Empty tuple conversion fails to compile

[jrade]

Automatic conversion between std::tuple<> and boost::json::value does not work.
The code

std::tuple<> t0;
boost::json::value v = boost::json::value_from(t0);
std::tuple<> t1 = boost::json::value_to<std::tuple<>>(v);

fails to compile with the error message

static assertion failed: 'No suitable tag_invoke overload found for the type'.

This seems counterintuitive. Obviously an empty C++ tuple should be converted to an empty JSON array and vice-versa.

Tested with Boost 1.92.0 and Visual Studio 2026 on Windows 11.

[grisumbras]

Yes, see this section:

... if std::tuple_size<T>::value is a positive number.

I currently don't remember the specific type which was incorrectly categorised as a tuple if I did not check for positve size, but there was something.

What's your use case for empty tuples?

[jrade]

No use case at all. I was just playing around with the library to learn how it works. It is rather a matter of logical consistency.