std::is_constructible<json, std::unordered_map<std::string, Data>> when to_json not defined

[cbegue]

Building the following code failed:

#include <string>
#include <tuple>
#include <unordered_map>
#include <nlohmann/json.hpp>

struct Data {
    int mydata;
    float myfloat;
};

int main() {
    using nlohmann::json;

    static_assert(!std::is_constructible<json, std::unordered_map<std::string, Data>>::value, "");
}

With the following error:

static assertion failed
     static_assert(!std::is_constructible<json, std::unordered_map<std::string, Data>>::value, "");

std::is_constructible<json, std::unordered_map<std::string, Data>>::value should be false

[cbegue]

I found the same kind of issue with std::tuple. Do you want I create a new issue with a new PR or can I add the fix for tuple in this PR?