[GHC] #7729: GHC panics. Invalid core

[GHC]

#7729: GHC panics. Invalid core
-----------------------------+----------------------------------------------
Reporter:  Khudyakov         |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.2             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------
 Following code snippet triggers panic:

 {{{
 {-# LANGUAGE FlexibleContexts, TypeFamilies #-}
 module Monad where

 class Monad m => PrimMonad m where
   type PrimState m

 class MonadTrans t where
   lift :: Monad m => m a -> t m a

 class (PrimMonad (BasePrimMonad m), Monad m) => MonadPrim m where
   type BasePrimMonad m :: * -> *
   liftPrim :: BasePrimMonad m a -> m a


 newtype Rand m a = Rand {
   runRand :: Maybe (m ()) -> m a
   }

 instance (Monad m) => Monad (Rand m) where
   return           = Rand . const . return
   (Rand rnd) >>= f = Rand $ \g -> (\x -> runRand (f x) g) =<< rnd g

 instance MonadTrans Rand where
   lift = Rand . const

 instance MonadPrim m => MonadPrim (Rand m) where
   type BasePrimMonad (Rand m) = BasePrimMonad m
   liftPrim = liftPrim . lift
 }}}

 GHC 7.6.2 panics
 {{{
 $ ghc-7.6.2 -c Monad.hs
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.2 for x86_64-unknown-linux):
         cgLookupPanic (probably invalid Core; try -dcore-lint)
     $dMonadTrans{v ahe} [lid]
     static binds for:
     local binds for:
     main:Monad.lift{v ra} [gid[ClassOp]]
     main:Monad.liftPrim{v rc} [gid[ClassOp]]
     main:Monad.$p1PrimMonad{v rgv} [gid[ClassOp]]
     main:Monad.$p1MonadPrim{v rgA} [gid[ClassOp]]
     main:Monad.$p2MonadPrim{v rgB} [gid[ClassOp]]

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

 whereas 7.4.2 reports type error
 {{{
 $ ghc-7.4.2 -c Monad.hs

 Monad.hs:28:14:
     Occurs check: cannot construct the infinite type: m0 = t0 m0
     Expected type: BasePrimMonad (Rand m) a -> Rand m a
       Actual type: m0 a -> Rand m a
     In the expression: liftPrim . lift
     In an equation for `liftPrim': liftPrim = liftPrim . lift
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7729>
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