[GHC] #7862: Could not deduce (A) from the context (A, ...)

[GHC]

#7862: Could not deduce (A) from the context (A, ...)
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Reporter:  alang9                     |          Owner:                         
    Type:  bug                        |         Status:  new                    
Priority:  normal                     |      Component:  Compiler (Type checker)
 Version:  7.6.2                      |       Keywords:                         
      Os:  Linux                      |   Architecture:  x86_64 (amd64)         
 Failure:  GHC rejects valid program  |      Blockedby:                         
Blocking:                             |        Related:                         
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 The following code doesn't compile and produces a strange error:

 {{{
 {-# LANGUAGE TypeFamilies, FlexibleContexts #-}

 module Numeric.AD.Internal.Tower () where

 type family Scalar t

 newtype Tower s a = Tower [a]

 type instance Scalar (Tower s a) = a

 class (Num (Scalar t), Num t) => Mode t where
     (<+>) :: t -> t -> t

 instance (Num a) => Mode (Tower s a) where
     Tower as <+> _ = undefined
       where
         _ = (Tower as) <+> (Tower as)

 instance Num a => Num (Tower s a) where
 }}}

 {{{
 src/Numeric/AD/Internal/Tower.hs:17:24:
     Could not deduce (Num (Scalar (Tower s a)))
       arising from a use of `<+>'
     from the context (Num (Scalar (Tower s a)), Num (Tower s a), Num a)
       bound by the instance declaration
       at src/Numeric/AD/Internal/Tower.hs:14:10-36
     Possible fix:
       add an instance declaration for (Num (Scalar (Tower s a)))
     In the expression: (Tower as) <+> (Tower as)
     In a pattern binding: _ = (Tower as) <+> (Tower as)
     In an equation for `<+>':
         (Tower as) <+> _
           = undefined
           where
               _ = (Tower as) <+> (Tower as)
 }}}

 Furthermore, Removing the {{{Num (Scalar t)}}} constraint from the
 {{{Mode}}} class produces a different strange error:
 {{{
 src/Numeric/AD/Internal/Tower.hs:17:24:
     Overlapping instances for Num (Tower s0 a)
       arising from a use of `<+>'
     Matching givens (or their superclasses):
       (Num (Tower s a))
         bound by the instance declaration
         at src/Numeric/AD/Internal/Tower.hs:14:10-36
     Matching instances:
       instance Num a => Num (Tower s a)
         -- Defined at src/Numeric/AD/Internal/Tower.hs:19:10
     (The choice depends on the instantiation of `a, s0')
     In the expression: (Tower as) <+> (Tower as)
     In a pattern binding: _ = (Tower as) <+> (Tower as)
     In an equation for `<+>':
         (Tower as) <+> _
           = undefined
           where
               _ = (Tower as) <+> (Tower as)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7862>
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