Type synonym impossible to fully apply in type application #4531

module Main where
import Prelude
import Type.Proxy (Proxy(..))

-- `i` will be determined when `x` is concrete
type F :: forall i . Row i -> Row i
type F x = ()

-- type checks; `i ~ Type`
y :: Proxy (F (() :: Row Type))
y = Proxy

-- fails:
--   Type synonym Main.F is partially applied.
--   Type synonyms must be applied to all of their type arguments.
z :: Proxy (F (() :: Row Type))
z = Proxy @(F (() :: Row Type))

type G = F (() :: Row Type)
-- also works
g :: Proxy G
g = Proxy @G