implicitly_convertible: don't accept enum targets (fixes #871)

Author: wjakob

include/nanobind/nb_class.h

@@ -801,6 +801,7 @@ template <typename T> class enum_ : public object {
 
 template <typename Source, typename Target> void implicitly_convertible() {
     using Caster = detail::make_caster<Source>;
+    static_assert(!std::is_enum_v<Target>, "implicitly_convertible(): 'Target' cannot be an enumeration.");
 
     if constexpr (detail::is_base_caster_v<Caster>) {
         detail::implicitly_convertible(&typeid(Source), &typeid(Target));