Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions

basic_json can be constructed from std::tuple<json&>, which it turns into
a one-element array. Because of this, std::tuple picks its converting
constructor that converts the whole source tuple instead of the
element-wise one. As a result, std::tuple<const json&> built from
std::forward_as_tuple(j) binds to a temporary (a compile error with libc++,
a dangling reference with other standard libraries), and std::tuple<json>
built the same way holds [j] instead of a copy of j.

The new opt-in macro JSON_DISABLE_TUPLE_REFERENCE_CONVERSION (CMake option
JSON_DisableTupleReferenceConversion) removes the conversion from a
one-element tuple holding a reference to the same basic_json type, so
std::tuple converts element-wise. It is off by default, so existing
behavior is unchanged. It does not change any function body and therefore
is not part of the ABI tag.

Fixes #2226

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/basic_json.md

@@ -159,6 +159,8 @@ basic_json(basic_json&& other) noexcept;
     - `CompatibleType` is not `basic_json` (to avoid hijacking copy/move constructors),
     - `CompatibleType` is not a different `basic_json` type (i.e. with different template arguments)
     - `CompatibleType` is not a `basic_json` nested type (e.g., `json_pointer`, `iterator`, etc.)
+    - if [`JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`](../macros/json_disable_tuple_reference_conversion.md) is defined
+      to `1`: `CompatibleType` is not a one-element `std::tuple` holding a reference to `basic_json`
     - `json_serializer<U>` (with `U = uncvref_t<CompatibleType>`) has a `to_json(basic_json_t&, CompatibleType&&)`
        method
 

docs/mkdocs/docs/api/macros/index.md

@@ -52,6 +52,7 @@ header. See also the [macro overview page](../../features/macros.md).
 
 - [**JSON_BRACE_INIT_COPY_SEMANTICS**](json_brace_init_copy_semantics.md) - opt in to copy/move semantics for single-element brace initialization
 - [**JSON_DISABLE_ENUM_SERIALIZATION**](json_disable_enum_serialization.md) - switch off default serialization/deserialization functions for enums
+- [**JSON_DISABLE_TUPLE_REFERENCE_CONVERSION**](json_disable_tuple_reference_conversion.md) - switch off conversion from a one-element tuple of a JSON reference
 - [**JSON_USE_IMPLICIT_CONVERSIONS**](json_use_implicit_conversions.md) - control implicit conversions
 
 ## Comparison behavior

docs/mkdocs/docs/api/macros/json_disable_tuple_reference_conversion.md

@@ -0,0 +1,114 @@
+# JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
+
+```cpp
+#define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION /* value */
+```
+
+When defined to `1`, a `basic_json` value can no longer be constructed from a one-element `std::tuple` whose element is
+a reference to that `basic_json` type, such as `std::tuple<json&>`, `std::tuple<const json&>`, or `std::tuple<json&&>`.
+These are the tuples created by `std::forward_as_tuple(j)`.
+
+## Default definition
+
+The default value is `0` (disabled — existing behavior is preserved).
+
+```cpp
+#define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0
+```
+
+## Notes
+
+!!! note "Background"
+
+    By default, `basic_json` can be constructed from any `std::tuple` whose elements can be converted to JSON; the result
+    is an array. This includes `std::tuple<json&>`, which becomes a one-element array.
+
+    `std::tuple` only converts another tuple element by element if its element type cannot be constructed from the whole
+    source tuple. Because `json` *can* be constructed from `std::tuple<json&>`, `std::tuple` instead converts the whole
+    tuple into a single `json` value. This has two surprising effects:
+
+    ```cpp
+    json j = true;
+
+    // rejected by some standard libraries (e.g., libc++); with others, the
+    // reference binds to a temporary that is destroyed right away
+    std::tuple<const json&> t1(std::forward_as_tuple(j));
+
+    // compiles, but std::get<0>(t2) is [true], not true
+    std::tuple<json> t2(std::forward_as_tuple(j));
+    ```
+
+    Enabling this macro removes the conversion, so both tuples are converted element by element: `std::get<0>(t1)`
+    refers to `j`, and `std::get<0>(t2)` is a copy of `j` (see [#2226](https://github.com/nlohmann/json/issues/2226)).
+
+!!! warning "Opt-in only"
+
+    This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no effect.
+
+!!! note "Affected conversions"
+
+    Only one-element tuples holding a reference to the **same** `basic_json` type are affected. Constructing a JSON value
+    from them no longer compiles:
+
+    ```cpp
+    json j = true;
+    json a = std::forward_as_tuple(j);  // error with the macro enabled
+    json b = json::array({j});          // use this instead: [true]
+    ```
+
+    Tuples holding a JSON value (`std::make_tuple(j)`), tuples with more than one element, and tuples holding references
+    to other types (including other `basic_json` specializations) are converted to arrays as before.
+
+!!! hint "CMake option"
+
+    This behavior can also be controlled with the CMake option
+    [`JSON_DisableTupleReferenceConversion`](../../integration/cmake.md#json_disabletuplereferenceconversion)
+    (`OFF` by default) which defines `JSON_DISABLE_TUPLE_REFERENCE_CONVERSION` accordingly.
+
+## Examples
+
+??? example "Default behavior (macro not defined)"
+
+    ```cpp
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    int main()
+    {
+        json j = true;
+
+        std::tuple<json> t(std::forward_as_tuple(j));
+        // std::get<0>(t) is [true] -- the whole tuple was converted
+    }
+    ```
+
+??? example "Conversion disabled (macro defined to 1)"
+
+    ```cpp
+    #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 1
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    int main()
+    {
+        json j = true;
+
+        std::tuple<json> t(std::forward_as_tuple(j));
+        // std::get<0>(t) is true -- a copy of j
+
+        std::tuple<const json&> r(std::forward_as_tuple(j));
+        // std::get<0>(r) refers to j
+    }
+    ```
+
+## See also
+
+- [**basic_json(CompatibleType&&)**](../basic_json/basic_json.md) - the affected constructor
+- [:simple-cmake: JSON_DisableTupleReferenceConversion](../../integration/cmake.md#json_disabletuplereferenceconversion) -
+  CMake option to control the macro
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/features/macros.md

@@ -83,6 +83,13 @@ When defined, default parse and serialize functions for enums are excluded and h
 
 See [full documentation of `JSON_DISABLE_ENUM_SERIALIZATION`](../api/macros/json_disable_enum_serialization.md).
 
+## `JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`
+
+When defined to `1`, a JSON value can no longer be created from a one-element `std::tuple` holding a reference to a JSON
+value, such as the result of `std::forward_as_tuple(j)`. This lets `std::tuple` convert such tuples element-wise.
+
+See [full documentation of `JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`](../api/macros/json_disable_tuple_reference_conversion.md).
+
 ## `JSON_NO_IO`
 
 When defined, headers `<cstdio>`, `<ios>`, `<iosfwd>`, `<istream>`, and `<ostream>` are not included and parse functions

docs/mkdocs/docs/integration/cmake.md

@@ -169,6 +169,12 @@ Enable position diagnostics by defining macro [`JSON_DIAGNOSTIC_POSITIONS`](../a
 Disable default `enum` serialization by defining the macro
 [`JSON_DISABLE_ENUM_SERIALIZATION`](../api/macros/json_disable_enum_serialization.md). This option is `OFF` by default.
 
+### `JSON_DisableTupleReferenceConversion`
+
+Disable the conversion from a one-element `std::tuple` holding a reference to a JSON value by defining the macro
+[`JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`](../api/macros/json_disable_tuple_reference_conversion.md). This option is
+`OFF` by default.
+
 ### `JSON_FastTests`
 
 Skip expensive/slow test suites. This option is `OFF` by default. Depends on `JSON_BuildTests`.

docs/mkdocs/mkdocs.yml

@@ -284,6 +284,7 @@ nav:
           - 'JSON_DIAGNOSTICS': api/macros/json_diagnostics.md
           - 'JSON_DIAGNOSTIC_POSITIONS': api/macros/json_diagnostic_positions.md
           - 'JSON_DISABLE_ENUM_SERIALIZATION': api/macros/json_disable_enum_serialization.md
+          - 'JSON_DISABLE_TUPLE_REFERENCE_CONVERSION': api/macros/json_disable_tuple_reference_conversion.md
           - 'JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20': api/macros/json_has_cpp_11.md
           - 'JSON_HAS_EXPERIMENTAL_FILESYSTEM, JSON_HAS_FILESYSTEM': api/macros/json_has_filesystem.md
           - 'JSON_HAS_RANGES': api/macros/json_has_ranges.md