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{-# LANGUAGE TypeFamilies, UndecidableInstances #-}

class D a where data DT a
class C a where data CT a

instance (D a, Eq (DT a)) => C (Maybe a) where
  data CT (Maybe a) = CTMaybe (DT a) deriving (Eq)

{-
$ stack --resolver=nightly-2017-10-20 exec -- ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.2.1

$ stack --resolver=nightly-2017-10-20 exec -- ghci Main.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
Loaded GHCi configuration from /home/mf/.ghci
[1 of 1] Compiling Main             ( Main.hs, interpreted )

Main.hs:7:48: error:
    • No instance for (Eq (DT a))
        arising from the first field of ‘CTMaybe’ (type ‘DT a’)
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Eq (CT (Maybe a)))
  |
7 |   data CT (Maybe a) = CTMaybe (DT a) deriving (Eq)
  |                                                ^^
Failed, 0 modules loaded.
(0.03 secs,)
Prelude>


-- if i remove the offending @deriving@ clause above and add this line, everything is fine.
-- use -XFlexibleInstances -XStandaloneDeriving for this.
deriving instance Eq (DT a) => Eq (CT (Maybe a))

-}

checked on linux with ghc8.0.2 and 8.2.1. thanks! and sorry if i've missed a previous report covering this.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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