extend_config vs .rubocop_todo.yml

[palkan]

Standard: 1.24.3
RuboCop: 1.44.1

I've been working on the blog post update, and found that it's not possible to migrate the project from .rubocop_todo.yml to standardrb without re-generating the TODO file with standard.

Reproduction script

Given the source code file (a.rb):

module A
  def foo; 'bar'; end
end

And the following configuration files:

# .rubocop.yml
inherit_mode:
  merge:
    - Exclude

require:
  - standard

inherit_gem:
  standard: config/base.yml

inherit_from:
  - .rubocop_todo.yml

AllCops:
  NewCops: disable
  SuggestExtensions: false
  TargetRubyVersion: 2.6

# .standard.yml
parallel: true
format: progress

extend_config:
  - .rubocop_todo.yml

The TODO file has been generated with RuboCop and contains:

Style/SingleLineMethods:
  Exclude:
    - 'a.rb'

Style/StringLiterals:
  EnforcedStyle: single_quotes

Adding .rubocop_todo.yml has no effect

I expect running standardrb to show no errors, but:

$ be standardrb

Inspecting 1 file
C

Offenses:

a.rb:2:3: C: [Correctable] Style/SingleLineMethods: Avoid single-line method definitions.
  def foo; 'bar'; end
  ^^^^^^^^^^^^^^^^^^^
a.rb:2:12: C: [Correctable] Style/StringLiterals: Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.
  def foo; 'bar'; end
           ^^^^^

Running rubocop works as expected: 1 file, no offenses detected.

Possible solutions

I think, it's okay to instruct users to run standardrb --generate-todo instead of loading the existing .rubocop_todo.yml. Thus, just a note in the documentation could be enough.

However, it's not clear why extending the configuration doesn't work in this case. Maybe, something related to the inherit_mode?

[searls]

I think that extend_config won't work here because the thing that merges in your extension will filter out all top-level keys that match the names of cops included in rubocop+performance. It does this to prevent people from extending the config so as to change the rules.

I think updating the doc to point people to standardrb --generate-todo is probably appropriate based on my read of what you want