[GHC] #14983: Have custom type errors imply Void

[GHC]

#14983: Have custom type errors imply Void
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  CustomTypeErrors
                                     |  QuantifiedConstraints wipT2893
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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Changes (by RyanGlScott):

 * keywords:  CustomTypeErrors => CustomTypeErrors QuantifiedConstraints
     wipT2893


Comment:

 Can't you just do this?

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE UndecidableSuperClasses #-}
 module Foo where

 import Data.Void
 import GHC.TypeLits

 class (forall x. x) => No where
   no :: Void

 class (TypeError a, forall x. x) => MyTypeError a

 instance MyTypeError (Text "Can't show functions") => Show (a -> b) where
   show = absurd no
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14983#comment:2>
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