haskell - 为什么 GHC 认为类型变量对于具有默认实现的类是不明确的？

Question

在下面的示例中，我希望 GHC 能够约束变量 a输入 A因为这就是我在 instance EAmbiguous A 中所说的.但是，当将其加载到 ghci 中时，我得到:

Ambiguous type variable ‘a0’ arising from a use of ‘Main.$dmcount’

有问题的代码:

{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}

import GHC.Generics (Rep, Generic)
import GHC.Base (Type)

class EAmbiguous a where
  count :: Int
  default count :: (Generic a, GenericAmbiguous (Rep a)) => Int
  count = genericCount @(Rep a)

class GenericAmbiguous (a :: Type -> Type) where
  genericCount :: Int

instance GenericAmbiguous (f p) where
  genericCount = 10

data A = A deriving Generic

-- The error happens when defining a body-less instance:
--     • Ambiguous type variable ‘a0’ arising from a use of ‘Main.$dmcount’
--      prevents the constraint ‘(Generic a0)’ from being solved.
instance EAmbiguous A

我已经研究了好一阵子了，我相信这应该行得通，但我显然遗漏了一些东西。所以我的问题是:为什么 GHC 不能解决 a至 A ？

Answer 1

这原来是 GHC 错误，已在 GHC 8.0.2 中修复:https://ghc.haskell.org/trac/ghc/ticket/12220