get_ref<const number_integer_t &>() fails on integers since 3.12.0 release

[trofi]

Description

When updating json from 3.11.3 to 3.12.0 in nixpkgs as NixOS/nixpkgs#399385 I noticed a regression in the nix test suite:

  3/194 libutil-tests / nix-util-tests                                      FAIL            0.24s   exit status 1
  4/194 libstore-tests / nix-store-tests                                    FAIL            0.26s   exit status 1

Bisecting json landed at commit 9f60e85 "Fix return value of get_ptr for unsigned integers (#4525)".

I extracted minimal example below that illustrates the problem. Can you help me understand if this API change intended or it's a bug?

Reproduction steps

Minimal example bellow contains exact sample to trigger the failure.

Expected vs. actual results

Minimal example below should run (I think).

Minimal code example

// $ g++ main.cc -o main -std=c++20 && ./main; echo $?
#include <nlohmann/json.hpp>

static const nlohmann::json & ensureType(
    const nlohmann::json & value,
    nlohmann::json::value_type expectedType
    )
{
    if (value.type() != expectedType)
        __builtin_trap();

    return value;
}

static const nlohmann::json::object_t & getObject(const nlohmann::json & value)
{
    return ensureType(value, nlohmann::json::value_t::object).get_ref<const nlohmann::json::object_t &>();
}

static const nlohmann::json & valueAt(
    const nlohmann::json::object_t & map,
    const std::string & key)
{
    if (!map.contains(key))
      __builtin_trap();

    return map.at(key);
}

const nlohmann::json::number_integer_t & getInteger(const nlohmann::json & value)
{
    return ensureType(value, nlohmann::json::value_t::number_integer).get_ref<const nlohmann::json::number_integer_t &>();
}

int main() {
  auto simple = R"({ "int": 0 })"_json;

  return getInteger(valueAt(getObject(simple), "int")) == 0;
}

Error messages

On `json-3.12.0` it fails as:


$ g++ main.cc -o main -std=c++20 && ./main; echo $?
terminate called after throwing an instance of 'nlohmann::json_abi_v3_12_0::detail::type_error'
  what():  [json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number
Aborted (core dumped)
134


On `json-3.11.3` it works as:


$ g++ main.cc -o main -std=c++20 && ./main; echo $?
1

Compiler and operating system

x86_64-linux gcc-14.2.0

Library version

3.12.0

Validation

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

[nlohmann]

It's expected and mentioned in the release notes for 3.12.0. This is the issue: #4475

[trofi]

Filed NixOS/nix#13046 to fix it on the nix side. Thank you!