[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
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by johnleo:

@@ -38,1 +38,1 @@
- Reproduces in 8.0.1 and 8.1.20160916.
+ Reproduces in 8.0.1 and 8.1.20161010.

New description:

 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 8.1.20161010.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12686#comment:1>
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