As far as I can tell, the --cabal-file FILE flag requires that FILE resolve to a non-hidden (ie, no leading dot) file with the extension .cabal in the current directory.
Why are there any requirements at all on the location or name of the cabal file? The user is explicitly specifying a file so I would think it would accept anything… you may ask why this matters: I would like to do some tooling that auto-generates the cabal file into a hidden directory. Thanks!