))

However, the cop did not handle the problem.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

👍