Overlapping type families, segafult

Not entirely sure what's going on here. I don't think this should type check; it appears to segfault whilst calling show on the wrong type.

This is probably not the absolute minimum required to reproduce.

I have reproduced on 7.8.3 and 7.9.20140727.

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE OverlappingInstances #-}
import Data.Monoid

class C x where
    data D x :: *
    makeD :: D x

instance Monoid x => C x where
    data D x = D1 (Either x ())
    makeD = D1 (Left mempty)

instance (Monoid x, Monoid y) => C (x, y) where
    data D (x,y) = D2 (x,y)
    makeD = D2 (mempty, mempty)

instance Show x => Show (D x) where
    show (D1 x) = show x


main = print (makeD :: D (String, String))

This does not segfault if you add:

  instance (Show x, Show y) => Show (D (x,y)) where
      show (D2 x) = show x

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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