GHC panic on type wildcard in left-hand side of data family
While playing around with data families I received the following output by ghc/ghci:
ghc: panic! (the 'impossible' happened) (GHC version 7.10.3 for x86_64-unknown-linux): rnHsTyKi HsWildcardTy Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
I had accidentally left a wildcard
_ on the left-hand side of a data instance definition.
To reproduce enter the following code into a file and try to load that file into ghci, or try to compile it with ghc:
data family Bug x data instance Bug _ = Bug
The expected behavior would be a (non-panic) error message of some sort.