Supporting ? as a string presentation type

[brevzin]

Formatting ranges prints strings escaped, this allows the user to do so directly outside of a range context (if so desired).

Most of this PR is simply moving the logic from fmt/ranges.h to fmt/core.h. The width-counting (to support like... "{:*^10?}") using a counting output iterator is... probably not the right way to do this, but I'm not sure what the right way is.

[brevzin]

So the issue here (as far as I can tell) is that when I moved the implementation from ranges.h to format.h, now it's suddenly getting used for detail::char8_type too in some contexts. In C++20 (where I was building locally), that's char8_t, which is convertible to char, so calls like this:

out = format_to(out, "\\x{:02x}", escape.cp);

work. But in C++11 (like some of the CI), it doesn't, complaining about how you can't call that:

../include/fmt/format.h:1661:26: error: no matching function for call to ‘format_to(std::back_insert_iterator<fmt::v8::detail::buffer<fmt::v8::detail::char8_type> >&, const char [9], uint32_t&)’
 1661 |           out = format_to(out, "\\U{:08x}", escape.cp);
      |                 ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

because the overload checks to see if the iterator is an output_iterator for char, and it's not if char8_type is just some enum.

The just-make-it-work solution (that I just pushed) is to only do the escaping logic for specifically char. That works on C++11 too. But like... obviously not the Right Thing To Do.

[vitaut]

Thanks for the PR!

[vitaut]

@brevzin, do you still plan working on this?

[brevzin]

@brevzin, do you still plan working on this?

I do intend on getting to it eventually, but if you (or someone else) wants to take over, I'm totally fine with that. As you can tell, I'm not really sure how to actually do some of these things :) (well, except for the formatting comments - I can format the formatting PR at least. much meta. such wow).

[vitaut]

I'm OK leaving the switch to code points for later (i.e. not this PR) and will merge the PR if you address the other (minor) comments.

[vitaut]

Thank you!