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

mentioned in issue #11509 (closed)

changed weight to 5

added Tbug Trac import labels

assigned to @rae

Urgh. UnboxedTupleRep is a lie, and liars never prosper.

This falls in my pile, but it will be a little while until I get to it, so if you (for any value of "you") are inspired, go for it.

Trac metadata

Trac field	Value
Differential revisions	- → D2557

changed milestone to %8.0.2

D2557 has been merged to master as [changeset:"23cf32da/ghc" 23cf32da/ghc].

added 1 deleted label

Merged to ghc-8.0 as c448d551.

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

mentioned in issue #13154 (closed)

mentioned in commit 9cb289ab

added compiler crash label

added Pnormal label