inherit_mode under Naming/MethodParameterName cop is not merging AllowedNames

[bbugh]

When using inherit_mode under Naming/MethodParameterName, it doesn't seem to work. inherit_mode at the root level works as expected.

---

Expected behavior

Based on the documentation, this seems like AllowedNames should be merged, and include the original defaults.

Naming/MethodParameterName:
  inherit_mode:
    merge:
      - AllowedNames
  AllowedNames:
    - t

Setting this at the root level works - but then it affects all of the cops.

inherit_mode:
  merge:
    - AllowedNames
    
Naming/MethodParameterName:
  AllowedNames:
    - t

Actual behavior

inherit_mode under the Naming/MethodParameterName cop still overwrites the existing AllowedNames

Steps to reproduce the problem

With the latest Rubocop:

Naming/MethodParameterName:
  inherit_mode:
    merge:
      - AllowedNames
  AllowedNames:
    - t

# This results in an error, but should not
def do_something(id: 'this should work')
end

Result:

Naming/MethodParameterName: Method parameter must be at least 3 characters long.
  def do_something(id: 'this should work')
                   ^^

RuboCop version

Tested with the latest rubocop 0.89.1:

$ [bundle exec] rubocop -V
0.89.1 (using Parser 2.7.1.4, rubocop-ast 0.3.0, running on ruby 2.6.5 x86_64-darwin18)

And the current version we're using (with Naming/UncommunicativeMethodParamName):

$ [bundle exec] rubocop -V
0.76.0 (using Parser 2.6.5.0, running on ruby 2.6.5 x86_64-darwin18)

[bbugh]

@bbatsov Hi 👋 sorry to tag you, I'm sure you're very busy. I don't know who else to ping. I didn't see anything in the CoC or Contributing guidelines about who manages the issues and can be contacted.

This problem bit me at another company since the first report. I am happy to PR this if someone can confirm that it's a bug and not expected to behavior.

[jonas054]

A fix for this problem was made in #9952 and released in rubocop 1.19.0.