Copy constructor changes semantics

[mbrucher]

Description

As far as I know, this was a known issue on gcc, but clang was OK, but this is now in clang 20. Using {} to construct the object adds an array around an original disctionary. This is IMHO a critical bug and now that gcc and clang are displaying this behavior, it should be fixed.

Reproduction steps

Just do

void Object::method(Json json)

call it with a dictionary, the new json object is now wrapped in an array.

Expected vs. actual results

The new object should not be wrapped in an array

Minimal code example

`void Object::method(Json json)`

Error messages

Compiler and operating system

clang 20, RH8

Library version

3.12

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]

You mean this issue? https://json.nlohmann.me/home/faq/#brace-initialization-yields-arrays

[haqqimm]

Hi everyone, I've been investigating this issue and successfully reproduced it using GCC/Clang.

json original_obj = {{"dev", "qi"}, {"dev", "haqqi"}};
// test_function({original_obj});
Result: [{"dev":"haqqi","dev":"qi"}] // Incorrectly wrapped in an array

Root Cause Analysis:
The issue stems from the basic_json(initializer_list_t init, ...) constructor. When passing a single basic_json object wrapped in braces {obj}, the init.size() is 1.

The is_an_object check (which looks for an array of two elements where the first is a string) evaluates to false. Consequently, the constructor falls back to the else block and forces the creation of an array type:

m_data.m_type = value_t::array;
m_data.m_value.array = create<array_t>(init.begin(), init.end());

I'd love to submit a PR to fix this. To ensure I follow the project's architectural guidelines, what would be the preferred approach here? Should we add an SFINAE constraint to disambiguate the copy constructor from the initializer_list constructor when size() == 1 and the element is already a basic_json object?

[nlohmann]

This issue cannot be fixed without breaking the behavior of existing code.