Proposal to use std::constructible_from in str_t to support objects with explicit conversion to string_view

[anders-wind]

Hi,

Thanks for the great library!
We have some strong types which wraps std::string. These provide explicit operator std::string_view(); However if this type is a key in a json dictionary, it will have extra quotes pre/appended to it (Foo{"hello"} -> "\"hello\"").

We traced the code to:

template <opts Opts, class Key, class Value, is_context Ctx>
GLZ_ALWAYS_INLINE void write_pair_content(const Key& key, Value&& value, Ctx& ctx, auto&&... args)
{
   if constexpr (str_t<Key> || char_t<Key> || glaze_enum_t<Key> || Opts.quoted_num) {
      write<JSON>::op<Opts>(key, ctx, args...);
   }
   else {
      write<JSON>::op<opt_false<Opts, &opts::raw_string>>(quoted_t<const Key>{key}, ctx, args...);
   }
...

and the definition of str_t to

template <class T>
concept str_t = (!std::same_as<std::nullptr_t, T> && std::convertible_to<std::decay_t<T>, std::string_view>) ||
                array_char_t<T>;

Due to our class not being implicitly convertible to string_view, we end up in the else block, getting the key quoted.

Would it be possible to use the std::constructible_from<std::string_view, T> instead of the std::convertible_to in the str_t definition, as it would allow structs with explicit conversions to also function as keys without quotes?

Godbolt example of the concepts: https://godbolt.org/z/KbKa8nY7c

Might be related to #1477 but the proposed solution is a bit different.

[stephenberry]

Thanks for suggesting this. I made the change and merged it in (#1486). Everything passes and I think it is a logical request.

[anders-wind]

Thanks!