Can't use superclass when type coercions are involved

GHC 6.10.4.

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
module Coercion where

class Nat n where
    toInt :: n -> Int

class (Nat (Arity f)) => Model f where
    type Arity f

ok :: Model f => f -> Arity f -> Int
ok _ n = toInt n

bug :: (Model f, Arity f ~ n) => f -> n -> Int
bug _ n = toInt n

I expect that (Model f) brings (Nat (Arity f)) into scope, and that (Arity f ~ n) means we can derive (Nat n). But the compiler disagrees.

Coercion.hs:14:10:
    Could not deduce (Nat n) from the context (Arity f ~ n, Model f)
      arising from a use of `toInt' at Coercion.hs:14:10-16
    Possible fix:
      add (Nat n) to the context of the type signature for `bug'
    In the expression: toInt n
    In the definition of `bug': bug _ n = toInt n

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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