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DerivingVia together with DeriveGeneric can generate wrong instances for Generic.

Consider the following:

{-# 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:

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:

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.

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Trac field	Value
Version
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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