ScopedTypeVariables unexpected

errors.txt

• Couldn't match type ‘a’ with ‘a1’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          mogel :: forall a. Show a => a -> (String, a)
        at src/Types.hs:105:1-48
      ‘a1’ is a rigid type variable bound by
        the type signature for:
          b :: forall a1. Show a1 => Maybe a1
        at src/Types.hs:108:5-39
      Expected type: Maybe a1
        Actual type: Maybe a
    • In the expression: Just a
      In an equation for ‘b’: b = Just a
      In the expression:
        let
          b :: forall a. (Show a) => Maybe a
          b = Just a
        in maybe ("", a) (\ aa -> (show aa, aa)) b
    • Relevant bindings include
        b :: Maybe a1 (bound at src/Types.hs:109:5)
        a :: a (bound at src/Types.hs:106:7)
        mogel :: a -> (String, a) (bound at src/Types.hs:106:1)
    |
109 |     b = Just a

problem.hs

{-# LANGUAGE ScopedTypeVariables #-}

fn :: forall a . (Show a) => a -> (String, a)
fn a =
  let
    b :: forall a . (Show a) => Maybe a
    b = Just a
  in maybe ("", a) (\aa -> (show aa, aa)) b