FlexibleContexts PolyKinds Type Families bug

A bug with TypeFamilies + FlexibleContexts + PolyKinds

{-# LANGUAGE PolyKinds, FlexibleContexts, TypeFamilies #-}
class Monoid (a :: k) where
	type Pr a :: *
	mempty :: Pr a
	mappend :: Pr a -> Pr a -> Pr a

instance Monoid [b] where 
	type Pr [b] = [b]
	mempty = [] 
	mappend = (++)

This is compilable. But this is not:

t :: (Monoid [b]) => b -> [b]
t b = [b] `mappend` mempty

    Could not deduce ([b] ~ Pr k0 a0)
    from the context (Monoid * [b])
      bound by the type signature for t :: Monoid * [b] => b -> [b]
      at t.hs:26:6-29
    The type variables `k0', `a0' are ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    In the expression: [b] `mappend` mempty
    In an equation for `t': t b = [b] `mappend` mempty

    Could not deduce (Pr k1 a1 ~ [b])
    from the context (Monoid * [b])
      bound by the type signature for t :: Monoid * [b] => b -> [b]
      at test4.hs:26:6-29
    The type variables `k1', `a1' are ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Expected type: Pr k0 a0
      Actual type: Pr k1 a1
    In the second argument of `mappend', namely `mempty'
    In the expression: [b] `mappend` mempty
    In an equation for `t': t b = [b] `mappend` mempty

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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