fix: treat single-element brace-init as copy/move instead of wrapping in array (#5074) (#5090)

* fix: treat single-element brace-init as copy/move

When passing a json value using brace initialization with a single element
(e.g., `json j{someObj}` or `foo({someJson})`), C++ always prefers the
initializer_list constructor over the copy/move constructor. This caused
the value to be unexpectedly wrapped in a single-element array.

This bug was previously compiler-dependent (GCC wrapped, Clang did not),
but Clang 20 started matching GCC behavior, making it a universal issue.

Fix: In the initializer_list constructor, when type deduction is enabled
and the list has exactly one element, copy/move it directly instead of
creating a single-element array.

Before:
  json obj = {{"key", 1}};
  json j{obj};   // -> [{"key":1}]  (wrong: array)
  foo({obj});    // -> [{"key":1}]  (wrong: array)

After:
  json j{obj};   // -> {"key":1}   (correct: copy)
  foo({obj});    // -> {"key":1}   (correct: copy)

To explicitly create a single-element array, use json::array({value}).

Fixes the issue #5074

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* fix: regenerate amalgamated single_include/nlohmann/json.hpp

- Add missing comment from include/nlohmann/json.hpp explaining the
  single-element brace-init fix (issue #5074)
- Fix extra 4-space indentation in embedded json_fwd.hpp section

Regenerated by running: make amalgamate

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* Revert brace-init semantics change and fix amalgamation

The single-element brace-init change was a breaking change that cannot be accepted upstream. Reverted all related source, test, and doc changes, then regenerated single_include with correct indentation to pass the amalgamation CI check.

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* Fix: add JSON_BRACE_INIT_COPY_SEMANTICS opt-in macro for issue #5074

Single-element brace initialization wrapping in an array cannot be fixed without breaking existing code. Added JSON_BRACE_INIT_COPY_SEMANTICS as an opt-in macro (default 0) so users can enable copy/move semantics for single-element brace init without affecting anyone relying on the current behavior.

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* docs: add dedicated macro page and CI test target for JSON_BRACE_INIT_COPY_SEMANTICS

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* fix: remove compiler-dependent assertions from #5074 regression test

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* fix: use defined() guard for JSON_BRACE_INIT_COPY_SEMANTICS to satisfy -Wundef

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* docs: fix section name in json_brace_init_copy_semantics.md to pass style check

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* docs: move Default definition section before Notes to fix style check order

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

---------

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

docs/mkdocs/docs/home/faq.md

@@ -38,6 +38,26 @@ for objects.
 
     To avoid any confusion and ensure portable code, **do not** use brace initialization with the types `basic_json`, `json`, or `ordered_json` unless you want to create an object or array as shown in the examples above.
 
+    To explicitly create a single-element array, use `json::array({value})`:
+
+    ```cpp
+    json j = json::array({true});  // [true]
+    ```
+
+**Opt-in copy semantics (since version 3.12.0)**
+
+If you define `JSON_BRACE_INIT_COPY_SEMANTICS` to `1` before including the library, single-element brace initialization is treated as copy/move instead of creating a single-element array:
+
+```cpp
+#define JSON_BRACE_INIT_COPY_SEMANTICS 1
+#include <nlohmann/json.hpp>
+
+json obj = {{"key", "value"}};
+json j{obj};   // -> {"key":"value"}  (copy, not array)
+```
+
+Without the macro (default behavior), `json j{obj}` creates `[{"key":"value"}]`. This opt-in macro fixes issue #5074 while preserving backwards compatibility for existing code.
+
 ## Limitations
 
 ### Relaxed parsing