The conversion operator template appears to not work for some types.
We just had a discussion about this on stack overflow (and an earlier one here).
Here's an example on Godbolt using GCC with -std=c++14. With -std=c++17 there's no error, but still a warning.

I would like to suggest a way for working around this.
Another version of the get function can be implemented like this:

template <typename T>
T& json::get(T& x) const { return x = get<T>(); }

so that the type may be deduced from the argument.

This would help particularly in situations where one has a reference to an object that needs to be given value from json, e.g.

auto& x = . . .;
json.get(x);

Which can be quite a bit less verbose than

auto& x = . . .;
x = json.get<std::decay_t<decltype(x)>>();

Thanks for the amazing library,
Ivan