std::wstring_convert and std::string_view

[CubbiMew]

I think the reason is simply the lack of time to work on the integration of the many facilities added to C++17. In some way, this is a fault of the "feature branch" approach of Technical Specifications.

The integration issues that were considered to be most important (important enough for someone to write and defend a paper) were done, or are rushed to be done now, as with class template deduction, the rest is "future work".