[GHC] #13738: TypeApplications-related GHC internal error

22 May 2017


      #13738: TypeApplications-related GHC internal error
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  TypeApplications                   |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This is reproducible with GHC 8.0.1, 8.0.2, 8.2.1, and HEAD:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 module Bug where

 import Data.Coerce

 class MFunctor t where
     hoist :: (Monad m) => (forall a . m a -> n a) -> t m b -> t n b

 newtype TaggedTrans tag trans m a = TaggedTrans (trans m a)

 instance MFunctor trans => MFunctor (TaggedTrans tag trans) where
  hoist = coerce
           @(forall (m :: * -> *)
                    (n :: * -> *)
                    (b :: k).
             Monad m =>
             (forall (a :: *).
              m a -> n a)
             -> trans m b -> trans n b)
           @(forall (m :: * -> *)
                    (n :: * -> *)
                    (b :: k).
             Monad m =>
             (forall (a :: *).
              m a -> n a)
             -> TaggedTrans tag trans m b
                -> TaggedTrans tag trans n b)
           hoist
 }}}

 {{{
 GHCi, version 8.3.20170516: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:18:26: error:
     • GHC internal error: ‘k’ is not in scope during type checking, but it
 passed the renamer
       tcl_env of environment: [a1tR :-> Type variable ‘m’ = m,
                                a1tS :-> Type variable ‘n’ = n, a1tT :->
 Type variable ‘b’ = b,
                                a1tV :-> Type variable ‘trans’ = trans,
                                a1tW :-> Type variable ‘tag’ = tag, a1tX
 :-> Type variable ‘m’ = m,
                                a1tY :-> Type variable ‘n’ = n, a1KE :->
 Type variable ‘k’ = k,
                                a1KF :-> Type variable ‘k’ = k]
     • In the kind ‘k’
       In the type ‘(forall (m :: * -> *) (n :: * -> *) (b :: k).
                     Monad m =>
                     (forall (a :: *). m a -> n a) -> trans m b -> trans n
 b)’
       In the expression:
         coerce
           @(forall (m :: * -> *) (n :: * -> *) (b :: k).
             Monad m => (forall (a :: *). m a -> n a) -> trans m b -> trans
 n b)
           @(forall (m :: * -> *) (n :: * -> *) (b :: k).
             Monad m =>
             (forall (a :: *). m a -> n a)
             -> TaggedTrans tag trans m b -> TaggedTrans tag trans n b)
           hoist
    |
 18 |                    (b :: k).
    |                          ^
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13738
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