haskell - 「from」の使用から生じる (Generic (fa)) のインスタンスなし

Question

次のコードに適した型制約を見つけるのに問題があります

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleContexts #-}
import GHC.Generics

data Value = One | Two deriving Generic

class Class a where
    cname :: a -> String -> Bool
    default cname :: (Generic a, GClass (Rep a))
               => a -> String -> Bool
    cname = gname . from

class GClass f where
    gname :: f a -> String -> Bool

instance GClass (f :+: g) where
    gname (L1 x) s | conName (from x) == s = True
                   | otherwise             = False
    gname (R1 x) s | conName (from x) == s = True
                   | otherwise             = False

で失敗します

No instance for (Generic (f a)) arising from a use of `from'

gnameこのように制約を追加する

instance (Generic (f a)) => GClass (f :+: g) where

で失敗します

Could not deduce (Generic (f a1)) arising from a use of `from'
from the context (Generic (f a))

編集:完全なスニペットの完全なエラー メッセージ

Generic.hs:19:31:
    No instance for (Generic (f a)) arising from a use of `from'
    Possible fix: add an instance declaration for (Generic (f a))
    In the first argument of `conName', namely `(from x)'
    In the first argument of `(==)', namely `conName (from x)'
    In the expression: conName (from x) == s

Generic.hs:21:31:
    No instance for (Generic (g a)) arising from a use of `from'
    Possible fix: add an instance declaration for (Generic (g a))
    In the first argument of `conName', namely `(from x)'
    In the first argument of `(==)', namely `conName (from x)'
    In the expression: conName (from x) == s

これは GHC 7.6.3 の場合です。