On Mon, Jul 1, 2019 at 8:46 PM Lyberta via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
Why can't std::span take std::optional<size_type>?

Because you can't have a non-type template parameter of type optional<size_type>
 

Why are we making the same mistake we did with std::basic_string::npos?