(https://github.com/ruby/irb/pull/897) Some features of irb do not work properly when using the old rdoc. I have compared several major versions and found that it works as intended from 4.0.0. This problem occurs when there is a Gemfile.lock is installed with the old rdoc. I don't know why this Gemfile.lock installs an older rdoc than the ruby bundled rdoc, but specifying the version in the gemspec will at least prevent the problem. NOTE: ruby/irb#704 problem does not occur with this change. The following is test code. ``` ### Usage: ruby __FILE__.rb # # input RDoc and Tab # >> RDoc<Tab> # ### Expect: Display document of RDoc ### Actual: <internal:marshal>:34:in `load': instance of RDoc::Constant needs to have method `marshal_load' (TypeError) require "bundler/inline" gemfile(true) do source "https://rubygems.org" git_source(:github) { |repo| "https://github.com/#{repo}.git" } gem 'irb' # gem 'rdoc', '~> 4.0.0' gem 'rdoc', '~> 3.12.0' end require 'rdoc' require 'irb' IRB.start ``` https://github.com/ruby/irb/commit/1a1fbba020