Style/MinMax cop fails to detect error

[asafbrukarz]

Given a file with

class MinMaxRange
  def min
    1
  end

  def max
    2
  end

  def to_s
    [min, max].uniq.join('-')
  end
end

Rubocop fails with:

Scanning minmax.rb
An error occurred while Style/MinMax cop was inspecting minmax.rb:11:4.
undefined method `source' for nil:NilClass
/.gem/ruby/2.4.2/gems/rubocop-0.51.0/lib/rubocop/cop/style/min_max.rb:38:in `message'

---

Expected behavior

Rubocop should either ignore this case, or suggest correcting to self.minmax...

Actual behavior

Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.51.0 (using Parser 2.4.0.2, running on ruby 2.4.2 x86_64-darwin16)

Steps to reproduce the problem

Run rubocop 0.51.0 on the above file with a default configuration

RuboCop version

$ rubocop -V
0.51.0 (using Parser 2.4.0.2, running on ruby 2.4.2 x86_64-darwin16)

[pocke]

Minimum code and stack trace.

[min, max]

$ rubocop -d
For /tmp/tmp.kaKlYAH35t: configuration from /home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/config/default.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/config/enabled.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/config/disabled.yml
Inspecting 1 file
Scanning /tmp/tmp.kaKlYAH35t/test.rb
undefined method `source' for nil:NilClass
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/style/min_max.rb:38:in `message'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/style/min_max.rb:25:in `block in on_array'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/style/min_max.rb:34:in `min_max_candidate'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/style/min_max.rb:21:in `on_array'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/commissioner.rb:44:in `block (2 levels) in on_array'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/commissioner.rb:109:in `with_cop_error_handling'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/commissioner.rb:43:in `block in on_array'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/commissioner.rb:42:in `each'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/commissioner.rb:42:in `on_array'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/ast/traversal.rb:12:in `walk'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/commissioner.rb:60:in `investigate'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/team.rb:114:in `investigate'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/team.rb:102:in `offenses'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cop/team.rb:44:in `inspect_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:258:in `inspect_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:205:in `block in do_inspection_loop'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:237:in `block in iterate_until_no_changes'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:230:in `loop'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:230:in `iterate_until_no_changes'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:201:in `do_inspection_loop'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:111:in `block in file_offenses'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:121:in `file_offense_cache'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:109:in `file_offenses'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:100:in `process_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:78:in `block in each_inspected_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:75:in `each'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:75:in `reduce'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:75:in `each_inspected_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:67:in `inspect_files'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/runner.rb:39:in `run'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cli.rb:128:in `execute_runner'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cli.rb:60:in `execute_runners'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/lib/rubocop/cli.rb:31:in `run'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/bin/rubocop:13:in `block in <top (required)>'
/usr/lib/ruby/2.4.0/benchmark.rb:308:in `realtime'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.51.0/bin/rubocop:12:in `<top (required)>'
/home/pocke/.gem/ruby/2.4.0/bin/rubocop:23:in `load'
/home/pocke/.gem/ruby/2.4.0/bin/rubocop:23:in `<main>'
.

1 file inspected, no offenses detected
Finished in 0.17434273899561958 seconds