tests/src/unit-diagnostics-optimized.cpp

//     __ _____ _____ _____
//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
// |  |  |__   |  |  | | | |  version 3.12.0
// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT

// Regression test for https://github.com/nlohmann/json/issues/5742: with
// JSON_DIAGNOSTICS, GCC (12 to at least 16) reported a false -Warray-bounds
// error in the inlined set_parents() at -O3. The type of a new string was set
// before the string was allocated, so GCC had to assume that operator new
// could change it again and checked the object branch of set_parents()
// against the string's allocation. Setting the type after creating the value
// avoids this. The warning depends on GCC's inlining decisions, so the
// sections cover two patterns that trigger it on different GCC versions
// (#4819 and #5742).
// On GCC, this file is compiled with -O3 -Werror=array-bounds (see
// tests/CMakeLists.txt), so the test fails to build if the warning returns.

#include "doctest_compatibility.h"

#ifdef JSON_DIAGNOSTICS
    #undef JSON_DIAGNOSTICS
#endif

#define JSON_DIAGNOSTICS 1

#include <nlohmann/json.hpp>
using nlohmann::json;

#include <algorithm>
#include <iterator>
#include <utility>
#include <vector>

// the static table in to_json has an exit-time destructor
DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")

namespace
{
enum class diag_color
{
    red,
    green,
    blue
};

void to_json(json& j, const diag_color& c)
{
    static const std::pair<diag_color, json> m[] = // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
    {
        {diag_color::red, "r"},
        {diag_color::green, "g"},
        {diag_color::blue, "b"},
    };
    const auto* it = std::find_if(std::begin(m), std::end(m), [c](const std::pair<diag_color, json>& p)
    {
        return p.first == c;
    });
    j = ((it != std::end(m)) ? it : std::begin(m))->second; // like NLOHMANN_JSON_SERIALIZE_ENUM
}
} // namespace

TEST_CASE("diagnostics with optimization")
{
    SECTION("issue #4819 - object in vector")
    {
        std::vector<json> jsons{};
        jsons.emplace_back(json({{"key", "value"}}));
        CHECK(jsons.back()["key"] == "value");
    }

    SECTION("issue #5742 - string values from a static table")
    {
        json j = json::array();
        j.push_back(diag_color::red);
        j.push_back(diag_color::green);
        j.push_back(diag_color::blue);
        CHECK(j.dump() == R"(["r","g","b"])");
        CHECK_THROWS_WITH_AS(j[1].get<int>(), "[json.exception.type_error.302] (/1) type must be number, but is string", json::type_error);
    }
}

DOCTEST_CLANG_SUPPRESS_WARNING_POP