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Empty tuple conversion fails to compile #1196
Description
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.
Activity
grisumbras commentedon Sep 9, 2026
Yes, see this section:
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 commentedon Sep 9, 2026
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.