This is cool! A frequent problem I've had with bundler is when the Gemfile lists a newer version of a gem (stringio) but the older version has already been activated. If you can get rid of such errors I would be so happy! Also I've had to frequently update bundler too often when updating a Rails site.
Yes, this is due to the effort to extract stdlib code into gems https://stdgems.org/
In your case, StringIO use to just be stdlib code so bundler (or rubygems) uses it. Later on it became a gem, so by requiring it before reading the Gemfile, bundler run into this problem of already having loaded the wrong version.
Everytime this happens the bundler team has to modify bundler, and as a user the fix is to upgrade bundler.
