[GHC] #9636: Function with type error accepted

[GHC]

#9636: Function with type error accepted
-------------------------------------+-------------------------------------
       Reporter:  augustss           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 The following program is accepted by ghc even though it is clearly wrong.
 The type family T is closed so T Bool cannot ever be reduced, so it must
 be a type error.

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Err62 where

 type family T a where
     T Int = ()

 x :: T Bool
 x = undefined

 f :: T Bool -> Bool
 f _ = True

 y :: Bool
 y = f x
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9636>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler