In Ruby >= 3.4, ReplTypeCompletor is a bundled gem, installed by default.
This pull request changes the default completor to type-completor if RUBY_VERSION >= 3.4.
If ReplTypeCompletor is not available (not installed or not added to Gemfile), fallbacks to regexp-baased completion.
The default isstill :regexp if RUBY_VERSION < 3.4

Run with bundler

With bundler and when repl_type_completor is not added to Gemfile, IRB will use regexp-based completion.
IRB shows warning message only If --type-completor or IRB.conf[:COMPLETOR] = :type is explicitly specified.

$ ruby -v
ruby 3.4.0dev

$ irb
irb(main):001> irb_info
(TypeCompletor)

$ echo "gem 'irb', path: 'path/to/irb'" > Gemfile

$ bundle exec irb
(no waarning)
irb(main):001> irb_info
(RegexpCompletor)

$ bundle exec irb --type-completor
TypeCompletor requires `gem repl_type_completor`: cannot load such file -- repl_type_completor
irb(main):001> irb_info
(RegexpCompletor)

Restriction with rdbg

ReplTypeCompletor uses background thread to load RBS type information which could take time.
Debug gem stops all other threads, so if IRB enters debug mode before loading RBS completes, type completor can't use type information.
Even without type information, type completor can still complete as good or better than regexp-based completion.