Safe integer conversions

[nebkat]

Integers are currently processed in an unsafe manner:

#include <cstdint>
#include <print>
#include <nlohmann/json.hpp>

int main() {
    nlohmann::json j = -123456;
    std::println(" int32_t: {}", j.get< int32_t>()); // -123456
    std::println("uint32_t: {}", j.get<uint32_t>()); // 4294843840
    std::println(" int16_t: {}", j.get< int16_t>()); // 7616
}

https://godbolt.org/z/7qEM1c7eY

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Proposal A: json::get_integer<std::integral T>()

Performs checks to ensure that the value is within std::numeric_limits<T> and throws an exception before returning.

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Proposal B: JSON_SAFE_INTEGER_CONVERSIONS

Opt-in to make this behavior default with json::get<std::integral T>().

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This doesn't address the implicit conversions from floats to integers, which it may also be desirable to restrict.