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

18 Apr 2018

      #15052: DeriveAnyClass instances may skip TypeError constraints
-------------------------------------+-------------------------------------
        Reporter:  jcpetruzza        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by jcpetruzza):

 I don't disagree with anything you are saying.

 The issue is that `deriving` is behaving differently from `instance` and
 from `deriving instance`. Consider this hopefully more clear example:

 {{{#!haskell
 {-# LANGUAGE ConstraintKinds      #-}
 {-# LANGUAGE DataKinds            #-}
 {-# LANGUAGE DefaultSignatures    #-}
 {-# LANGUAGE DeriveAnyClass       #-}
 {-# LANGUAGE TypeFamilies         #-}
 {-# LANGUAGE StandaloneDeriving   #-}
 {-# LANGUAGE UndecidableInstances #-}
 module T2 where

 import Data.Kind(Constraint)
 import GHC.TypeLits(TypeError, ErrorMessage(..))

 class D a where
   f :: a

   default f :: DeferError a  => a
   f = error "unreachable"

 type family DeferError a :: Constraint where
     DeferError a = TypeError ('Text "Boom")

 data X
   deriving D             -- < -- ACCEPTED

 data Y
 instance D Y             -- < -- REJECTED

 data Z
 deriving instance D Z    -- < -- REJECTED
 }}}

 Do you aggree that either the three instances should be accepted here or
 the three instances should be rejected?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15052#comment:11
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler