Projects like Rails use thor by require 'thor/group' rather than require 'thor'. This can cause a problem because the deprecation_warning method added in version 1.0.0 is defined in "ib/thor.rb", which doesn't get required, but is referenced in "lib/thor/base.rb" and "lib/thor/parser/option.rb".

The safest solution would be to move the deprecation_warning method to "lib/thor/base.rb" and let the Thor class get it when "thor/base" is required.

A temporary workaround to get thor 1.0.0 working is to add an explicit gem 'thor' reference to your Gemfile, which will require "thor" as a side effect.