[GHC] #12866: GHC internal error while building darcsden

[GHC]

#12866: GHC internal error while building darcsden
-------------------------------------+-------------------------------------
        Reporter:  simonmic          |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12867            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => fixed
 * related:   => #12867


Comment:

 Here's (what I believe to be) a minimal test case:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE ConstraintKinds #-}
 -- {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE ImplicitParams #-}

 module T12866 where

 import Control.Monad.IO.Class

 type BT bt = (BackendTransient bt, ?backendTransient :: bt)
 type BTIO bt = (BT bt, MonadIO (BackendTransientM bt))

 class (Functor (BackendTransientM bt), Monad (BackendTransientM bt))
      => BackendTransient bt where

     type BackendTransientM bt :: * -> *
 }}}

 Running this with GHC 8.0.1/8.0.2 yields:

 {{{
 [1 of 1] Compiling T12866           ( Bug.hs, interpreted )

 Bug.hs:11:33: error:
     • GHC internal error: ‘BackendTransientM’ is not in scope during type
 checking, but it passed the renamer
       tcl_env of environment: [a142 :-> Type variable ‘bt’ = bt]
     • In the first argument of ‘MonadIO’, namely ‘BackendTransientM bt’
       In the type ‘(BT bt, MonadIO (BackendTransientM bt))’
       In the type declaration for ‘BTIO’

 Bug.hs:13:1: error:
     • Non type-variable argument
         in the constraint: Functor (BackendTransientM bt)
       (Use FlexibleContexts to permit this)
     • In the context: (Functor (BackendTransientM bt),
                        Monad (BackendTransientM bt))
       While checking the super-classes of class ‘BackendTransient’
       In the class declaration for ‘BackendTransient’
 }}}

 But in GHC HEAD, there's no internal error:

 {{{
 [1 of 1] Compiling T12866           ( Bug.hs, interpreted )

 Bug.hs:11:1: error:
     • Non type-variable argument
         in the constraint: MonadIO (BackendTransientM bt)
       (Use FlexibleContexts to permit this)
     • In the type synonym declaration for ‘BTIO’

 Bug.hs:13:1: error:
     • Non type-variable argument
         in the constraint: Functor (BackendTransientM bt)
       (Use FlexibleContexts to permit this)
     • In the context: (Functor (BackendTransientM bt),
                        Monad (BackendTransientM bt))
       While checking the super-classes of class ‘BackendTransient’
       In the class declaration for ‘BackendTransient’
 }}}

 The workaround is simple: just enable `FlexibleContexts`. That makes it
 compile without issue on GHC 8.0.1, 8.0.2, and HEAD.

 For completeness's sake, I was able to reproduce the issue when building
 `darcsden` with GHC 8.0.1, and I confirmed that enabling
 `FlexibleContexts` at the top of the file made the internal error go away.
 I don't have time to build everything again with GHC HEAD, but I think the
 story would be pretty similar.

 In conclusion, this is a much more elaborate example of #12867.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12866#comment:5>
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