derivedArgDict doesn't work when an index is another parameter

[bglgwyng]

There are issues with derivedArgDict here and I think I found another one.
I want to define Foo with two type parameters a and b. And the usage of a(b is ok) in the place of b causes the issue.

data Foo a b where
  Foo :: Foo a Int -- ok
  Bar :: Foo a (a) -- not ok

deriveGEq ''Foo
deriveGCompare ''Foo
deriveGShow ''Foo
deriveArgDict ''Foo -- fails

So I expanded TH.

data Foo a b where
  Foo :: Foo a Int -- ok
  Bar :: Foo a [a] -- not ok

deriveGEq ''Foo
deriveGCompare ''Foo
deriveGShow ''Foo
instance (c_a3QI3 Int, c_a3QI3 [a_a3QzU]) =>
         Has c_a3QI3 (Foo a_a3QI2) where
  argDict
    = \case
        Foo {} -> Dict
        Bar {} -> Dict

I think the a_a3QzU is not correct. So I corrected it as follows.

data Foo a b where
  Foo :: Foo a Int -- ok
  Bar :: Foo a [a] -- not ok

deriveGEq ''Foo
deriveGCompare ''Foo
deriveGShow ''Foo
instance (c Int, c [a]) =>
         Has c (Foo a) where
  argDict
    = \case
        Foo {} -> Dict
        Bar {} -> Dict

Then it worked!

First, is the hand-written instance correct?
Also, can the instance be derived?