New clang-tidy modernize-nlohmann-json-explicit-conversions check

[mikecrowe]

I've written a somewhat rudimentary clang-tidy check that will automatically convert existing implicit conversions to explicit conversions. In other words, it turns:

void f(const nlohmann::json &j1, const nlohmann::json &j2)
{
    int i = j1;
    double d = j2.at("value");
    std::cout << i << " " << d << "\n";
}

into

void f(const nlohmann::json &j1, const nlohmann::json &j2)
{
    int i = j1.get<int>();
    double d = j2.at("value").get<double>();
    std::cout << i << " " << d << "\n";
}

by knowing what the target type is for the implicit conversion and turning that into an explicit call to the get method with that type as the template parameter. Further examples can be found in the tests for the check.

I've used this check to replace 220 of 221 implicit conversions in our code. It was incapable of recognising an implicit conversion used to construct std::optional and left it alone. Even if it can't do the whole job, I think it's better than nothing.

Using this check may ease the path towards removing these implicit conversions in a future version of the library.

I raise it here in case:

1. you can see any flaws in it or suggest more test cases.

2. you think it's worth of a link from mkdocs/docs/api/macros/json_use_implicit_conversions.md.

I haven't tried to submit it to the upstream llvm-project. I'm not sure whether they'll accept checks for third-party (non-Google) libraries. If the response here is favourable then I can try.

[nlohmann]

Wow, this is awesome! I will give it a try soon. At first glance, I would definitely love to mention this in the documentation for implicit conversions (and also on https://json.nlohmann.me/integration/migration_guide). And yes, please try to submit this upstream! In any case, your project is an excellent starting point for integrating other changes that would make the transition to the next major version as smooth as possible.