An unfortunate bug in rdoc-5.0 causes a build failure on epel8. This is
fixed in ruby-2.6/rdoc-6.0.2¹. Until/unless the fix is backported to
RHEL-8 (which seems unlikely), we can only patch around this.
Thanks, as usual, to Dan Allen for going above and beyond to patch this
upstream.
¹ https://github.com/ruby/rdoc/pull/586https://github.com/ruby/rdoc/commit/a1631aa