GHC無法正確解析類型類方法中的約束（ghc錯誤/限制？）

Question

GHC在某些類成員的默認實現中不解析/傳播約束。 這種行為真的很奇怪，在我看來，這是一個錯誤。

有人可以幫我/向我解釋什么地方嗎？

1. 我怎么能告訴GHC統一a從類聲明和a從類方法，這樣的定義是沒有歧義
2. 有什么我不懂的嗎？

碼：

module Foo where
import Data.Proxy

data Stuff a = Stuff
  {content :: String}

class HasStuff a where
  stuff :: Stuff a

  -- This works
  useStuffOK :: Proxy a -> (Stuff a)
  useStuffOK _ = (stuff)

  -- those don't work, 
  -- (but I think ghc has all the information necessary to figure it out)
  useStuffBAD :: Proxy a -> (Stuff a, String)
  useStuffBAD _ = (stuff, content (stuff :: Stuff a))
  -- Could not deduce (HasStuff a1) arising from a use of ‘stuff’
  -- from the context (HasStuff a)
  --   bound by the class declaration for ‘HasStuff’
  --   at Test.hs:(7,1)-(17,45)

  useStuffBAD2 :: Proxy a -> String
  useStuffBAD2 _ = content (stuff :: Stuff a)
  -- Could not deduce (HasStuff a1) arising from a use of ‘stuff’
  -- from the context (HasStuff a)
  --   bound by the class declaration for ‘HasStuff’
  --   at Test.hs:(7,1)-(17,45)

instance HasStuff Int where
  stuff = Stuff "ok"

-- inference works here
x :: Stuff Int
x = stuff

-- works here too
x :: String
x = content (stuff :: Stuff Int)

謝謝

Answer 1

正如@epsilonhalbe和@ user2407038指出的那樣，我缺少ScopedTypeVariables擴展