Merge branch 'develop' into claude/fix-3669-fa2492

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

.github/workflows/ubuntu.yml

@@ -100,7 +100,7 @@ jobs:
     container: ubuntu:focal
     strategy:
       matrix:
-        target: [ci_cmake_flags, ci_test_diagnostics, ci_test_diagnostic_positions, ci_test_noexceptions, ci_test_noimplicitconversions, ci_test_legacycomparison, ci_test_noglobaludls, ci_test_disableenumserialization, ci_test_disabletuplereferenceconversion, ci_test_skiplibraryversioncheck, ci_test_simdutf, ci_test_strict_nul_handling, ci_test_no_thread_local]
+        target: [ci_cmake_flags, ci_test_diagnostics, ci_test_diagnostic_positions, ci_test_noexceptions, ci_test_noimplicitconversions, ci_test_legacycomparison, ci_test_noglobaludls, ci_test_disableenumserialization, ci_test_skiplibraryversioncheck, ci_test_simdutf, ci_test_strict_nul_handling, ci_test_no_thread_local]
     steps:
       - name: Install build-essential
         run: apt-get update ; apt-get install -y build-essential unzip wget git libssl-dev

CMakeLists.txt

@@ -55,7 +55,6 @@ option(JSON_Diagnostic_Positions           "Enable diagnostic positions." OFF)
 option(JSON_GlobalUDLs                     "Place user-defined string literals in the global namespace." ON)
 option(JSON_ImplicitConversions            "Enable implicit conversions." ON)
 option(JSON_DisableEnumSerialization       "Disable default integer enum serialization." OFF)
-option(JSON_DisableTupleReferenceConversion "Disable conversion from a one-element tuple of a JSON reference." OFF)
 option(JSON_LegacyDiscardedValueComparison "Enable legacy discarded value comparison." OFF)
 option(JSON_Install                        "Install CMake targets during install step." ${MAIN_PROJECT})
 option(JSON_MultipleHeaders                "Use non-amalgamated version of the library." ON)
@@ -102,10 +101,6 @@ if (JSON_DisableEnumSerialization)
     message(STATUS "Enum integer serialization is disabled (JSON_DISABLE_ENUM_SERIALIZATION=1)")
 endif()
 
-if (JSON_DisableTupleReferenceConversion)
-    message(STATUS "Tuple reference conversion is disabled (JSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1)")
-endif()
-
 if (JSON_LegacyDiscardedValueComparison)
     message(STATUS "Legacy discarded value comparison enabled (JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1)")
 endif()
@@ -148,7 +143,6 @@ target_compile_definitions(
     $<$<NOT:$<BOOL:${JSON_GlobalUDLs}>>:JSON_USE_GLOBAL_UDLS=0>
     $<$<NOT:$<BOOL:${JSON_ImplicitConversions}>>:JSON_USE_IMPLICIT_CONVERSIONS=0>
     $<$<BOOL:${JSON_DisableEnumSerialization}>:JSON_DISABLE_ENUM_SERIALIZATION=1>
-    $<$<BOOL:${JSON_DisableTupleReferenceConversion}>:JSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1>
     $<$<BOOL:${JSON_Diagnostics}>:JSON_DIAGNOSTICS=1>
     $<$<BOOL:${JSON_Diagnostic_Positions}>:JSON_DIAGNOSTIC_POSITIONS=1>
     $<$<BOOL:${JSON_LegacyDiscardedValueComparison}>:JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1>

cmake/ci.cmake

@@ -276,20 +276,6 @@ add_custom_target(ci_test_disableenumserialization
     COMMENT "Compile and test with enum serialization disabled"
 )
 
-###############################################################################
-# Disable conversion from a one-element tuple of a JSON reference.
-###############################################################################
-
-add_custom_target(ci_test_disabletuplereferenceconversion
-    COMMAND ${CMAKE_COMMAND}
-    -DCMAKE_BUILD_TYPE=Debug -GNinja
-    -DJSON_BuildTests=ON -DJSON_FastTests=ON -DJSON_DisableTupleReferenceConversion=ON
-    -S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_disabletuplereferenceconversion
-    COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_disabletuplereferenceconversion
-    COMMAND cd ${PROJECT_BINARY_DIR}/build_disabletuplereferenceconversion && ${CMAKE_CTEST_COMMAND} --parallel ${N} --output-on-failure
-    COMMENT "Compile and test with tuple reference conversion disabled"
-)
-
 ###############################################################################
 # Skip the multiple-inclusion library version check.
 ###############################################################################

docs/docset/docSet.sql

@@ -210,7 +210,6 @@ INSERT INTO searchIndex(name, type, path) VALUES ('JSON_CATCH_USER', 'Macro', 'a
 INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DIAGNOSTICS', 'Macro', 'api/macros/json_diagnostics/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DIAGNOSTIC_POSITIONS', 'Macro', 'api/macros/json_diagnostic_positions/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DISABLE_ENUM_SERIALIZATION', 'Macro', 'api/macros/json_disable_enum_serialization/index.html');
-INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DISABLE_TUPLE_REFERENCE_CONVERSION', 'Macro', 'api/macros/json_disable_tuple_reference_conversion/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('JSON_HAS_CPP_11', 'Macro', 'api/macros/json_has_cpp_11/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('JSON_HAS_CPP_14', 'Macro', 'api/macros/json_has_cpp_11/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('JSON_HAS_CPP_17', 'Macro', 'api/macros/json_has_cpp_11/index.html');

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

@@ -159,8 +159,6 @@ 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

@@ -52,7 +52,6 @@ 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

@@ -1,114 +0,0 @@
-# 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/arbitrary_types.md

@@ -79,6 +79,7 @@ Some important things:
 * When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
 * In function `from_json`, use function [`at()`](../api/basic_json/at.md) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
 * You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
+* If you control the type, consider defining `to_json`/`from_json` as `friend` functions inside the class ("hidden friends"). Argument-dependent lookup then only finds them for your type, which also avoids a [GCC < 11 compilation error](../home/faq.md#incomplete-detector-type-with-gcc-11).
 
 
 ## Simplify your life with macros

docs/mkdocs/docs/features/macros.md

@@ -83,13 +83,6 @@ 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/home/faq.md

@@ -307,6 +307,51 @@ APP_CPPFLAGS += -frtti -fexceptions
 The code compiles successfully with [Android NDK](https://developer.android.com/ndk/index.html?hl=ml), Revision 9 - 11 (and possibly later) and [CrystaX's Android NDK](https://www.crystax.net/en/android/ndk) version 10.
 
 
+### Incomplete `detector` type with GCC < 11
+
+!!! question
+
+    Why does GCC 10 or older fail with `invalid use of incomplete type 'struct nlohmann::detail::detector<..., to_json_function, ...>'` for a type that holds an `optional` member?
+
+This happens with GCC 10 and older in C++11/C++14 mode when all of these hold:
+
+- a class `Holder` has an `optional<Dummy>` member (e.g., `boost::optional`),
+- `Dummy` has a constructor taking a `json` value, and
+- `to_json` for `Holder` is a free function in the namespace of `Dummy`.
+
+```cpp
+class Dummy {
+  public:
+    explicit Dummy(const nlohmann::json& j);
+};
+
+class Holder {
+    boost::optional<Dummy> d;
+};
+
+void to_json(nlohmann::json& j, const Holder& h);  // triggers the error
+```
+
+To decide whether `Dummy` is copyable, the compiler checks whether a `Dummy` can be converted to `json`. That check
+looks up `to_json` via argument-dependent lookup, finds the unrelated `to_json` for `Holder`, and eventually asks again
+whether `Dummy` is copyable. GCC before version 11 turns this cycle into a hard error; GCC 11 and later, Clang, and
+C++17 mode compile the code. The same error shows up without this library whenever a constrained converting constructor
+is involved, so the library can't avoid it.
+
+To work around this, define `to_json` (and `from_json`) as a *hidden friend* inside the class. That way,
+argument-dependent lookup only finds it for `Holder`:
+
+```cpp
+class Holder {
+    boost::optional<Dummy> d;
+
+    friend void to_json(nlohmann::json& j, const Holder& h) { /* ... */ }
+};
+```
+
+The [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](../api/macros/nlohmann_define_type_intrusive.md) macros define hidden friends as
+well. See [#3669](https://github.com/nlohmann/json/issues/3669) for details.
+
 ### Missing STL function
 
 !!! question "Questions"

docs/mkdocs/docs/integration/cmake.md

@@ -169,12 +169,6 @@ 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

@@ -287,7 +287,6 @@ 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

include/nlohmann/detail/conversions/to_json.hpp

@@ -471,13 +471,11 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
     j = { std::get<Idx>(t)... };
 }
 
-// A one-element braced list does not reliably wrap its element: with
-// JSON_BRACE_INIT_COPY_SEMANTICS it copies it, which would serialize
-// std::tuple<int>{5} as 5 rather than [5], and some compilers (e.g., Apple clang
-// 15 and 16) copy an element that is itself a basic_json even without it, so
-// std::tuple<json>{true} became true rather than [true]. Build what the default
-// deduction builds instead: an object if the element is a [string, value] pair,
-// a one-element array otherwise.
+#if JSON_BRACE_INIT_COPY_SEMANTICS
+// JSON_BRACE_INIT_COPY_SEMANTICS makes a one-element braced list copy its
+// element instead of wrapping it, which would serialize std::tuple<int>{5} as 5
+// rather than [5]. Build what the default deduction builds instead: an object
+// if the element is a [string, value] pair, a one-element array otherwise.
 template<typename BasicJsonType, typename Tuple>
 inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<0> /*unused*/)
 {
@@ -495,6 +493,7 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
         j = BasicJsonType::array({std::move(element)});
     }
 }
+#endif
 
 template<typename BasicJsonType, typename Tuple>
 inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& /*unused*/, index_sequence<> /*unused*/)

include/nlohmann/detail/macro_scope.hpp

@@ -916,10 +916,6 @@ void templated_json_throw(ExceptionType exception)
     #define JSON_DISABLE_ENUM_SERIALIZATION 0
 #endif
 
-#ifndef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-    #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0
-#endif
-
 #ifndef JSON_USE_GLOBAL_UDLS
     #define JSON_USE_GLOBAL_UDLS 1
 #endif

include/nlohmann/detail/macro_unscope.hpp

@@ -25,7 +25,6 @@
 #undef JSON_INLINE_VARIABLE
 #undef JSON_NO_UNIQUE_ADDRESS
 #undef JSON_DISABLE_ENUM_SERIALIZATION
-#undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
 #undef JSON_USE_GLOBAL_UDLS
 
 #ifndef JSON_TEST_KEEP_MACROS

include/nlohmann/detail/meta/type_traits.hpp

@@ -636,18 +636,6 @@ template<typename BasicJsonType, typename CompatibleType>
 struct is_compatible_type
     : is_compatible_type_impl<BasicJsonType, CompatibleType> {};
 
-// a one-element std::tuple holding a reference to BasicJsonType, as created by
-// std::forward_as_tuple(j); see JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-template<typename BasicJsonType, typename T>
-struct is_basic_json_reference_tuple : std::false_type {};
-
-template<typename BasicJsonType, typename T>
-struct is_basic_json_reference_tuple<BasicJsonType, std::tuple<T>>
-{
-    static constexpr bool value =
-        std::is_reference<T>::value && std::is_same<uncvref_t<T>, BasicJsonType>::value;
-};
-
 template<typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_binary_type
 {

include/nlohmann/json.hpp

@@ -1595,12 +1595,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     template < typename CompatibleType,
                typename U = detail::uncvref_t<CompatibleType>,
                detail::enable_if_t <
-                   !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value
-#if JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-                   // see https://github.com/nlohmann/json/issues/2226
-                   && !detail::is_basic_json_reference_tuple<basic_json_t, U>::value
-#endif
-                   , int > = 0 >
+                   !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value, int > = 0 >
     basic_json(CompatibleType && val) noexcept(noexcept( // NOLINT(bugprone-forwarding-reference-overload,bugprone-exception-escape)
                 JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
                                            std::forward<CompatibleType>(val))))

single_include/nlohmann/json.hpp

@@ -3327,10 +3327,6 @@ void templated_json_throw(ExceptionType exception)
     #define JSON_DISABLE_ENUM_SERIALIZATION 0
 #endif
 
-#ifndef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-    #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0
-#endif
-
 #ifndef JSON_USE_GLOBAL_UDLS
     #define JSON_USE_GLOBAL_UDLS 1
 #endif
@@ -4650,18 +4646,6 @@ template<typename BasicJsonType, typename CompatibleType>
 struct is_compatible_type
     : is_compatible_type_impl<BasicJsonType, CompatibleType> {};
 
-// a one-element std::tuple holding a reference to BasicJsonType, as created by
-// std::forward_as_tuple(j); see JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-template<typename BasicJsonType, typename T>
-struct is_basic_json_reference_tuple : std::false_type {};
-
-template<typename BasicJsonType, typename T>
-struct is_basic_json_reference_tuple<BasicJsonType, std::tuple<T>>
-{
-    static constexpr bool value =
-        std::is_reference<T>::value && std::is_same<uncvref_t<T>, BasicJsonType>::value;
-};
-
 template<typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_binary_type
 {
@@ -7016,13 +7000,11 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
     j = { std::get<Idx>(t)... };
 }
 
-// A one-element braced list does not reliably wrap its element: with
-// JSON_BRACE_INIT_COPY_SEMANTICS it copies it, which would serialize
-// std::tuple<int>{5} as 5 rather than [5], and some compilers (e.g., Apple clang
-// 15 and 16) copy an element that is itself a basic_json even without it, so
-// std::tuple<json>{true} became true rather than [true]. Build what the default
-// deduction builds instead: an object if the element is a [string, value] pair,
-// a one-element array otherwise.
+#if JSON_BRACE_INIT_COPY_SEMANTICS
+// JSON_BRACE_INIT_COPY_SEMANTICS makes a one-element braced list copy its
+// element instead of wrapping it, which would serialize std::tuple<int>{5} as 5
+// rather than [5]. Build what the default deduction builds instead: an object
+// if the element is a [string, value] pair, a one-element array otherwise.
 template<typename BasicJsonType, typename Tuple>
 inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<0> /*unused*/)
 {
@@ -7040,6 +7022,7 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
         j = BasicJsonType::array({std::move(element)});
     }
 }
+#endif
 
 template<typename BasicJsonType, typename Tuple>
 inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& /*unused*/, index_sequence<> /*unused*/)
@@ -27495,12 +27478,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     template < typename CompatibleType,
                typename U = detail::uncvref_t<CompatibleType>,
                detail::enable_if_t <
-                   !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value
-#if JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-                   // see https://github.com/nlohmann/json/issues/2226
-                   && !detail::is_basic_json_reference_tuple<basic_json_t, U>::value
-#endif
-                   , int > = 0 >
+                   !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value, int > = 0 >
     basic_json(CompatibleType && val) noexcept(noexcept( // NOLINT(bugprone-forwarding-reference-overload,bugprone-exception-escape)
             JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
                                        std::forward<CompatibleType>(val))))
@@ -32550,7 +32528,6 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
 #undef JSON_INLINE_VARIABLE
 #undef JSON_NO_UNIQUE_ADDRESS
 #undef JSON_DISABLE_ENUM_SERIALIZATION
-#undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
 #undef JSON_USE_GLOBAL_UDLS
 
 #ifndef JSON_TEST_KEEP_MACROS

tests/src/unit-disable-tuple-reference-conversion.cpp

@@ -1,79 +0,0 @@
-//     __ _____ _____ _____
-//  __|  |   __|     |   | |  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
-
-#include "doctest_compatibility.h"
-
-// This file tests the opt-in JSON_DISABLE_TUPLE_REFERENCE_CONVERSION, so it
-// defines the macro itself rather than relying on a -D flag, and runs in every
-// build.
-#ifdef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-    #undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-#endif
-
-#define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 1
-
-#include <nlohmann/json.hpp>
-using nlohmann::json;
-using nlohmann::ordered_json;
-
-#include <string>
-#include <tuple>
-#include <type_traits>
-#include <utility>
-
-TEST_CASE("JSON_DISABLE_TUPLE_REFERENCE_CONVERSION")
-{
-    SECTION("json is not constructible from a one-element tuple of a json reference")
-    {
-        CHECK_FALSE(std::is_constructible<json, std::tuple<json&>>::value);
-        CHECK_FALSE(std::is_constructible<json, std::tuple<const json&>>::value);
-        CHECK_FALSE(std::is_constructible < json, std::tuple < json && >>::value);
-        CHECK_FALSE(std::is_constructible<json, const std::tuple<json&>&>::value);
-        CHECK_FALSE(std::is_constructible<ordered_json, std::tuple<ordered_json&>>::value);
-    }
-
-    SECTION("issue #2226 - tuple<const json&> from tuple<json&> keeps the reference")
-    {
-        json j = true;
-        const std::tuple<const json&> tup(std::forward_as_tuple(j));
-        CHECK(&std::get<0>(tup) == &j);
-    }
-
-    SECTION("tuple<json> from tuple<json&> copies the element")
-    {
-        const json j = {{"key", "value"}};
-        const std::tuple<json> t1(std::forward_as_tuple(j));
-        CHECK(std::get<0>(t1) == j);
-
-        json j2 = "text";
-        const std::tuple<json> t2(std::forward_as_tuple(std::move(j2)));
-        CHECK(std::get<0>(t2) == "text");
-    }
-
-    SECTION("other tuple conversions are not affected")
-    {
-        const json j = true;
-
-        // one-element tuple holding a json value
-        CHECK(json(std::make_tuple(j)) == json::array({true}));
-
-        // tuples with more than one element, even when holding references
-        int i = 1;
-        CHECK(json(std::forward_as_tuple(i, j)) == json::array({1, true}));
-        CHECK(json(std::forward_as_tuple(j, j)) == json::array({true, true}));
-
-        // one-element tuples holding references to other types
-        std::string s = "text";
-        CHECK(json(std::forward_as_tuple(s)) == json::array({"text"}));
-        CHECK(json(std::forward_as_tuple(i)) == json::array({1}));
-
-        // a reference to a different basic_json specialization
-        ordered_json oj = true;
-        CHECK(json(std::forward_as_tuple(oj)) == json::array({true}));
-    }
-}

tests/src/unit-regression2.cpp

@@ -18,11 +18,6 @@
 // for some reason including this after the json header leads to linker errors with VS 2017...
 #include <locale>
 
-// skip tests if JSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1 (#2226)
-#if defined(JSON_DISABLE_TUPLE_REFERENCE_CONVERSION) && (JSON_DISABLE_TUPLE_REFERENCE_CONVERSION == 1)
-    #define SKIP_TESTS_FOR_TUPLE_REFERENCE_CONVERSION
-#endif
-
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using json = nlohmann::json;
@@ -33,7 +28,6 @@ using ordered_json = nlohmann::ordered_json;
 
 #include <cstdio>
 #include <list>
-#include <tuple>
 #include <type_traits>
 #include <utility>
 
@@ -247,6 +241,52 @@ class my_allocator : public std::allocator<T>
     };
 };
 
+/////////////////////////////////////////////////////////////////////
+// for #3669
+/////////////////////////////////////////////////////////////////////
+
+// mimics boost::optional's converting constructor, whose SFINAE check asks
+// whether T is constructible from const U&
+template<class T, class Arg>
+struct issue3669_is_constructible
+{
+    template<class T2, class A2, class = decltype(T2(std::declval<A2>()))>
+    static char test(int);
+    template<class, class>
+    static long test(...);
+    static constexpr bool value = sizeof(test<T, Arg>(0)) == 1;
+};
+
+template<class T>
+class issue3669_optional
+{
+  public:
+    issue3669_optional() = default;
+    template<class U>
+    issue3669_optional(const issue3669_optional<U>& /*unused*/, // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+                       typename std::enable_if<issue3669_is_constructible<T, const U&>::value, bool>::type /*unused*/ = true) {}
+};
+
+class Issue3669Dummy
+{
+  public:
+    explicit Issue3669Dummy(const json& /*unused*/) {}
+};
+
+class Issue3669Holder
+{
+    issue3669_optional<Issue3669Dummy> d{};
+
+    // GCC < 11 (C++11/14) rejects a free to_json(json&, const Issue3669Holder&)
+    // here, because ADL for Issue3669Dummy finds it and closes an instantiation
+    // cycle; a hidden friend is only visible to ADL for Issue3669Holder
+    friend void to_json(json& j, const Issue3669Holder& h)
+    {
+        static_cast<void>(h.d); // silence -Wunused-private-field
+        j = "holder";
+    }
+};
+
 TEST_CASE("regression tests 2")
 {
     SECTION("issue #1001 - Fix memory leak during parser callback")
@@ -548,18 +588,6 @@ TEST_CASE("regression tests 2")
                       )));
     }
 
-#ifndef SKIP_TESTS_FOR_TUPLE_REFERENCE_CONVERSION
-    SECTION("issue #2226 - std::tuple dangling reference - implicit conversion")
-    {
-        // by default, a one-element tuple holding a json reference converts to
-        // a one-element array; JSON_DISABLE_TUPLE_REFERENCE_CONVERSION removes
-        // this conversion (see unit-disable-tuple-reference-conversion.cpp)
-        const json j = true;
-        CHECK(std::is_constructible<json, std::tuple<const json&>>::value);
-        CHECK(json(std::forward_as_tuple(j)) == json::array({true}));
-    }
-#endif
-
     SECTION("PR #2181 - regression bug with lvalue")
     {
         // see https://github.com/nlohmann/json/pull/2181#issuecomment-653326060
@@ -784,6 +812,14 @@ TEST_CASE("regression tests 2")
         CHECK(j == k);
     }
 
+    SECTION("issue #3669 - invalid use of incomplete type with optional member and to_json")
+    {
+        const Issue3669Holder h{};
+        const Issue3669Holder h2(h); // NOLINT(performance-unnecessary-copy-initialization)
+        const json j = h2;
+        CHECK(j == "holder");
+    }
+
 }
 
 TEST_CASE("regression test - parser callback must not lose a duplicate key's prior value")