Expansion of type synonyms is not well kinded

[RyanGlScott]

I'm not sure if this is a bug that can realistically be fixed, but it did actually bite me recently, so I thought I should report it. Consider the following code:

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Data.Kind
import Data.Proxy
import Language.Haskell.TH (pprint, stringE)
import Language.Haskell.TH.Desugar

type MyProxy = (Proxy :: k -> Type)

$(pure [])

test :: String
test = $(expandType (DConT ''MyProxy `DAppT` DConT ''Int) >>= stringE . pprint . typeToTH)

This expands the type synonyms in the type MyProxy Int. However, the result you get is:

λ> test
"(Data.Proxy.Proxy :: k_0 -> *) GHC.Types.Int"

Which is not well kinded. The reason this happens is that when MyProxy is reified during type synonym expansion, a kind signature of k_0 -> Type is placed on it to clarify what its kind argument is. Unfortunately, this later bites us when MyProxy is applied to an argument of kind Type (instead of kind k_0).

I'm not sure what to do here. Should we add a Known Bugs section to the README and mention this there?