[GHC] #15434: DerivingVia (and perhaps even GND) works badly with DeriveGeneric

24 Jul 2018

      #15434: DerivingVia (and perhaps even GND) works badly with DeriveGeneric
-------------------------------------+-------------------------------------
           Reporter:  konn           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:
           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:
-------------------------------------+-------------------------------------
 `DerivingVia` together with `DeriveGeneric` can generate wrong instances
 for `Generic`.

 Consider the following:

 {{{#!haskell
 {-# LANGUAGE DeriveGeneric, DerivingStrategies, DerivingVia, GADTs
 #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies,
 UndecidableInstances #-}
 module Data.Foldable.Bad where
 import GHC.Generics

 newtype Bad a = Bad a deriving (Generic)
 data Foo = Foo Int
   deriving (Read, Show, Eq, Ord)
   deriving (Generic) via Bad Foo
 }}}

 which gives the following representation, which is considered to be wrong
 for `Foo`:

 {{{#!haskell
 ghci> from $ Foo 12
 M1 {unM1 = M1 {unM1 = M1 {unM1 = K1 {unK1 = Foo 12}}}}
 ghci> :t it
 it
   :: D1
        ('MetaData "Bad" "Data.Foldable.Bad" "main" 'True)
        (C1
           ('MetaCons "Bad" 'PrefixI 'False)
           (S1
              ('MetaSel
                 'Nothing 'NoSourceUnpackedness 'NoSourceStrictness
 'DecidedLazy)
              (Rec0 Foo)))
        x
 }}}

 Also, `DerivingStrategies` + GND + `DeriveGeneric` already can generate
 wrong instance:

 {{{#!haskell
 newtype Bad2 = Bad2 Bool
   deriving newtype (Generic)

 {-

 ghci> from $ Bad2 False
 M1 {unM1 = L1 (M1 {unM1 = U1})}
 ghci> :t it
 it
   :: D1
        ('MetaData "Bool" "GHC.Types" "ghc-prim" 'False)
        (C1 ('MetaCons "False" 'PrefixI 'False) U1
         :+: C1 ('MetaCons "True" 'PrefixI 'False) U1)
        x
 -}

 }}}

 I tested this against GHC 8.6.1-alpha1.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15434
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