在类型系列中显示自定义类型错误

Question

GHC 8.0具有用户定义类型错误的功能.我正在尝试,但我不能完全做到我想要的:

{-# 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))

工作的前四个定义main就像我期望的那样:

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,我想显示Unsupported type: Bool,但GHC显示:

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

我很困惑为什么GHC得到前三个错误是正确的,但是没有在最终定义中显示我的类型错误.我正在寻找一个简要的原因解释,如果可能的话还有一个解决方法.这只是自定义类型错误实现的限制吗？