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

Read the cmp implementation in ext/pathname/pathname.c and the related cases in test/pathname/test_pathname.rb, then reproduce the differing Pathname#<=> and string-order results described in the issue. Done means the intended ordering behavior is established and the relevant tests or documentation clearly reflect that conclusion.