Make change as suggested by @laramiel. This makes it much more obviou…

…s that the latest implementation of `smart_holder_from_unique_ptr()` accepts all existing `return_value_policy` enum values except `copy`.

include/pybind11/detail/type_caster_base.h

@@ -528,13 +528,8 @@ handle smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&src,
  return_value_policy policy,
  handle parent,
  const std::pair<const void *, const type_info *> &st) {
- if (policy != return_value_policy::automatic
- && policy != return_value_policy::automatic_reference
- && policy != return_value_policy::take_ownership && policy != return_value_policy::move
- && policy != return_value_policy::reference
- && policy != return_value_policy::reference_internal) {
- // SMART_HOLDER_WIP: IMPROVABLE: Error message.
- throw cast_error("Invalid return_value_policy for unique_ptr.");
+ if (policy == return_value_policy::copy) {
+ throw cast_error("return_value_policy::copy is invalid for unique_ptr.");
  }
  if (!src) {
  return none().release();