Merge branch 'develop' into claude/fix-issue-2649-738aba

Conflicts:
- tests/src/unit-alt-string.cpp: kept both new sections (this PR's
  "conversion between basic_json specializations (#2649)" and develop's
  "strict enum" regression test for #5667)

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
 
@@ -274,6 +272,15 @@ basic_json(basic_json&& other) noexcept;
         When used without parentheses around an empty initializer list, `basic_json()` is called instead of this
         function, yielding the JSON `#!json null` value.
 
+- Overload 4:
+
+    !!! info "Implicit conversion"
+
+        The conversion is implicit unless [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
+        is defined to `0` and `BasicJsonType::string_t` differs from `string_t`. In that case, the constructor is
+        `explicit`, so a JSON value with a different string type is no longer silently converted, for example when it is
+        passed to a function taking `#!cpp const json&`. Write `#!cpp json(other)` or `#!cpp other.get<json>()` instead.
+
 - Overload 7:
 
     !!! info "Preconditions"
@@ -422,7 +429,8 @@ basic_json(basic_json&& other) noexcept;
 1. Since version 1.0.0.
 2. Since version 1.0.0.
 3. Since version 2.1.0.
-4. Since version 3.2.0.
+4. Since version 3.2.0. Explicit for different string types if `JSON_USE_IMPLICIT_CONVERSIONS` is `0` since
+   version 3.13.0.
 5. Since version 1.0.0.
 6. Since version 1.0.0.
 7. Since version 1.0.0. Fixed in version 3.13.0 to also check the iterator range for binary values; before, a range

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

@@ -53,7 +53,6 @@ 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/api/macros/json_use_implicit_conversions.md

@@ -5,7 +5,9 @@
 ```
 
 When defined to `0`, implicit conversions are switched off. By default, implicit conversions are switched on. The
-value directly affects [`operator ValueType`](../basic_json/operator_ValueType.md).
+value directly affects [`operator ValueType`](../basic_json/operator_ValueType.md) and the
+[converting constructor](../basic_json/basic_json.md) from a `basic_json` specialization with a different string
+type (overload 4).
 
 ## Default definition
 
@@ -57,6 +59,25 @@ By default, implicit conversions are enabled.
     auto s = j.get<std::string>();
     ```
 
+??? example "Conversion between `basic_json` specializations"
+
+    A `basic_json` specialization with a different string type is also no longer converted implicitly when
+    `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`:
+
+    ```cpp
+    using wjson = nlohmann::basic_json<std::map, std::vector, std::wstring>;
+
+    void load(const nlohmann::json& j);
+
+    wjson wj = /* ... */;
+    load(wj);                            // error: no implicit conversion
+    load(nlohmann::json(wj));            // OK: explicit conversion
+    load(wj.get<nlohmann::json>());      // OK: explicit conversion
+    ```
+
+    Specializations that share the same string type, such as `json` and `ordered_json`, remain implicitly
+    convertible.
+
 ## See also
 
 - [**operator ValueType**](../basic_json/operator_ValueType.md) - get a value (implicit)
@@ -66,3 +87,4 @@ By default, implicit conversions are enabled.
 ## Version history
 
 - Added in version 3.9.0.
+- Also affects the conversion between `basic_json` specializations with different string types since version 3.13.0.

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_AUTOMATIC_UDLS`
 
 When defined, `<nlohmann/json.hpp>` does not include `<nlohmann/json_literals.hpp>` with the user-defined string literals

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

@@ -291,7 +291,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_not
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {
@@ -471,13 +473,11 @@ 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
+#if JSON_BRACE_INIT_COPY_SEMANTICS
-// JSON_BRACE_INIT_COPY_SEMANTICS it copies it, which would serialize
+// JSON_BRACE_INIT_COPY_SEMANTICS makes a one-element braced list copy its
-// std::tuple<int>{5} as 5 rather than [5], and some compilers (e.g., Apple clang
+// element instead of wrapping it, which would serialize std::tuple<int>{5} as 5
-// 15 and 16) copy an element that is itself a basic_json even without it, so
+// rather than [5]. Build what the default deduction builds instead: an object
-// std::tuple<json>{true} became true rather than [true]. Build what the default
+// if the element is a [string, value] pair, a one-element array otherwise.
-// 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 +495,7 @@ 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

@@ -915,7 +915,3 @@ void templated_json_throw(ExceptionType exception)
 #ifndef JSON_DISABLE_ENUM_SERIALIZATION
     #define JSON_DISABLE_ENUM_SERIALIZATION 0
 #endif
-
-#ifndef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
-    #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0
-#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
 
 #ifndef JSON_TEST_KEEP_MACROS
     #undef JSON_CATCH

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

@@ -906,11 +906,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /*!
     @brief how many levels the operation going on in this thread has descended into
 
-    Copying a value, converting one from another specialization, and comparing
+    Copying a value and comparing two values share this count. The library never
-    two values share this count. The library never nests one of them inside
+    nests one inside the other - copying a value does not compare one, and
-    another - none of them does either of the other two on the way - and where
+    comparing two values does not copy them - and where user code nests them
-    user code nests them anyway, sharing the count only ends a descent sooner
+    anyway, sharing the count only ends a descent sooner than it had to, which
-    than it had to, which costs a little speed and is never wrong.
+    costs a little speed and is never wrong.
 
     A byte is enough: the count never exceeds the limit by more than the single
     level that notices the limit has been reached.
@@ -1267,222 +1267,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         copy_iteratively(src);
     }
 
-    /*!
-    @brief convert the value @a val of another specialization into this null
-           value; @a val must be neither an object nor an array
-
-    Converting such a value never descends, so both ways of converting an
-    object or an array (@ref convert_structured) leave their elements of this
-    kind to the converting constructor, which leaves them to this.
-    */
-    template<typename BasicJsonType>
-    void convert_leaf(const BasicJsonType& val)
-    {
-        using other_boolean_t = typename BasicJsonType::boolean_t;
-        using other_number_float_t = typename BasicJsonType::number_float_t;
-        using other_number_integer_t = typename BasicJsonType::number_integer_t;
-        using other_number_unsigned_t = typename BasicJsonType::number_unsigned_t;
-        using other_string_t = typename BasicJsonType::string_t;
-        using other_binary_t = typename BasicJsonType::binary_t;
-
-        switch (val.type())
-        {
-            case value_t::boolean:
-                JSONSerializer<other_boolean_t>::to_json(*this, val.template get<other_boolean_t>());
-                break;
-            case value_t::number_float:
-                JSONSerializer<other_number_float_t>::to_json(*this, val.template get<other_number_float_t>());
-                break;
-            case value_t::number_integer:
-                JSONSerializer<other_number_integer_t>::to_json(*this, val.template get<other_number_integer_t>());
-                break;
-            case value_t::number_unsigned:
-                JSONSerializer<other_number_unsigned_t>::to_json(*this, val.template get<other_number_unsigned_t>());
-                break;
-            case value_t::string:
-                JSONSerializer<other_string_t>::to_json(*this, val.template get_ref<const other_string_t&>());
-                break;
-            case value_t::binary:
-                JSONSerializer<other_binary_t>::to_json(*this, val.template get_ref<const other_binary_t&>());
-                break;
-            case value_t::null:
-                // m_data.m_type is already value_t::null
-                break;
-            case value_t::discarded:
-                m_data.m_type = value_t::discarded;
-                break;
-            case value_t::object: // LCOV_EXCL_LINE
-            case value_t::array:  // LCOV_EXCL_LINE
-            default:              // LCOV_EXCL_LINE
-                JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
-        }
-    }
-
-    /// scratch space for the converted elements of the arrays that
-    /// @ref convert_iteratively has yet to create
-    using convert_scratch_t = std::vector<basic_json, AllocatorType<basic_json>>;
-
-    /*!
-    @brief create the object or array @a val converted into this null value
-
-    Its converted elements are the last `val.size()` entries of @a elements (an
-    array) or of @a members (an object); they are moved into the container in
-    one go and then removed.
-    */
-    template<typename BasicJsonType>
-    void convert_level(const BasicJsonType& val, convert_scratch_t& elements, copy_scratch_t& members)
-    {
-        if (val.is_object())
-        {
-            const auto first = members.end() - static_cast<typename copy_scratch_t::difference_type>(val.size());
-            m_data.m_value.object = create<object_t>(std::make_move_iterator(first),
-                                    std::make_move_iterator(members.end()));
-            // only now that the object exists may this stop being a null value
-            m_data.m_type = value_t::object;
-            members.erase(first, members.end());
-        }
-        else
-        {
-            const auto first = elements.end() - static_cast<typename convert_scratch_t::difference_type>(val.size());
-            m_data.m_value.array = create<array_t>(std::make_move_iterator(first),
-                                                   std::make_move_iterator(elements.end()));
-            // only now that the array exists may this stop being a null value
-            m_data.m_type = value_t::array;
-            elements.erase(first, elements.end());
-        }
-
-        set_parents();
-    }
-
-    /*!
-    @brief convert the object or array @a val of another specialization into
-           this null value without recursing
-
-    The containers whose conversion has begun are kept on an explicit stack
-    rather than on the call stack. Unlike @ref copy_iteratively, this builds
-    every container from the bottom up: all its elements are converted first,
-    and the container is then created from them in one go, the way the range
-    constructor that converts the levels above the bound does. The two object
-    types need not enumerate their members in the same order, so the members
-    could not be paired up by position anyway, and building from a range keeps
-    what the range constructor does with keys that become equal on conversion.
-
-    Every value is complete before it is handed on, and a container gets its
-    type only once it exists, so whatever throws, every value left behind can
-    be destroyed.
-    */
-    template<typename BasicJsonType>
-    void convert_iteratively(const BasicJsonType& val)
-    {
-        using other_const_iterator = typename BasicJsonType::const_iterator;
-
-        // the containers whose conversion has begun, innermost last, each with
-        // its element to convert next
-        std::vector<std::pair<const BasicJsonType*, other_const_iterator>> pending;
-
-        // the converted elements of the pending arrays and the converted
-        // members of the pending objects, those of the innermost one last
-        convert_scratch_t elements;
-        copy_scratch_t members;
-
-        pending.emplace_back(&val, val.cbegin());
-
-        for (;;)
-        {
-            const BasicJsonType& container = *pending.back().first;
-            // a copy, as descending below can reallocate pending; the
-            // iterator kept in pending is only advanced through pending.back()
-            const other_const_iterator next = pending.back().second;
-
-            if (next != container.cend())
-            {
-                if (next->is_structured())
-                {
-                    // convert its elements first; next stays where it is until
-                    // the converted container is handed back to this one
-                    pending.emplace_back(&*next, next->cbegin());
-                    continue;
-                }
-
-                // the converting constructor does not descend into this value
-                if (container.is_object())
-                {
-                    members.emplace_back(next.key(), *next);
-                }
-                else
-                {
-                    elements.emplace_back(*next);
-                }
-                ++pending.back().second;
-                continue;
-            }
-
-            // all elements of the container are converted: create it
-            pending.pop_back();
-
-            if (pending.empty())
-            {
-                convert_level(container, elements, members);
-                return;
-            }
-
-            basic_json converted;
-            converted.convert_level(container, elements, members);
-#if JSON_DIAGNOSTIC_POSITIONS
-            converted.start_position = container.start_pos();
-            converted.end_position = container.end_pos();
-#endif
-
-            // hand it to the container it is an element of
-            if (pending.back().first->is_object())
-            {
-                members.emplace_back(pending.back().second.key(), std::move(converted));
-            }
-            else
-            {
-                elements.push_back(std::move(converted));
-            }
-            ++pending.back().second;
-        }
-    }
-
-    /*!
-    @brief convert the object or array @a val of another specialization into
-           this null value
-
-    Converting a container converts its elements, so a value nested deeply
-    enough used to exhaust the call stack. The descent is bounded here as in
-    @ref copy_structured: the first `detail::recursion_depth_limit()` levels
-    are converted by the containers' range constructors, just as they always were,
-    and anything below that is converted without the call stack by
-    @ref convert_iteratively.
-
-    @sa https://github.com/nlohmann/json/issues/5650
-    */
-    template<typename BasicJsonType>
-    void convert_structured(const BasicJsonType& val)
-    {
-        const nesting_depth_guard guard;
-
-        if (JSON_HEDLEY_LIKELY(guard.okay()))
-        {
-            // every element comes back to the converting constructor
-            if (val.is_object())
-            {
-                using other_object_t = typename BasicJsonType::object_t;
-                JSONSerializer<other_object_t>::to_json(*this, val.template get_ref<const other_object_t&>());
-            }
-            else
-            {
-                using other_array_t = typename BasicJsonType::array_t;
-                JSONSerializer<other_array_t>::to_json(*this, val.template get_ref<const other_array_t&>());
-            }
-            return;
-        }
-
-        convert_iteratively(val);
-    }
-
 
     /// the result of comparing two values, including values that cannot be
     /// ordered at all, such as a discarded value or a NaN
@@ -1812,12 +1596,7 @@ 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
+                   !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value, int > = 0 >
-#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 >
     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))))
@@ -1827,24 +1606,90 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  private:
+    /// whether a basic_json specialization can be converted implicitly into this one;
+    /// with JSON_USE_IMPLICIT_CONVERSIONS set to 0, this is only the case if both share
+    /// the same string type (see https://github.com/nlohmann/json/issues/2649)
+    template<typename BasicJsonType>
+    using is_implicitly_convertible_basic_json = std::integral_constant < bool,
+          (JSON_USE_IMPLICIT_CONVERSIONS != 0)
+          || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
+
+    /// tag to select the constructor that performs the conversion from another basic_json specialization
+    struct convert_basic_json_tag {};
+
+  public:
     /// @brief create a JSON value from an existing one
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     template < typename BasicJsonType,
                detail::enable_if_t <
-                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
     basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+    /// @brief create a JSON value from an existing one
+    /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
+    template < typename BasicJsonType,
+               detail::enable_if_t <
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && !is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
+    explicit basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+  private:
+    template<typename BasicJsonType>
+    basic_json(const BasicJsonType& val, convert_basic_json_tag /*unused*/)
 #if JSON_DIAGNOSTIC_POSITIONS
         : start_position(val.start_pos()),
           end_position(val.end_pos())
 #endif
     {
-        if (val.is_structured())
+        using other_boolean_t = typename BasicJsonType::boolean_t;
+        using other_number_float_t = typename BasicJsonType::number_float_t;
+        using other_number_integer_t = typename BasicJsonType::number_integer_t;
+        using other_number_unsigned_t = typename BasicJsonType::number_unsigned_t;
+        using other_string_t = typename BasicJsonType::string_t;
+        using other_object_t = typename BasicJsonType::object_t;
+        using other_array_t = typename BasicJsonType::array_t;
+        using other_binary_t = typename BasicJsonType::binary_t;
+
+        switch (val.type())
         {
-            convert_structured(val);
+            case value_t::boolean:
-        }
+                JSONSerializer<other_boolean_t>::to_json(*this, val.template get<other_boolean_t>());
-        else
+                break;
-        {
+            case value_t::number_float:
-            convert_leaf(val);
+                JSONSerializer<other_number_float_t>::to_json(*this, val.template get<other_number_float_t>());
+                break;
+            case value_t::number_integer:
+                JSONSerializer<other_number_integer_t>::to_json(*this, val.template get<other_number_integer_t>());
+                break;
+            case value_t::number_unsigned:
+                JSONSerializer<other_number_unsigned_t>::to_json(*this, val.template get<other_number_unsigned_t>());
+                break;
+            case value_t::string:
+                JSONSerializer<other_string_t>::to_json(*this, val.template get_ref<const other_string_t&>());
+                break;
+            case value_t::object:
+                JSONSerializer<other_object_t>::to_json(*this, val.template get_ref<const other_object_t&>());
+                break;
+            case value_t::array:
+                JSONSerializer<other_array_t>::to_json(*this, val.template get_ref<const other_array_t&>());
+                break;
+            case value_t::binary:
+                JSONSerializer<other_binary_t>::to_json(*this, val.template get_ref<const other_binary_t&>());
+                break;
+            case value_t::null:
+                *this = nullptr;
+                break;
+            case value_t::discarded:
+                m_data.m_type = value_t::discarded;
+                break;
+            default:            // LCOV_EXCL_LINE
+                JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
         }
         JSON_ASSERT(m_data.m_type == val.type());
 
@@ -1852,6 +1697,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  public:
     /// @brief create a container (array or object) from an initializer list
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(initializer_list_t init,
@@ -2618,7 +2464,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!
@@ -2757,7 +2603,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 

single_include/nlohmann/json.hpp

@@ -3329,10 +3329,6 @@ 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
-
 #if JSON_HAS_THREE_WAY_COMPARISON
     #include <compare> // partial_ordering
 #endif
@@ -4648,18 +4644,6 @@ 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
 {
@@ -6838,7 +6822,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_not
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {
@@ -7018,13 +7004,11 @@ 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
+#if JSON_BRACE_INIT_COPY_SEMANTICS
-// JSON_BRACE_INIT_COPY_SEMANTICS it copies it, which would serialize
+// JSON_BRACE_INIT_COPY_SEMANTICS makes a one-element braced list copy its
-// std::tuple<int>{5} as 5 rather than [5], and some compilers (e.g., Apple clang
+// element instead of wrapping it, which would serialize std::tuple<int>{5} as 5
-// 15 and 16) copy an element that is itself a basic_json even without it, so
+// rather than [5]. Build what the default deduction builds instead: an object
-// std::tuple<json>{true} became true rather than [true]. Build what the default
+// if the element is a [string, value] pair, a one-element array otherwise.
-// 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*/)
 {
@@ -7042,6 +7026,7 @@ 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*/)
@@ -27850,11 +27835,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /*!
     @brief how many levels the operation going on in this thread has descended into
 
-    Copying a value, converting one from another specialization, and comparing
+    Copying a value and comparing two values share this count. The library never
-    two values share this count. The library never nests one of them inside
+    nests one inside the other - copying a value does not compare one, and
-    another - none of them does either of the other two on the way - and where
+    comparing two values does not copy them - and where user code nests them
-    user code nests them anyway, sharing the count only ends a descent sooner
+    anyway, sharing the count only ends a descent sooner than it had to, which
-    than it had to, which costs a little speed and is never wrong.
+    costs a little speed and is never wrong.
 
     A byte is enough: the count never exceeds the limit by more than the single
     level that notices the limit has been reached.
@@ -28211,222 +28196,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         copy_iteratively(src);
     }
 
-    /*!
-    @brief convert the value @a val of another specialization into this null
-           value; @a val must be neither an object nor an array
-
-    Converting such a value never descends, so both ways of converting an
-    object or an array (@ref convert_structured) leave their elements of this
-    kind to the converting constructor, which leaves them to this.
-    */
-    template<typename BasicJsonType>
-    void convert_leaf(const BasicJsonType& val)
-    {
-        using other_boolean_t = typename BasicJsonType::boolean_t;
-        using other_number_float_t = typename BasicJsonType::number_float_t;
-        using other_number_integer_t = typename BasicJsonType::number_integer_t;
-        using other_number_unsigned_t = typename BasicJsonType::number_unsigned_t;
-        using other_string_t = typename BasicJsonType::string_t;
-        using other_binary_t = typename BasicJsonType::binary_t;
-
-        switch (val.type())
-        {
-            case value_t::boolean:
-                JSONSerializer<other_boolean_t>::to_json(*this, val.template get<other_boolean_t>());
-                break;
-            case value_t::number_float:
-                JSONSerializer<other_number_float_t>::to_json(*this, val.template get<other_number_float_t>());
-                break;
-            case value_t::number_integer:
-                JSONSerializer<other_number_integer_t>::to_json(*this, val.template get<other_number_integer_t>());
-                break;
-            case value_t::number_unsigned:
-                JSONSerializer<other_number_unsigned_t>::to_json(*this, val.template get<other_number_unsigned_t>());
-                break;
-            case value_t::string:
-                JSONSerializer<other_string_t>::to_json(*this, val.template get_ref<const other_string_t&>());
-                break;
-            case value_t::binary:
-                JSONSerializer<other_binary_t>::to_json(*this, val.template get_ref<const other_binary_t&>());
-                break;
-            case value_t::null:
-                // m_data.m_type is already value_t::null
-                break;
-            case value_t::discarded:
-                m_data.m_type = value_t::discarded;
-                break;
-            case value_t::object: // LCOV_EXCL_LINE
-            case value_t::array:  // LCOV_EXCL_LINE
-            default:              // LCOV_EXCL_LINE
-                JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
-        }
-    }
-
-    /// scratch space for the converted elements of the arrays that
-    /// @ref convert_iteratively has yet to create
-    using convert_scratch_t = std::vector<basic_json, AllocatorType<basic_json>>;
-
-    /*!
-    @brief create the object or array @a val converted into this null value
-
-    Its converted elements are the last `val.size()` entries of @a elements (an
-    array) or of @a members (an object); they are moved into the container in
-    one go and then removed.
-    */
-    template<typename BasicJsonType>
-    void convert_level(const BasicJsonType& val, convert_scratch_t& elements, copy_scratch_t& members)
-    {
-        if (val.is_object())
-        {
-            const auto first = members.end() - static_cast<typename copy_scratch_t::difference_type>(val.size());
-            m_data.m_value.object = create<object_t>(std::make_move_iterator(first),
-                                    std::make_move_iterator(members.end()));
-            // only now that the object exists may this stop being a null value
-            m_data.m_type = value_t::object;
-            members.erase(first, members.end());
-        }
-        else
-        {
-            const auto first = elements.end() - static_cast<typename convert_scratch_t::difference_type>(val.size());
-            m_data.m_value.array = create<array_t>(std::make_move_iterator(first),
-                                                   std::make_move_iterator(elements.end()));
-            // only now that the array exists may this stop being a null value
-            m_data.m_type = value_t::array;
-            elements.erase(first, elements.end());
-        }
-
-        set_parents();
-    }
-
-    /*!
-    @brief convert the object or array @a val of another specialization into
-           this null value without recursing
-
-    The containers whose conversion has begun are kept on an explicit stack
-    rather than on the call stack. Unlike @ref copy_iteratively, this builds
-    every container from the bottom up: all its elements are converted first,
-    and the container is then created from them in one go, the way the range
-    constructor that converts the levels above the bound does. The two object
-    types need not enumerate their members in the same order, so the members
-    could not be paired up by position anyway, and building from a range keeps
-    what the range constructor does with keys that become equal on conversion.
-
-    Every value is complete before it is handed on, and a container gets its
-    type only once it exists, so whatever throws, every value left behind can
-    be destroyed.
-    */
-    template<typename BasicJsonType>
-    void convert_iteratively(const BasicJsonType& val)
-    {
-        using other_const_iterator = typename BasicJsonType::const_iterator;
-
-        // the containers whose conversion has begun, innermost last, each with
-        // its element to convert next
-        std::vector<std::pair<const BasicJsonType*, other_const_iterator>> pending;
-
-        // the converted elements of the pending arrays and the converted
-        // members of the pending objects, those of the innermost one last
-        convert_scratch_t elements;
-        copy_scratch_t members;
-
-        pending.emplace_back(&val, val.cbegin());
-
-        for (;;)
-        {
-            const BasicJsonType& container = *pending.back().first;
-            // a copy, as descending below can reallocate pending; the
-            // iterator kept in pending is only advanced through pending.back()
-            const other_const_iterator next = pending.back().second;
-
-            if (next != container.cend())
-            {
-                if (next->is_structured())
-                {
-                    // convert its elements first; next stays where it is until
-                    // the converted container is handed back to this one
-                    pending.emplace_back(&*next, next->cbegin());
-                    continue;
-                }
-
-                // the converting constructor does not descend into this value
-                if (container.is_object())
-                {
-                    members.emplace_back(next.key(), *next);
-                }
-                else
-                {
-                    elements.emplace_back(*next);
-                }
-                ++pending.back().second;
-                continue;
-            }
-
-            // all elements of the container are converted: create it
-            pending.pop_back();
-
-            if (pending.empty())
-            {
-                convert_level(container, elements, members);
-                return;
-            }
-
-            basic_json converted;
-            converted.convert_level(container, elements, members);
-#if JSON_DIAGNOSTIC_POSITIONS
-            converted.start_position = container.start_pos();
-            converted.end_position = container.end_pos();
-#endif
-
-            // hand it to the container it is an element of
-            if (pending.back().first->is_object())
-            {
-                members.emplace_back(pending.back().second.key(), std::move(converted));
-            }
-            else
-            {
-                elements.push_back(std::move(converted));
-            }
-            ++pending.back().second;
-        }
-    }
-
-    /*!
-    @brief convert the object or array @a val of another specialization into
-           this null value
-
-    Converting a container converts its elements, so a value nested deeply
-    enough used to exhaust the call stack. The descent is bounded here as in
-    @ref copy_structured: the first `detail::recursion_depth_limit()` levels
-    are converted by the containers' range constructors, just as they always were,
-    and anything below that is converted without the call stack by
-    @ref convert_iteratively.
-
-    @sa https://github.com/nlohmann/json/issues/5650
-    */
-    template<typename BasicJsonType>
-    void convert_structured(const BasicJsonType& val)
-    {
-        const nesting_depth_guard guard;
-
-        if (JSON_HEDLEY_LIKELY(guard.okay()))
-        {
-            // every element comes back to the converting constructor
-            if (val.is_object())
-            {
-                using other_object_t = typename BasicJsonType::object_t;
-                JSONSerializer<other_object_t>::to_json(*this, val.template get_ref<const other_object_t&>());
-            }
-            else
-            {
-                using other_array_t = typename BasicJsonType::array_t;
-                JSONSerializer<other_array_t>::to_json(*this, val.template get_ref<const other_array_t&>());
-            }
-            return;
-        }
-
-        convert_iteratively(val);
-    }
-
 
     /// the result of comparing two values, including values that cannot be
     /// ordered at all, such as a discarded value or a NaN
@@ -28756,12 +28525,7 @@ 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
+                   !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value, int > = 0 >
-#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 >
     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))))
@@ -28771,24 +28535,90 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  private:
+    /// whether a basic_json specialization can be converted implicitly into this one;
+    /// with JSON_USE_IMPLICIT_CONVERSIONS set to 0, this is only the case if both share
+    /// the same string type (see https://github.com/nlohmann/json/issues/2649)
+    template<typename BasicJsonType>
+    using is_implicitly_convertible_basic_json = std::integral_constant < bool,
+        (JSON_USE_IMPLICIT_CONVERSIONS != 0)
+        || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
+
+    /// tag to select the constructor that performs the conversion from another basic_json specialization
+    struct convert_basic_json_tag {};
+
+  public:
     /// @brief create a JSON value from an existing one
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     template < typename BasicJsonType,
                detail::enable_if_t <
-                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
     basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+    /// @brief create a JSON value from an existing one
+    /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
+    template < typename BasicJsonType,
+               detail::enable_if_t <
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && !is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
+    explicit basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+  private:
+    template<typename BasicJsonType>
+    basic_json(const BasicJsonType& val, convert_basic_json_tag /*unused*/)
 #if JSON_DIAGNOSTIC_POSITIONS
         : start_position(val.start_pos()),
           end_position(val.end_pos())
 #endif
     {
-        if (val.is_structured())
+        using other_boolean_t = typename BasicJsonType::boolean_t;
+        using other_number_float_t = typename BasicJsonType::number_float_t;
+        using other_number_integer_t = typename BasicJsonType::number_integer_t;
+        using other_number_unsigned_t = typename BasicJsonType::number_unsigned_t;
+        using other_string_t = typename BasicJsonType::string_t;
+        using other_object_t = typename BasicJsonType::object_t;
+        using other_array_t = typename BasicJsonType::array_t;
+        using other_binary_t = typename BasicJsonType::binary_t;
+
+        switch (val.type())
         {
-            convert_structured(val);
+            case value_t::boolean:
-        }
+                JSONSerializer<other_boolean_t>::to_json(*this, val.template get<other_boolean_t>());
-        else
+                break;
-        {
+            case value_t::number_float:
-            convert_leaf(val);
+                JSONSerializer<other_number_float_t>::to_json(*this, val.template get<other_number_float_t>());
+                break;
+            case value_t::number_integer:
+                JSONSerializer<other_number_integer_t>::to_json(*this, val.template get<other_number_integer_t>());
+                break;
+            case value_t::number_unsigned:
+                JSONSerializer<other_number_unsigned_t>::to_json(*this, val.template get<other_number_unsigned_t>());
+                break;
+            case value_t::string:
+                JSONSerializer<other_string_t>::to_json(*this, val.template get_ref<const other_string_t&>());
+                break;
+            case value_t::object:
+                JSONSerializer<other_object_t>::to_json(*this, val.template get_ref<const other_object_t&>());
+                break;
+            case value_t::array:
+                JSONSerializer<other_array_t>::to_json(*this, val.template get_ref<const other_array_t&>());
+                break;
+            case value_t::binary:
+                JSONSerializer<other_binary_t>::to_json(*this, val.template get_ref<const other_binary_t&>());
+                break;
+            case value_t::null:
+                *this = nullptr;
+                break;
+            case value_t::discarded:
+                m_data.m_type = value_t::discarded;
+                break;
+            default:            // LCOV_EXCL_LINE
+                JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
         }
         JSON_ASSERT(m_data.m_type == val.type());
 
@@ -28796,6 +28626,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  public:
     /// @brief create a container (array or object) from an initializer list
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(initializer_list_t init,
@@ -29562,7 +29393,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!
@@ -29701,7 +29532,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 
@@ -33774,7 +33605,6 @@ 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
 
 #ifndef JSON_TEST_KEEP_MACROS
     #undef JSON_CATCH

tests/src/unit-allocator.cpp

@@ -479,77 +479,6 @@ TEST_CASE("deep copy uses the provided allocator")
     CHECK(copy == j);
 }
 
-namespace
-{
-// the number of constructions countdown_allocator lets happen, including the
-// one that fails; 0 means none ever fails
-std::size_t constructions_until_failure = 0;
-
-template<class T>
-struct countdown_allocator : std::allocator<T>
-{
-    using std::allocator<T>::allocator;
-
-    template<class U, class... Args>
-    void construct(U* p, Args&& ... args)
-    {
-        if (constructions_until_failure != 0 && --constructions_until_failure == 0)
-        {
-            throw std::bad_alloc();
-        }
-
-        ::new (static_cast<void*>(p)) U(std::forward<Args>(args)...);
-    }
-
-    template <class U>
-    struct rebind
-    {
-        using other = countdown_allocator<U>;
-    };
-};
-} // namespace
-
-TEST_CASE("converting a deeply nested value from another specialization fails cleanly (#5650)")
-{
-    using countdown_json = nlohmann::basic_json<std::map,
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          countdown_allocator>;
-
-    // deeper than the 128 levels the converting constructor descends into, so
-    // that failures land on both sides of the bound - or, built with
-    // JSON_NO_THREAD_LOCAL, all in the iterative conversion
-    json j = {1, "two", {{"three", 3}}};
-    for (std::size_t i = 0; i < 150; ++i)
-    {
-        j = json{{"a", json::array({j, "sibling"})}};
-    }
-
-    // Fail every construction in turn. Each failure has to reach the caller,
-    // and everything built until then has to be destroyed cleanly.
-    std::size_t failures = 0;
-    for (std::size_t n = 1;; ++n)
-    {
-        constructions_until_failure = n;
-        try
-        {
-            const countdown_json converted = j;
-            constructions_until_failure = 0;
-            CHECK(converted.dump() == j.dump());
-            break;
-        }
-        catch (const std::bad_alloc&)
-        {
-            ++failures;
-        }
-    }
-    CHECK(failures > 0);
-}
-
 namespace
 {
 template<class T>

tests/src/unit-alt-string.cpp

@@ -13,6 +13,7 @@
 
 #include <cstdint>
 #include <string>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -419,6 +420,41 @@ TEST_CASE("alternative string type")
         CHECK(j2.dump() == R"({"/foo/0":"bar","/foo/1":"baz"})");
     }
 
+    SECTION("conversion between basic_json specializations (#2649)")
+    {
+        // explicit conversions are always possible
+        CHECK(std::is_constructible<nlohmann::json, alt_json>::value);
+        CHECK(std::is_constructible<alt_json, nlohmann::json>::value);
+        CHECK(std::is_constructible<nlohmann::json, nlohmann::ordered_json>::value);
+        CHECK(std::is_constructible<nlohmann::ordered_json, nlohmann::json>::value);
+
+        // specializations with the same string type are implicitly convertible
+        CHECK(std::is_convertible<nlohmann::ordered_json, nlohmann::json>::value);
+        CHECK(std::is_convertible<nlohmann::json, nlohmann::ordered_json>::value);
+
+        // specializations with different string types are only implicitly convertible
+        // if implicit conversions are enabled
+#if JSON_USE_IMPLICIT_CONVERSIONS
+        CHECK(std::is_convertible<alt_json, nlohmann::json>::value);
+        CHECK(std::is_convertible<nlohmann::json, alt_json>::value);
+#else
+        CHECK_FALSE(std::is_convertible<alt_json, nlohmann::json>::value);
+        CHECK_FALSE(std::is_convertible<nlohmann::json, alt_json>::value);
+#endif
+
+        // get<BasicJsonType>() works in either case
+        const nlohmann::json j = {{"foo", 1}, {"bar", true}};
+        CHECK(j.get<nlohmann::ordered_json>() == nlohmann::ordered_json(j));
+        // (only a number is converted here, as objects and strings are affected by #3425)
+        CHECK(nlohmann::json(42).get<alt_json>() == 42);
+        CHECK(alt_json(nlohmann::json(42)) == 42);
+
+        // get_to() also works in either case
+        alt_json a;
+        nlohmann::json(42).get_to(a);
+        CHECK(a == 42);
+    }
+
     SECTION("strict enum")
     {
         // regression test for #5667: NLOHMANN_JSON_SERIALIZE_ENUM_STRICT's from_json

tests/src/unit-diagnostic-positions.cpp

@@ -141,58 +141,6 @@ TEST_CASE("Better diagnostics with positions")
         check_objects(300);
     }
 
-    SECTION("converting keeps the positions of nested values (#5650)")
-    {
-        // Values nested deeper than the converting constructor's descent bound
-        // are converted without the call stack, on a path that has to carry the
-        // positions of every value over itself. Objects and arrays take turns,
-        // and the innermost value is null, which used to lose its positions.
-        const auto check_conversion = [](std::size_t depth)
-        {
-            CAPTURE(depth)
-
-            std::string text;
-            std::string closing;
-            for (std::size_t i = 0; i < depth; ++i)
-            {
-                text += (i % 2 == 0) ? "[12, " : R"({"b":1, "a":)";
-                closing += (i % 2 == 0) ? ']' : '}';
-            }
-            text += "null";
-            text.append(closing.rbegin(), closing.rend());
-
-            const json original = json::parse(text);
-            const nlohmann::ordered_json converted = original;
-
-            const json* o = &original;
-            const nlohmann::ordered_json* c = &converted;
-            for (std::size_t level = 0; level <= depth; ++level)
-            {
-                CAPTURE(level)
-                REQUIRE(c->start_pos() == o->start_pos());
-                REQUIRE(c->end_pos() == o->end_pos());
-
-                if (level < depth)
-                {
-                    // the number beside the value nested next
-                    const json& o_number = o->is_object() ? o->at("b") : o->at(0);
-                    const nlohmann::ordered_json& c_number = c->is_object() ? c->at("b") : c->at(0);
-                    REQUIRE(c_number.start_pos() == o_number.start_pos());
-                    REQUIRE(c_number.end_pos() == o_number.end_pos());
-
-                    o = o->is_object() ? &o->at("a") : &o->at(1);
-                    c = c->is_object() ? &c->at("a") : &c->at(1);
-                }
-            }
-        };
-
-        check_conversion(1);
-        check_conversion(127);
-        check_conversion(128);
-        check_conversion(129);
-        check_conversion(300);
-    }
-
     SECTION("JSON patch add to primitive parent (#4292)")
     {
         // the JSON Patch "add" target /foo/bar/baz has a string parent

tests/src/unit-diagnostics.cpp

@@ -341,36 +341,6 @@ TEST_CASE("Regression tests for extended diagnostics")
         }
     }
 
-    SECTION("Regression test for issue #5650 - converting keeps the parents of nested values")
-    {
-        // A value nested deeper than the converting constructor's descent bound
-        // is converted without the call stack. Every container that path creates
-        // has to have the parents of its children set, or the JSON Pointer in the
-        // diagnostic is cut short. Objects and arrays take turns.
-        const std::size_t pairs = 150;
-
-        json j = "not a number";
-        std::string pointer;
-        for (std::size_t i = 0; i < pairs; ++i)
-        {
-            j = json{{"a", json::array({j})}};
-            pointer += "/a/0";
-        }
-
-        const nlohmann::ordered_json converted = j;
-
-        const nlohmann::ordered_json* inner = &converted;
-        for (std::size_t i = 0; i < pairs; ++i)
-        {
-            inner = &inner->at("a").at(0);
-        }
-
-        std::string const expected = "[json.exception.type_error.302] (" + pointer + ") type must be number, but is string";
-        int i = 0;
-        CHECK_THROWS_WITH_AS(i = inner->get<int>(), expected.c_str(), nlohmann::ordered_json::type_error);
-        CHECK(i == 0);
-    }
-
     SECTION("Regression test for issue #5668 - wrong path for std::map/unordered_map with non-string keys")
     {
         // a map with non-string keys is read from an array of [key, value] arrays;

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

@@ -1,93 +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>
-
-// clang before 4 and GCC before 5 cannot create a std::tuple of basic_json
-// references at all, with or without JSON_DISABLE_TUPLE_REFERENCE_CONVERSION:
-// the tuple constructors make them instantiate basic_json's conversion operator
-// for libstdc++'s internal tuple bases, which fails hard
-#if (defined(__clang__) && __clang_major__ < 4) || (!defined(__clang__) && defined(__GNUC__) && __GNUC__ < 5)
-    #define SKIP_TESTS_FOR_JSON_REFERENCE_TUPLES
-#endif
-
-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);
-    }
-
-#ifndef SKIP_TESTS_FOR_JSON_REFERENCE_TUPLES
-    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");
-    }
-#endif
-
-    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;
-#ifndef SKIP_TESTS_FOR_JSON_REFERENCE_TUPLES
-        CHECK(json(std::forward_as_tuple(i, j)) == json::array({1, true}));
-        CHECK(json(std::forward_as_tuple(j, j)) == json::array({true, true}));
-#endif
-
-        // 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}));
-
-#ifndef SKIP_TESTS_FOR_JSON_REFERENCE_TUPLES
-        // a reference to a different basic_json specialization
-        ordered_json oj = true;
-        CHECK(json(std::forward_as_tuple(oj)) == json::array({true}));
-#endif
-    }
-}

tests/src/unit-large_json.cpp

@@ -13,7 +13,6 @@ using nlohmann::json;
 
 #include <algorithm>
 #include <string>
-#include <vector>
 
 TEST_CASE("tests on very large JSONs")
 {
@@ -54,24 +53,6 @@ const json* innermost_value(const json& j, std::size_t& depth)
     return current;
 }
 
-// The text of a value nested depth levels deep around the number 0. Level i is
-// an array if pattern[i % pattern.size()] is '[', and otherwise an object with
-// the single member "a", which every object type enumerates in the same order.
-std::string nested_text(std::size_t depth, const std::string& pattern)
-{
-    std::string text;
-    std::string closing;
-    for (std::size_t i = 0; i < depth; ++i)
-    {
-        const bool array = pattern[i % pattern.size()] == '[';
-        text += array ? "[" : "{\"a\":";
-        closing += array ? ']' : '}';
-    }
-    text += '0';
-    text.append(closing.rbegin(), closing.rend());
-    return text;
-}
-
 } // namespace
 
 TEST_CASE("tests on deeply nested JSONs")
@@ -243,114 +224,5 @@ TEST_CASE("tests on deeply nested JSONs")
             CHECK(*innermost_value(j, unused) == 0);
         }
     }
-
-    SECTION("issue #5650 - stack overflow converting between specializations")
-    {
-        const std::vector<std::string> patterns = {"[", "{", "[{"};
-
-        SECTION("json to ordered_json")
-        {
-            for (const auto& pattern : patterns)
-            {
-                CAPTURE(pattern);
-                const std::string text = nested_text(depth, pattern);
-                const json j = json::parse(text);
-
-                const nlohmann::ordered_json converted = j;
-                CHECK(converted.dump() == text);
-            }
-        }
-
-        SECTION("ordered_json to json")
-        {
-            for (const auto& pattern : patterns)
-            {
-                CAPTURE(pattern);
-                const std::string text = nested_text(depth, pattern);
-                const nlohmann::ordered_json o = nlohmann::ordered_json::parse(text);
-
-                const json converted = o;
-                CHECK(converted.dump() == text);
-            }
-        }
-
-        SECTION("get<ordered_json>()")
-        {
-            for (const auto& pattern : patterns)
-            {
-                CAPTURE(pattern);
-                const std::string text = nested_text(depth, pattern);
-                const json j = json::parse(text);
-
-                CHECK(j.get<nlohmann::ordered_json>().dump() == text);
-            }
-        }
-
-        SECTION("depths around the bound of the recursive descent")
-        {
-            for (std::size_t d = 1; d <= 300; ++d)
-            {
-                CAPTURE(d);
-                for (const auto& pattern : patterns)
-                {
-                    CAPTURE(pattern);
-                    const std::string text = nested_text(d, pattern);
-                    const json j = json::parse(text);
-
-                    const nlohmann::ordered_json converted = j;
-                    CHECK(converted.dump() == text);
-                    const json back = converted;
-                    CHECK(back.dump() == text);
-                }
-            }
-        }
-
-        SECTION("values below the bound are converted as values above it")
-        {
-            // Bury a value below the bound, where it is converted without the
-            // call stack, and compare it with the same value converted on its
-            // own by the containers' range constructors. Its objects have
-            // members that the two object types enumerate in different orders.
-            const auto bury = [](nlohmann::ordered_json value)
-            {
-                for (std::size_t i = 0; i < 200; ++i)
-                {
-                    value = nlohmann::ordered_json::array({std::move(value)});
-                }
-                return value;
-            };
-            const auto dig = [](const json & value)
-            {
-                const json* current = &value;
-                for (std::size_t i = 0; i < 200; ++i)
-                {
-                    current = &current->at(0);
-                }
-                return current;
-            };
-
-            nlohmann::ordered_json value = nlohmann::ordered_json::object();
-            value["z"] = {1, -2, 3U, 4.5, true, nullptr, "six", nlohmann::ordered_json::binary({7, 8}, 9),
-                          nlohmann::ordered_json::binary({10}), nlohmann::ordered_json::array(), nlohmann::ordered_json::object()
-                         };
-            value["y"] = {{"x", {{"w", 1}, {"v", 2}}}, {"u", {3, {{"t", 4}, {"s", 5}}}}};
-            value["r"] = nlohmann::ordered_json::array({nlohmann::ordered_json(nlohmann::ordered_json::value_t::discarded)});
-
-            const json converted_above = value;
-            const json buried = bury(value);
-            const json& converted_below = *dig(buried);
-
-            CHECK(converted_below.dump() == converted_above.dump());
-            CHECK(converted_below.at("z").at(7).get_binary().subtype() == 9);
-            CHECK_FALSE(converted_below.at("z").at(8).get_binary().has_subtype());
-            CHECK(converted_below.at("r").at(0).is_discarded());
-
-            // a discarded value is never equal to anything, so compare the rest
-            value.erase("r");
-            const json without_discarded_above = value;
-            const json without_discarded_buried = bury(value);
-            CHECK(*dig(without_discarded_buried) == without_discarded_above);
-        }
-    }
 }
 

tests/src/unit-regression2.cpp

@@ -18,19 +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
-
-// clang before 4 and GCC before 5 cannot create a std::tuple of basic_json
-// references at all, with or without JSON_DISABLE_TUPLE_REFERENCE_CONVERSION:
-// the tuple constructors make them instantiate basic_json's conversion operator
-// for libstdc++'s internal tuple bases, which fails hard
-#if (defined(__clang__) && __clang_major__ < 4) || (!defined(__clang__) && defined(__GNUC__) && __GNUC__ < 5)
-    #define SKIP_TESTS_FOR_JSON_REFERENCE_TUPLES
-#endif
-
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using json = nlohmann::json;
@@ -41,7 +28,6 @@ using ordered_json = nlohmann::ordered_json;
 
 #include <cstdio>
 #include <list>
-#include <tuple>
 #include <type_traits>
 #include <utility>
 
@@ -556,20 +542,6 @@ 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);
-#ifndef SKIP_TESTS_FOR_JSON_REFERENCE_TUPLES
-        CHECK(json(std::forward_as_tuple(j)) == json::array({true}));
-#endif
-    }
-#endif
-
     SECTION("PR #2181 - regression bug with lvalue")
     {
         // see https://github.com/nlohmann/json/pull/2181#issuecomment-653326060