Error

How.hs:24:13: error:
    • Could not deduce (Storable t0)
      from the context: Storable t
        bound by the type signature for:
                   method :: forall t. Storable t => Int
        at How.hs:24:13-44
      The type variable ‘t0’ is ambiguous
    • When checking that instance signature for ‘method’
        is more general than its signature in the class
        Instance sig: forall t. Storable t => Int
           Class sig: forall t. Storable t => Int
      In the instance declaration for ‘Class String’
   |
24 |   method :: forall t . Storable t => Int t
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.

How.hs

{-# LANGUAGE NoStarIsType #-}
{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}

import Foreign.Storable

class Class where
  method :: forall t . Storable t => Int

instance Class where
  method :: forall t . Storable t => Int
  method = sizeOf $ (undefined :: t)