SPECIALISE fails with a cryptic warning

[rl]

Example:

type family T a
type instance T Int = Bool

foo :: Num a => a -> T a
foo = undefined

{-# SPECIALISE foo :: Int -> Bool #-}

GHC produces this warning:

    RULE left-hand side too complicated to desugar
      case cobox of _ { GHC.Types.Eq# cobox ->
      (foo @ Int $dNum)
      `cast` (<Int> -> cobox :: (Int -> T Int) ~# (Int -> Bool))
      }

Given that rewrite rules don't reliably work in the presence of type families, I somewhat suspect that GHC won't be able to generate a sensible specialisation here but it should produce a better diagnostic.