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

[GHC]

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

 Here's a somewhat more minimal example:

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

 import Data.Coerce

 newtype Wrap f a = Wrap (f a)

 class C f where
   c :: f a

 instance C f => C (Wrap f) where
   c = coerce @(forall (a :: k). f a)
              @(forall (a :: k). C f a)
       c
 }}}

 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive Bug.hs
 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:15:29: error:
     • GHC internal error: ‘k’ is not in scope during type checking, but it
 passed the renamer
       tcl_env of environment: [a1tN :-> Type variable ‘a’ = a,
                                a1tQ :-> Type variable ‘f’ = f, a1uU :->
 Type variable ‘k’ = k]
     • In the kind ‘k’
       In the type ‘(forall (a :: k). f a)’
       In the expression:
         coerce @(forall (a :: k). f a) @(forall (a :: k). C f a) c
    |
 15 |   c = coerce @(forall (a :: k). f a)
    |
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13738#comment:1>
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