The pull request we use does not suffice any longer because it does not update
the version constraint on base in the Cabal file.
Generally speaking, it might be a good idea to use
https://github.com/ChrisKuklewicz/regex-tdfa/pull/16 instead of the old one,
because it looks much cleaner. (It might have the same problem w/r to base,
though).
Pinging our resident GHC 8.4.1 guru @deepfire.