T17841.hs:8:45: error:
    • Expected a type, but ‘t’ has kind ‘k’
      ‘k’ is a rigid type variable bound by
        the class declaration for ‘Foo’
        at T17841.hs:8:12-17
    • In the kind ‘t’
      In the first argument of ‘Proxy’, namely ‘(a :: t)’
      In the type signature: foo :: Proxy (a :: t)