An error occurred while Style/SwapValues cop was inspecting bug.rb

[pvdb]

The following error message is printed when processing an otherwise perfectly valid Ruby file:

1 error occurred:
An error occurred while Style/SwapValues cop was inspecting /private/tmp/rubocop/bug.rb:3:5.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop/rubocop/issues

This appears to be caused - after a bit of digging - by the following exception:

An error occurred while Style/SwapValues cop was inspecting /private/tmp/rubocop/bug.rb:3:5.
undefined method `type' for :do_it:Symbol
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/style/swap_values.rb:61:in `simple_assignment?'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/style/swap_values.rb:53:in `swapping_values?'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/style/swap_values.rb:35:in `on_asgn'
...

---

Expected behavior

Consider the following Ruby script named bug.rb:

# frozen_string_literal: true

def (blegga = Object.new).do_it
  puts 42
end

blegga.do_it

It is completely valid (but possibly questionable) 😅 Ruby syntax:

> ruby -W2 -c bug.rb
Syntax OK
> _

It runs correctly:

> ruby bug.rb
42
> _

It lints correctly:

> rubocop --lint bug.rb
Inspecting 1 file
.

1 file inspected, no offenses detected
> _

Actual behavior

The Style/SwapValues cop chokes on it:

> rubocop --only Style/SwapValues bug.rb
An error occurred while Style/SwapValues cop was inspecting /private/tmp/rubocop/bug.rb:3:5.
To see the complete backtrace run rubocop -d.
Inspecting 1 file
An error occurred while Style/SwapValues cop was inspecting /private/tmp/rubocop/bug.rb:3:5.
To see the complete backtrace run rubocop -d.
.

1 file inspected, no offenses detected

...

> _

Steps to reproduce the problem

1. create a file named bug.rb with the above content
2. run rubocop --only Style/SwapValues bug.rb

Alternatively, run the following one-liner (in a shell that supports so-called "here strings"):

rubocop --debug --stdin /dev/null --only Style/SwapValues <<<"def (_ = Object.new).do_it() puts 42; end"

Running this one-liner includes the stacktrace, and points at the root cause of the bug:

For /private/tmp/rubocop: Default configuration from /usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/config/default.yml
Inspecting 1 file
Scanning /dev/null
An error occurred while Style/SwapValues cop was inspecting /dev/null:1:5.
undefined method `type' for :do_it:Symbol
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/style/swap_values.rb:61:in `simple_assignment?'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/style/swap_values.rb:53:in `swapping_values?'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/style/swap_values.rb:35:in `on_asgn'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:100:in `public_send'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:100:in `block (2 levels) in trigger_responding_cops'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:160:in `with_cop_error_handling'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:99:in `block in trigger_responding_cops'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:98:in `each'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:98:in `trigger_responding_cops'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:69:in `on_lvasgn'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-ast-1.15.1/lib/rubocop/ast/traversal.rb:155:in `on_defs'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:71:in `on_defs'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-ast-1.15.1/lib/rubocop/ast/traversal.rb:20:in `walk'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/commissioner.rb:86:in `investigate'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/team.rb:155:in `investigate_partial'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cop/team.rb:83:in `investigate'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:309:in `inspect_file'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:253:in `block in do_inspection_loop'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:287:in `block in iterate_until_no_changes'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:280:in `loop'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:280:in `iterate_until_no_changes'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:249:in `do_inspection_loop'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:130:in `block in file_offenses'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:155:in `file_offense_cache'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:129:in `file_offenses'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:120:in `process_file'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:101:in `block in each_inspected_file'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:100:in `each'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:100:in `reduce'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:100:in `each_inspected_file'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:86:in `inspect_files'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/runner.rb:47:in `run'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli/command/execute_runner.rb:26:in `block in execute_runner'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli/command/execute_runner.rb:52:in `with_redirect'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli/command/execute_runner.rb:25:in `execute_runner'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli/command/execute_runner.rb:17:in `run'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli/command.rb:11:in `run'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli/environment.rb:18:in `run'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli.rb:71:in `run_command'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli.rb:78:in `execute_runners'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/lib/rubocop/cli.rb:47:in `run'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/exe/rubocop:12:in `block in <top (required)>'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/2.6.0/benchmark.rb:308:in `realtime'
/usr/local/var/rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/rubocop-1.25.0/exe/rubocop:12:in `<top (required)>'
/usr/local/var/rbenv/versions/2.6.9/bin/rubocop:23:in `load'
/usr/local/var/rbenv/versions/2.6.9/bin/rubocop:23:in `<main>'
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Style/SwapValues cop was inspecting /dev/null:1:5.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop/rubocop/issues

Mention the following information in the issue report:
1.25.0 (using Parser 3.1.0.0, rubocop-ast 1.15.1, running on ruby 2.6.9 x86_64-darwin19)
Finished in 0.12142199999652803 seconds

RuboCop version

> rubocop -V
1.25.0 (using Parser 3.1.0.0, rubocop-ast 1.15.1, running on ruby 2.6.9 x86_64-darwin19)
> _