defoverridable makes overridable only function but not a corresponding spec

[Alexey Nikitin]

I face this issue when adding spec for `template_not_found` for my Phoenix ErrorView

Dialyzer says

```

lib/my_app_web/views/error_view.ex:2:overlapping_contract
Overloaded contract for MyAppWeb.ErrorView.template_not_found/2 has
overlapping domains; such contracts are currently unsupported and
are simply ignored.

```

due to default implementation of template_not_found already has a spec. See https://github.com/phoenixframework/phoenix/pull/1583

I'm not sure I understand the problem correctly. Any help would be appreciated