cmp different from the string representation

There's an ordering difference when using the built-in <=> vs the string representation one:

> Pathname.new(File.expand_path('.')).glob('**/*.rb').sort.first(5)
=> [#<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/chain.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/error_generator.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/expect_chain_chain.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/expectation_chain.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/message_chains.rb>]
> Pathname.new(File.expand_path('.')).glob('**/*.rb').sort_by(&:to_s).first(5)
=> [#<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/chain.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/error_generator.rb>,
 #<Pathname:/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/expect_chain_chain.rb>]

And the latter corresponds to the classic alternative:

> Dir[File.expand_path('**/*.rb')].sort.first(5)
=> ["/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks.rb",
 "/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance.rb",
 "/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/chain.rb",
 "/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/error_generator.rb",
 "/Users/pirj/source/rspec-dev/repos/rspec-mocks/lib/rspec/mocks/any_instance/expect_chain_chain.rb"]

Just by looking at the cmp implementation and tests I couldn't understand, is this the designed behaviour, or the behaviour is undefined?

Semantically, it would make sense to load rspec/mocks.rb first, and then rspec/mocks/any_instance.rb, not the other way around.
Some discussion in rspec-rails regarding the issues it may cause.
rspec-rails PR to change the helper template to use Pathname#glob instead of Dir.[].

A workaround rspec-rails plans to take is to sort_by(&:to_s) to get back to the string representation-like ordering.

(semi-)Related cops:

• https://docs.rubocop.org/rubocop-rails/cops_rails.html#railsrootpathnamemethods
• https://docs.rubocop.org/rubocop/1.65/cops_lint.html#lintnondeterministicrequireorder

ext/pathname/pathname.c の cmp 実装と、test/pathname/test_pathname.rb の関連するケースを読み、issue に記載されている Pathname#<=> と文字列の順序付けの異なる結果を再現してください。意図された順序付けの動作が確立され、関連するテストまたはドキュメントにその結論が明確に反映されていれば完了です。