UnboxedTupleRep-related panic involving typeclasses

You can trigger a GHC panic by abusing UnboxedTupleRep in just the right way:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UnboxedTuples #-}

import GHC.Exts

class Wat (a :: TYPE 'UnboxedTupleRep)
deriving instance Wat (# a, b #)

$ /opt/ghc/8.0.1/bin/ghc Bug.hs 
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
        newTyConEtadArity (#,#)

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

This is probably a leak that #11723 (closed) didn't plug, as it only considered datatypes, not typeclasses.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire
Operating system
Architecture