T14880.hs:12:5: error:
    • Cannot generalise type; skolem ‘arg’ would escape its scope
      if I tried to quantify (a0 :: arg) in this type:
        forall x arg.
        ((Foo arg @a0 :: (Proxy @{arg} a0 -> *))
         ~ (Foo arg @a0 :: (Proxy @{arg} a0 -> *))) =>
        Bar x
      (Indeed, I sometimes struggle even printing this correctly,
       due to its ill-scoped nature.)
    • In the definition of data constructor ‘MkBar’
      In the data declaration for ‘Bar’