在类型系列中显示自定义类型错误

Question

GHC 8.0具有user-defined type errors的功能。我正在尝试，但我不能完全做到我想做的事情：

{-# LANGUAGE DataKinds, FlexibleContexts, FlexibleInstances, MultiParamTypeClasses,
             TypeFamilies, TypeOperators, UndecidableInstances #-}

import Data.Proxy
import GHC.TypeLits

data DoubleD
data IntD

type family CTypeOf x where
  CTypeOf (a,b) = EqCType (CTypeOf a) (CTypeOf b)
  CTypeOf Double = DoubleD
  CTypeOf Int = IntD

  CTypeOf a = TypeError (Text "Unsupported type: " :<>: ShowType a)

type family EqCType a b where
  EqCType a a = a
  EqCType a b = TypeError (Text "Pair error")

class (repr ~ CTypeOf r) => Dispatch' repr r
instance (CTypeOf r ~ DoubleD) => Dispatch' DoubleD r
instance (CTypeOf r ~ IntD) => Dispatch' IntD r

foo :: (Dispatch' (CTypeOf a) a) => Proxy a -> Int
foo _ = 3

main :: IO ()
--main = print $ foo (Proxy::Proxy (Int, Int))
--main = print $ foo (Proxy::Proxy (Int, Double))
--main = print $ foo (Proxy::Proxy Bool)
--main = print $ foo (Proxy::Proxy (Bool, Bool))
main = print $ foo (Proxy::Proxy (Bool, Double))

main的前四个定义与我期望的一样：

1. 编译
2. 错误：Pair error
3. 错误：Unsupported type: Bool
4. 错误：Unsupported type: Bool

对于第五个定义，我希望显示Unsupported type: Bool，但GHC显示：

1. 错误：No instance for (Dispatch' (EqCType (TypeError ...) DoubleD) (Bool, Double))

我很困惑为什么GHC得到前三个错误，但是没有在最终定义中显示我的类型错误。我正在寻找为什么的简要说明，以及可行的解决方法。这只是自定义类型错误实现的限制吗？