Defaulting not used enough with type families

Consider this module

{-# LANGUAGE TypeFamilies, ExtendedDefaultRules #-}
module Bug3 where

class C t where
    type T t

instance C Integer where
    type T Integer = Integer

data (C t, Num t) => D t = D (T t)
instance Show (D a) where show _ = ""

Now let's try to ask for the type of (D 5)

*Bug3> :t D 5
D 5 :: (Num (T t), C t, Num t) => D t

That's great. Now if we try to show this value it should have type

(Num (T t), C t, Num t) => String

And now the defaulting mechanism should set t to Integer, which would resolve the rest of the context. But instead we get this:

*Bug3> :t show (D 5)
<interactive>:1:8:
    No instance for (Num (T t))
      arising from the literal `5' at <interactive>:1:8
    Possible fix: add an instance declaration for (Num (T t))
    In the first argument of `D', namely `5'
    In the first argument of `show', namely `(D 5)'
    In the expression: show (D 5)

It looks like the defaulting never got used.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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