Haskell 默认约束类型将被忽略

我在定义默认约束时遇到了一个奇怪的问题。如果约束为unit，则不会选择默认实例。在所有其他情况下，它都按预期工作

{-# LANGUAGE TypeFamilies, ConstraintKinds #-}
import qualified GHC.Exts as E

class Expression a where
  type Constr a v :: E.Constraint
  --type Constr a v = ()         -- with this line compilation fails
  --type Constr a v = v ~ v      -- compiles
  wrap :: Constr a v => a -> Maybe v

instance Expression () where
  wrap () = Just undefined

main = print (wrap () :: Maybe Int)

---

有人能解释一下typechecker行为的原因吗？

这不是一个真正的答案，但这不是关于

ConstraintKinds

class Expression a where
   type Type a v
   type Type a v = ()
   wrap :: (Type a v) ~ () => a -> Maybe v

instance Expression () where
   wrap () = Just undefined

main = print (wrap () :: Maybe Int)

不编译，但

class Expression a where
   type Type a v
   type Type a v = v
   wrap :: (Type a v) ~ v => a -> Maybe v

instance Expression () where
   wrap () = Just undefined

main = print (wrap () :: Maybe Int)

---

是否

这是一个与7.4.1中的默认类型相关联的错误。几周前，我在#haskell上被告知这是一个已修复的已知错误，但我在GHC trac上找不到它的提及。

猜测一下，因为

v

类型与关联类型映射解析的内容之间没有联系？可能相关：