[libc++] `basic_string_view` is exposed and depended by `bitset` before C++17

[frederick-vs-ja]

Enabling basic_string_view before C++17 seems to be an intended extension but doesn't seem documented. I'm not sure whether we should provide a more pedantic option to disable it before C++17.

Also, libc++'s bitset depends on basic_string_view before C++17. It's possibly better (for the purpose of throughput) to avoid such dependency by decompose the basic_string_view parameter of __init_from_string_view into pointer and size. Should we make such change?

[philnik777]

I don't think we should provide such an option. That seems like a lot of hassle for very little gain from our side. We still need to provide it, since way too many people rely on the extension. Re. throughput I'd really like to see some compelling numbers there. That seems like a change with little gain compared to added maintenance costs.

[ldionne]

string_view has been provided as an extension since its introduction. This has caused some difficulties in the past (e.g. I remember this broke nlohmann.json due to overload resolution differences), however due to the prevalence of the type in people's code, I agree with @philnik777 that removing that extension is unfortunately out of question.

Feel free to explore changing __init_from_string_view if you believe there is a benefit -- that's definitely something we can discuss in a PR. I'll close this now since I think the main question has been answered.