[GHC] #12867: Internal error while typechecking invalid program

[GHC]

#12867: Internal error while typechecking invalid program
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           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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 While trying to reduce #12866 I encountered this program,
 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}

 module T12866 where

 type Test2 a = (Eq (TestM a))

 class Test a where
     type TestM :: *
 }}}

 which gives rise to this internal GHC error during typechecking,
 {{{
 $ ghc Hi2.hs

 Hi2.hs:7:21: error:
     • GHC internal error: ‘TestM’ is not in scope during type checking,
 but it passed the renamer
       tcl_env of environment: [ap0 :-> Type variable ‘a’ = a]
     • In the first argument of ‘Eq’, namely ‘TestM a’
       In the type ‘Eq (TestM a)’
       In the type declaration for ‘Test2’

 Hi2.hs:9:1: error:
     • The associated type ‘TestM’
       mentions none of the type or kind variables of the class ‘Test a’
     • In the class declaration for ‘Test’
 }}}

 Clearly the program is bogus, but the first error message is quite
 suspicious.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12867>
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