a.casecmp(b).zero? is not an universal replacement for a.downcase == b.downcase

casecmp only supports characters A-Za-z. downcase supports special characters as well "ÄÖÜ".downcase == "äöü"

Expected behavior

Rubocop does not change the semantics of my program.

Actual behavior

Rubocop breaks my code.

Steps to reproduce the problem

foo.rb

# frozen_string_literal: true

if 'Grün'.downcase == 'GRÜN'.downcase
  puts 'this should be the same'
else
  puts 'rubocop broke my code'
end

rubocop -a foo.rb

foo.rb

# frozen_string_literal: true

if 'Grün'.casecmp('GRÜN').zero?
  puts 'this should be the same'
else
  puts 'rubocop broke my code'
end

RuboCop version

> bundle exec rubocop -V
0.80.0 (using Parser 2.7.0.3, running on ruby 2.5.3 x86_64-linux)