The package in bionic-proposed correctly builds from source now. I did the following to confirm that:
$ dget -u https://launchpad.net/ubuntu/+archive/primary/+sourcefiles/ruby2.5/2.5.1-1ubuntu1.7/ruby2.5_2.5.1-1ubuntu1.7.dsc $ sbuild -d bionic ruby2.5_2.5.1-1ubuntu1.7.dsc
The package in bionic-proposed correctly builds from source now. I did the following to confirm that:
$ dget -u https:/ /launchpad. net/ubuntu/ +archive/ primary/ +sourcefiles/ ruby2.5/ 2.5.1-1ubuntu1. 7/ruby2. 5_2.5.1- 1ubuntu1. 7.dsc 2.5.1-1ubuntu1. 7.dsc
$ sbuild -d bionic ruby2.5_