[GHC] #12686: Attempt to promote a value to a type results in an internal error

[GHC]

#12686: Attempt to promote a value to a type results in an internal error
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           Reporter:  johnleo        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Trying to compile
 {{{#!hs
 {-# LANGUAGE GADTs, DataKinds, KindSignatures #-}

 data Nat = Zero | Succ Nat

 data Vec :: * -> Nat -> * where
   Nil  :: Vec a 'Zero
   Cons :: a -> Vec a n -> Vec a ('Succ n)

 data Bad = Bad {
   a :: Nat,
   b :: Vec Int a}
 }}}
 results in `error: Not in scope: type variable ‘a’`.

 Change the last structure to
 {{{#!hs
 data Bad = Bad {
   a :: Nat,
   b :: Vec Int 'a}
 }}}
 and the result is

 {{{
     • GHC internal error: ‘a’ is not in scope during type checking, but it
 passed the renamer
       tcl_env of environment: [r15Y :-> ATcTyCon Bad,
                                r17P :-> APromotionErr RecDataConPE]
     • In the second argument of ‘Vec’, namely ‘a’
       In the type ‘Vec Int a’
       In the definition of data constructor ‘Bad’
 }}}

 The user is attempting to promote a value to a type, which cannot be done,
 but the error message should be more informative.

 Reproduces in 8.0.1 and 8.1.20160916.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12686>
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