why I can return tuple as json?

[tesla1060]

Description

I am wonder why below code can be compiled

#include "json.hpp"
#include <iostream>
using json = nlohmann::json;

std::tuple<bool, json> test() {
    json j;
    j["name"]="a";
    return std::make_tuple(true, j);
}

int main() {
    json j;
    j = test();
    std::cout << j.contains("name") << std::endl;
}

Reproduction steps

code supplied description.

Expected vs. actual results

I am expecting a compile time failure, while the above code can actually compiled and run

Minimal code example

No response

Error messages

No response

Compiler and operating system

ubuntu 20

Library version

gcc 9

Validation

• [ ] The bug also occurs if the latest version from the develop branch is used.
• [X] I can successfully compile and run the unit tests.

[nlohmann]

See https://json.nlohmann.me/api/macros/json_use_implicit_conversions/

Can you try to compile with

#define JSON_USE_IMPLICIT_CONVERSIONS 0