message: Don't define specializations for absl::optional when it aliases std::optional.
  Other files already define these specializations for std::optional, and these
  definitions conflict.
diff: @@ -64,12 +64,16 @@ StatusOr<PythonBufferTree> GetPythonBufferTree(
  namespace pybind11 {
  namespace detail {
  +// When absl::optional is an alias for std::optional, the type_caster
  +// specializations are provided by pybind11.
  +#ifndef ABSL_HAVE_STD_OPTIONAL
  // absl::optional
  template <typename T>
  struct type_caster<absl::optional<T>> : optional_caster<absl::optional<T>> {};
  template <>
  struct type_caster<absl::nullopt_t> : public void_caster<absl::nullopt_t> {};
  +#endif
  // absl::Span
  template <typename T>
  