Motivating use case for nonthrowing value_to

[pdimov]

Suppose that a user wants to represent variant<T1, T2, T3> as just the active alternative; that is, when the variant contains T1, its value_from function just forwards to T1's value_from, and similarly for T2 and T3. When converting from a value, it tries the alternatives in order, and uses the first one that matches. (https://godbolt.org/z/GqKPYx4Tq)

This occurs in practice when the JSON schema is externally specified and the types of some values can vary. For instance, Boost's meta/libraries.json allows both a string and an array of strings in the authors and maintainers fields. This can be represented by variant<string, vector<string>>.

The solution in the above CE link works, but it throws sizeof...(T) / 2 times on average on each value_to call, which kills performance. It would be much better if a nonthrowing alternative of value_to existed that returned the parse/type errors using error_code. In fact, the code above emulates this nonthrowing alternative with the try_value_to function.

There are still few users for the tag_invoke mechanism so switching to a nonthrowing support function now won't break much existing code, although the longer we wait, the less true this becomes.