Consistently referer dependencies
It seems that the default gems interdependencies in Ruby are mess. Years ago, when JSON was merged into StdLib, there was big movement and everybody dropped their references to JSON "because it is part of StdLib and therefore it is not needed". I always thought that removing the references was mistake.
Now, there are other interesting cases. Let me name two I know about:
REXML is going to be removed from default gems in Ruby 2.8, so some packages already started to introduce the dependency explicitly 1. So once somebody uses Kramdown on older Ruby, the external REXML of whatever version is going to be used.
There are also gems in StdLib, such as IRB, which are specifying their dependencies in .gemspec file.
This is unfortunately causing very inconsistent user experience, depending if RubyGems are enabled/disabled, if one is using Bundler or not, if somebody explicitly states something somewhere and what dependencies are transitively pulled in.
I would really appreciate, if Ruby upstream finally paid attention to this problem. My suggestion is that if some gem depends on some other gem, this dependency should be always explicitly stated in the .gemspec file. This would provide clear precedence and guideline to others. This would save all possible surprises and hidden issues, suddenly using dependency of different version, which is pulled in transitively.