gcc 14.2 bug: array subscript out of bounds with `JSON_DIAGNOSTICS`

[ryanjmulder]

Description

std::vector< json > jsons{};
jsons.emplace_back( json( { { "key", "value" } } ) );

yields error: array subscript is partly outside array bounds of 'unsigned char [32]' [-Werror=array-bounds=]
When built with JSON_DIAGNOSTICS and -O3 and -Wall

It appears to be a gcc bug fixed in 15, but I didn't know if you wanted to implement a work-around or not.

Reproduction steps

See compiler explorer here: https://godbolt.org/z/fnrzsqKeT

Expected vs. actual results

compiles

Minimal code example

Error messages

Compiler and operating system

Ubuntu 24.04, gcc 14

Library version

3.12.0 and HEAD today (568b708)

Validation

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

[gregmarr]

See #4704
We do not want to try to fix this in the library.

[nlohmann]

FYI: The issue is fixed with GCC 15. There,

#include <nlohmann/json.hpp>
#include <vector>

int main() {
    std::vector<nlohmann::json> jsons{};
    jsons.emplace_back(nlohmann::json({{"key", "value"}}));
    return 0;
}

Compiles with: g++ -std=c++17 -DJSON_DIAGNOSTICS -O3 -Wall -Werror=array-bounds.