Re: [GHC] #15052: DeriveAnyClass instances may skip TypeError constraints

18 Apr 2018

      #15052: DeriveAnyClass instances may skip TypeError constraints
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        Reporter:  jcpetruzza        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      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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Comment (by jcpetruzza):

 Sorry that I wasn't clear. I meant, using `instance` and `deriving
 instance` without providing the instance head.

 {{{#!haskell
 {-# LANGUAGE DeriveAnyClass       #-}
 {-# LANGUAGE DataKinds            #-}
 {-# LANGUAGE FlexibleContexts     #-}
 {-# LANGUAGE StandaloneDeriving   #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE UndecidableSuperClasses #-}
 module T where

 import GHC.TypeLits(TypeError, ErrorMessage(..))

 class TypeError ('Text "BOOM") => C a where
     f :: a -> ()
     f _ = ()

 data T1 = T1
    deriving C

 data T2 = T2
 instance C T2 -- won't typecheck

 data T3 = T3
 deriving instance C T3 -- won't typecheck
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15052#comment:2
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