Style/RedundantFetchBlock causes undefined method error.

[richardkmichael]

I am enabling the recent Style/RedundantFetchBlock and {}.fetch(:key) { } (here in Rails) causes an undefined method error.

Expected behavior

As with other suggestions to #fetch() with a block, Rubocop should suggest the implicit nil returned by the block:

{}.fetch(:key, nil)

Actual behavior

 $ rubocop --force-default-config -d --only 'Style/RedundantFetchBlock' empty_block.rb
Default configuration from /Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/config/default.yml
Inspecting 1 file
Scanning ./empty_block.rb
An error occurred while Style/RedundantFetchBlock cop was inspecting ./empty_block.rb:2:0.
undefined method `basic_literal?' for nil:NilClass
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/style/redundant_fetch_block.rb:76:in `basic_literal?'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/style/redundant_fetch_block.rb:53:in `redundant_fetch_block_candidate?'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/style/redundant_fetch_block.rb:48:in `on_block'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:56:in `block (2 levels) in trigger_responding_cops'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:115:in `with_cop_error_handling'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:55:in `block in trigger_responding_cops'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:54:in `each'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:54:in `trigger_responding_cops'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:32:in `block (2 levels) in <class:Commissioner>'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-ast-0.1.0/lib/rubocop/ast/traversal.rb:59:in `block in on_begin'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-ast-0.1.0/lib/rubocop/ast/traversal.rb:59:in `each'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-ast-0.1.0/lib/rubocop/ast/traversal.rb:59:in `on_begin'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:33:in `block (2 levels) in <class:Commissioner>'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-ast-0.1.0/lib/rubocop/ast/traversal.rb:14:in `walk'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/commissioner.rb:43:in `investigate'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/team.rb:149:in `investigate'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/team.rb:137:in `offenses'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cop/team.rb:75:in `inspect_file'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:299:in `inspect_file'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:247:in `block in do_inspection_loop'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:279:in `block in iterate_until_no_changes'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:272:in `loop'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:272:in `iterate_until_no_changes'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:243:in `do_inspection_loop'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:122:in `block in file_offenses'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:146:in `file_offense_cache'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:120:in `file_offenses'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:111:in `process_file'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:90:in `block in each_inspected_file'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:89:in `each'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:89:in `reduce'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:89:in `each_inspected_file'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:78:in `inspect_files'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/runner.rb:39:in `run'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cli/command/execute_runner.rb:21:in `execute_runner'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cli/command/execute_runner.rb:13:in `run'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cli/command.rb:10:in `run'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cli/environment.rb:17:in `run'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cli.rb:65:in `run_command'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cli.rb:72:in `execute_runners'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/lib/rubocop/cli.rb:41:in `run'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/exe/rubocop:13:in `block in <top (required)>'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'
/Users/testuser/.rbenv/versions/2.5.8/lib/ruby/gems/2.5.0/gems/rubocop-0.86.0/exe/rubocop:12:in `<top (required)>'
/Users/testuser/.rbenv/versions/2.5.8/bin/rubocop:23:in `load'
/Users/testuser/.rbenv/versions/2.5.8/bin/rubocop:23:in `<main>'
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Style/RedundantFetchBlock cop was inspecting ./empty_block.rb:2:0.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop-hq/rubocop/issues

Mention the following information in the issue report:
0.86.0 (using Parser 2.7.1.4, rubocop-ast 0.1.0, running on ruby 2.5.8 x86_64-darwin18)
Finished in 0.13840800000252784 seconds

Steps to reproduce the problem

echo '{}.fetch(:key) { }' > empty_block.rb
rubocop --force-default-config -d --only 'Style/RedundantFetchBlock' empty_block.rb

RuboCop version

0.86.0 (using Parser 2.7.1.4, rubocop-ast 0.1.0, running on ruby 2.5.8 x86_64-darwin18)

Presumably related to the helpful cop added in #8147 by @fatkodima (thank you!).

[fatkodima]

Thanks for reporting this. Will look at it tomorrow, if no one gets ahead of me 😃 .

[jonathanhefner]

I believe this was already fixed by #8197.

[richardkmichael]

@fatkodima You're welcome. It was brought to my attention in rails/rails#39765, so you might consider bumping your own rails/rails#39527 when fixed so Rails could use it.

[richardkmichael]

@jonathanhefner Thanks again. :-) It is fast today!