This is what I assume to be a fairly minor change to the library's logic, but would represent a large improvement in usability.

Feature request

C++17 adds std::optional<T>. Let std::optional<T> convert to nlohmann::json.

If std::optional<T> is empty, I would expect this to result in null - if std::optional<T>, I would expect it to result in the contained value converted.

Use case example

The change would simplify (especially nested) uses of std::optional.

For example, imagine a simple key-value map, where each integer key can optionally have a localized string representation for each of 8 locales. The client dynamically selects the "best" available locale depending on user preferences.

In server code, one could express this as a std::vector<std::array<std::optional<std::string>, 8>>.
Currently, this type does not convert to nlohmann::json because std::optional<std::string> does not - resulting in needing to allocate a bunch of temporaries so the inner conversions can be done "manually".

Allowing std::optional to convert to nlohmann::json would simplify this logic greatly.