[GHC] #15215: GHC HEAD internal error when FlexibleContexts isn't enabled

2 Jun 2018

      #15215: GHC HEAD internal error when FlexibleContexts isn't enabled
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
  (Type checker)                     |
           Keywords:                 |  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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 The following code produces a GHC internal error on GHC HEAD:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Data.Kind

 data A :: Type -> Type where
   MkA :: Show (Maybe a) => A a

 data SA :: forall a. A a -> Type where
   SMkA :: SA MkA
 }}}
 {{{
 $ ~/Software/ghc5/inplace/bin/ghc-stage2 Bug.hs[1 of 1] Compiling Bug
 ( Bug.hs, Bug.o )

 Bug.hs:9:3: error:
     • Non type-variable argument in the constraint: Show (Maybe a)
       (Use FlexibleContexts to permit this)
     • In the definition of data constructor ‘MkA’
       In the data type declaration for ‘A’
   |
 9 |   MkA :: Show (Maybe a) => A a
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Bug.hs:12:14: error:
     • GHC internal error: ‘MkA’ is not in scope during type checking, but
 it passed the renamer
       tcl_env of environment: [rqU :-> ATcTyCon SA :: forall a. A a -> *,
                                rsV :-> APromotionErr RecDataConPE]
     • In the first argument of ‘SA’, namely ‘MkA’
       In the type ‘SA MkA’
       In the definition of data constructor ‘SMkA’
    |
 12 |   SMkA :: SA MkA
    |              ^^^
 }}}

 Enabling `FlexibleContexts` causes the internal error to go away.

 This is a regression from GHC 8.4, which does not give an internal error.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15215
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