Wrong parsing of int64 values nearest of limit

[gysevvlad]

What is the issue you have?

For number "9223372036854775808" (int64_t::max() + 1) I expects that json.get<nlohmann::json::number_integer_t>() throws exception.

Can you provide a small but working code example?

https://godbolt.org/z/v4ejoPszf

#include "nlohmann/json.hpp"
#include <iostream>

int main(int, char **) {
  auto json = nlohmann::json::parse(R"({ "package size": 9223372036854775808 })");
  auto [key, number] = *json.items().begin();
  std::cout << number.get<nlohmann::json::number_integer_t>() << std::endl;
  return 0;
}

What is the expected behavior?

I expects to get exception.

And what is the actual behavior instead?

Program stdout
-9223372036854775808

Which compiler and operating system are you using?

• Compiler: clang-12
• Operating system: Ubuntu, RedHat

Which version of the library did you use?

• latest release version 3.10.4
• other release - please state the version: ___
• the develop branch

[gysevvlad]

Also it's relevant for is_number_integer check:

#include "nlohmann/json.hpp"
#include <iostream>

int main(int, char **) {
  auto json = nlohmann::json::parse(R"({ "package size": 9223372036854775808 })");
  auto [key, number] = *json.items().begin();
  if (number.is_number_integer()) {
    std::cout << number.get<nlohmann::json::number_integer_t>() << std::endl;
  }
  return 0;
}

bash :> ./build/default/main
-9223372036854775808

[nlohmann]

The library differentiates between signed and unsigned integers (see https://json.nlohmann.me/features/types/number_handling/). is_number_integer() returns true for either integer type (see https://json.nlohmann.me/api/basic_json/is_number_integer/).

So with get<nlohmann::json::number_integer_t>() you actually cast an std::uint64_t to a std::int64_t.

#include <nlohmann/json.hpp>

int main() {
    const auto j = nlohmann::json::parse("9223372036854775808");
    assert(j.is_number_unsigned());
    std::uint64_t n = j;
    assert(n == 9223372036854775808U);
}