Polymorphic instances aren't automatically specialised

[reinerp]

Related to #255. Given (roughly) the example from that ticket:

f :: (Storable a, Eq a) => T a

g :: T (Ptr a)
g = f

we find that g is not specialised. Adding a SPECIALISE pragma fixes this, but it ought not to be necessary.

The following module is a complete example:

module C where

class C a where f :: a -> a

newtype Id a = Id a
instance C (Id a) where f = id

g :: C a => Int -> a -> a
g 0 a = a
g n a = g (n-1) (f a)

h :: Int -> Id a -> Id a
h = g

j :: Int -> Id Int -> Id Int
j = g

We find that h passes a dictionary to g:

C.h =
  \ (@ a_alq) (w_smq :: Int) (w1_smu :: C.Id a_alq) ->
    case w_smq of _ { I# ww_sms ->
    C.$wg @ (C.Id a_alq) (C.$fCId @ a_alq) ww_sms w1_smu
    }

whereas j is specialised as desired, getting its own worker and no dictionaries:

C.j =
  \ (w_smg :: Int) (w1_smk :: C.Id Int) ->
    case w_smg of _ { I# ww_smi ->
    (C.$wj ww_smi w1_smk)
    `cast` (sym (C.NTCo:Id Int)
            :: Int ~ C.Id Int)
    }

If we add

{-# SPECIALISE g :: Int -> Id a -> Id a #-}

then h is specialised as desired.

[igloo]

Thanks for the report.