UselessAssignment cop chokes on ||= in argument list

[sds]

The following is valid ruby (e.g. ruby -c test.rb returns "Syntax OK").

function(foo ||= :bar)

Running Rubocop's master branch (07279a1) against this using rubocop -d test.rb results in the following:

Inspecting 1 file
Scanning /home/user/some-project/test.rb
For /home/user/some-project: configuration from /home/user/.rubocop.yml
Default configuration from /home/user/rubocop/config/default.yml
Inheriting configuration from /home/user/rubocop/config/enabled.yml
Inheriting configuration from /home/user/rubocop/config/disabled.yml
An error occurred while UselessAssignment cop was inspecting /home/user/some-project/test.rb.
undefined method `type' for nil:NilClass
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:72:in `return_value_node_of_scope'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:58:in `message_for_useless_assignment'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:37:in `block in check_for_unused_assignments'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:34:in `each'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:34:in `check_for_unused_assignments'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:26:in `block in after_leaving_scope'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:25:in `each_value'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:25:in `after_leaving_scope'
/home/user/rubocop/lib/rubocop/cop/variable_inspector/variable_table.rb:17:in `invoke_hook'
/home/user/rubocop/lib/rubocop/cop/variable_inspector/variable_table.rb:36:in `pop_scope'
/home/user/rubocop/lib/rubocop/cop/variable_inspector.rb:65:in `inspect_variables_in_scope'
/home/user/rubocop/lib/rubocop/cop/variable_inspector.rb:58:in `inspect_variables'
/home/user/rubocop/lib/rubocop/cop/lint/useless_assignment.rb:21:in `investigate'
/home/user/rubocop/lib/rubocop/cop/commissioner.rb:94:in `block (2 levels) in invoke_cops_callback'
/home/user/rubocop/lib/rubocop/cop/commissioner.rb:100:in `with_cop_error_handling'
/home/user/rubocop/lib/rubocop/cop/commissioner.rb:93:in `block in invoke_cops_callback'
/home/user/rubocop/lib/rubocop/cop/commissioner.rb:85:in `each'
/home/user/rubocop/lib/rubocop/cop/commissioner.rb:85:in `invoke_cops_callback'
/home/user/rubocop/lib/rubocop/cop/commissioner.rb:55:in `investigate'
/home/user/rubocop/lib/rubocop/cop/team.rb:42:in `inspect_file'
/home/user/rubocop/lib/rubocop/file_inspector.rb:55:in `inspect_file'
/home/user/rubocop/lib/rubocop/file_inspector.rb:27:in `block in process_files'
/home/user/rubocop/lib/rubocop/file_inspector.rb:21:in `each'
/home/user/rubocop/lib/rubocop/file_inspector.rb:21:in `process_files'
/home/user/rubocop/lib/rubocop/cli.rb:31:in `run'
bin/rubocop:14:in `block in <main>'
/home/user/.rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/benchmark.rb:296:in `realtime'
bin/rubocop:13:in `<main>'
.

1 file inspected, no offences detected

1 error occurred:
An error occurred while UselessAssignment cop was inspecting /home/user/some-project/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:
0.16.0 (using Parser 2.1.1, running on ruby 2.0.0 x86_64-linux)
Finished in 0.011428515 seconds

[bbatsov]

@yujinakayama would you have a look at this?

[yujinakayama]

OK.