Define function that accepts only specific multi data constructors on a type

gistfile1.hs

{-# LANGUAGE GADTs #-}

data MyString
data MyInt
data MyNil

data MyData tag where
  MyString :: String -> MyData MyString
  MyInt :: Int -> MyData MyInt
  MyNil :: MyData MyNil

class HasValue a
instance HasValue MyString
instance HasValue MyInt

-- accept all data constructor
test1 :: MyData a -> String
test1 (MyString s) = "MyString " ++ s
test1 (MyInt i) = "MyInt " ++ show i
test1 MyNil = "MyNil"

-- accept only data constructor that conform HasValue
test2 :: (HasValue a) => MyData a -> String
test2 _ = "test2"


{-
   ghci> test1 (MyString "hello")
   "MyString hello"
   ghci> test1 (MyInt 22)
   "MyInt 22"
   ghci> test1 MyNil
   "MyNil"
                
   ghci> test2 (MyString "hello")
   "test2"
   ghci> test2 (MyInt 55)
   "test2"
   ghci> test2 MyNil
   <interactive>:34:1:
       No instance for (HasValue MyNil)
         arising from a use of `test2'
       Possible fix: add an instance declaration for (HasValue MyNil)
       In the expression: test2 MyNil
       In an equation for `it': it = test2 MyNil
-}