Convert one-element tuples to arrays on every compiler

to_json for std::tuple assigns a braced list, j = { std::get<Idx>(t)... }.
With a single element that is itself a basic_json, Apple clang 15 and 16
treat j = {x} as a copy of x, so std::tuple<json>{true} became true
instead of [true]. The macOS jobs (Xcode 15.1, 16.1) failed the new
checks in unit-disable-tuple-reference-conversion and unit-regression2.

The one-element overload that already handles
JSON_BRACE_INIT_COPY_SEMANTICS builds the array (or object, for a
[string, value] element) explicitly, the same way the initializer-list
constructor does. Use it unconditionally. The output is unchanged on
compilers that already wrapped the element.

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

.github/workflows/ubuntu.yml

@@ -31,45 +31,6 @@ jobs:
       - name: Build
         run: cmake --build build --target ci_test_gcc
 
-  ci_meson_install:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
-        with:
-          persist-credentials: false
-      - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@fffaaafeea488556c2c12dad60690008bc1caacb # v4.4.2
-      - name: Install Meson
-        run: pip install meson
-      - name: Install with Meson
-        run: |
-          meson setup build-meson --prefix=${{ github.workspace }}/install
-          meson install -C build-meson
-      - name: Use the installed package with find_package
-        run: |
-          cmake -S tests/cmake_import/project -B build-import -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install
-          cmake --build build-import
-      - name: Install with Meson and non-default options
-        run: |
-          meson setup build-meson-options --prefix=${{ github.workspace }}/install-options -DMultipleHeaders=true -DDiagnostics=true -DGlobalUDLs=false
-          meson install -C build-meson-options
-      - name: Check that the options reach the installed files
-        run: |
-          test -d install-options/include/nlohmann/detail
-          cflags=$(PKG_CONFIG_PATH=${{ github.workspace }}/install-options/share/pkgconfig pkg-config --cflags nlohmann_json)
-          echo "$cflags"
-          echo "$cflags" | grep -q -- '-DJSON_DIAGNOSTICS=1'
-          echo "$cflags" | grep -q -- '-DJSON_USE_GLOBAL_UDLS=0'
-          grep -q 'JSON_USE_GLOBAL_UDLS=0;JSON_DIAGNOSTICS=1' install-options/share/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
-          cmake -S tests/cmake_import/project -B build-import-options -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-options
-          cmake --build build-import-options
-      - name: Install with Meson and the include directory outside the prefix
-        run: |
-          meson setup build-meson-split --prefix=${{ github.workspace }}/install-split --includedir=${{ github.workspace }}/install-split-dev/include
-          meson install -C build-meson-split
-          cmake -S tests/cmake_import/project -B build-import-split -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-split
-          cmake --build build-import-split
-
   ci_infer:
     runs-on: ubuntu-latest
     steps:
@@ -139,7 +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_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_disabletuplereferenceconversion, 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

@@ -55,6 +55,7 @@ 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)
@@ -101,6 +102,10 @@ 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()
@@ -143,6 +148,7 @@ 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>
@@ -168,30 +174,7 @@ if (MSVC)
     )
 endif()
 
-# Install a pkg-config file, so other tools can find this. It carries the same
-# compile definitions as the target above.
-set(NLOHMANN_JSON_PKGCONFIG_CFLAGS "")
-if (NOT JSON_GlobalUDLs)
-    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_GLOBAL_UDLS=0")
-endif()
-if (NOT JSON_ImplicitConversions)
-    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_IMPLICIT_CONVERSIONS=0")
-endif()
-if (JSON_DisableEnumSerialization)
-    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DISABLE_ENUM_SERIALIZATION=1")
-endif()
-if (JSON_Diagnostics)
-    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DIAGNOSTICS=1")
-endif()
-if (JSON_Diagnostic_Positions)
-    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DIAGNOSTIC_POSITIONS=1")
-endif()
-if (JSON_LegacyDiscardedValueComparison)
-    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1")
-endif()
-if (JSON_StrictNulHandling)
-    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_STRICT_NUL_HANDLING=1")
-endif()
+# Install a pkg-config file, so other tools can find this.
 configure_file(
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake/pkg-config.pc.in"
     "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"

FILES.md

@@ -260,29 +260,9 @@ make BUILD.bazel
 
 The "Check amalgamation" workflow fails if the file is out of date.
 
-### `meson.build` and `meson_options.txt`
+### `meson.build`
 
-Meson build definitions suitable for use as a subproject ("wrap" in Meson terminology).
-
-Projects wishing to use the wrap can execute:
-```sh
-meson wrap install nlohmann_json
-```
-
-Which allows Meson to build from source when a system provided dependency isn't available.
-
-To build directly:
-```sh
-meson setup builddir
-ninja -C builddir
-```
-
-`meson_options.txt` defines the options, which mirror the CMake options that change the library's target (for example,
-`-DDiagnostics=true`). Meson requires this file next to `meson.build`, so it is also part of `include.zip`.
-
-When installing, `meson.build` installs the headers, a pkg-config file, and the CMake package config files, so that
-`find_package(nlohmann_json)` works. As Meson cannot generate `nlohmann_jsonTargets.cmake` itself, it is created from
-the template `cmake/nlohmann_jsonTargets.cmake.in`, which is only used by Meson.
+The build definition for the [Meson](https://mesonbuild.com) build system.
 
 ### `Package.swift`
 

Makefile

@@ -222,7 +222,7 @@ json.tar.xz:
 # We use `-X` to make the resulting ZIP file reproducible, see
 # <https://content.pivotal.io/blog/barriers-to-deterministic-reproducible-zip-files>.
 include.zip: BUILD.bazel
-	zip -9 --recurse-paths -X include.zip $(SRCS) $(AMALGAMATED_FILE) $(AMALGAMATED_FWD_FILE) BUILD.bazel MODULE.bazel meson.build meson_options.txt LICENSE.MIT
+	zip -9 --recurse-paths -X include.zip $(SRCS) $(AMALGAMATED_FILE) $(AMALGAMATED_FWD_FILE) BUILD.bazel MODULE.bazel meson.build LICENSE.MIT
 
 # Create the files for a release and add signatures and hashes.
 release: include.zip json.tar.xz

cmake/ci.cmake

@@ -276,6 +276,20 @@ 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.
 ###############################################################################

cmake/nlohmann_jsonTargets.cmake.in

@@ -1,38 +0,0 @@
-# Imported target for installations made with Meson (see meson.build).
-#
-# CMake installations generate this file with install(EXPORT ...). Meson cannot
-# do that, but as the library is header-only, the target only needs an include
-# directory, the C++ standard, and the compile definitions of the options that
-# differ from their defaults. Paths are computed relative to this file so that
-# the installation can be relocated (e.g., into a sysroot), unless includedir or
-# datadir is outside the prefix.
-
-if(TARGET @PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@)
-    return()
-endif()
-
-get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_DIR}/@NLOHMANN_JSON_CONFIG_TO_PREFIX@" ABSOLUTE)
-
-add_library(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ INTERFACE IMPORTED)
-set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
-    INTERFACE_INCLUDE_DIRECTORIES "@NLOHMANN_JSON_INCLUDE_DIR@"
-)
-if(CMAKE_VERSION VERSION_LESS 3.8)
-    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
-        INTERFACE_COMPILE_FEATURES cxx_range_for
-    )
-else()
-    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
-        INTERFACE_COMPILE_FEATURES cxx_std_11
-    )
-endif()
-
-set(_NLOHMANN_JSON_COMPILE_DEFINITIONS "@NLOHMANN_JSON_COMPILE_DEFINITIONS@")
-if(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
-    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
-        INTERFACE_COMPILE_DEFINITIONS "${_NLOHMANN_JSON_COMPILE_DEFINITIONS}"
-    )
-endif()
-
-unset(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
-unset(_IMPORT_PREFIX)

cmake/pkg-config.pc.in

@@ -4,4 +4,4 @@ includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
 Name: @PROJECT_NAME@
 Description: JSON for Modern C++
 Version: @PROJECT_VERSION@
-Cflags: -I${includedir}@NLOHMANN_JSON_PKGCONFIG_CFLAGS@
+Cflags: -I${includedir}

docs/docset/docSet.sql

@@ -210,6 +210,7 @@ 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

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

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

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

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

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

docs/mkdocs/docs/features/macros.md

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

docs/mkdocs/docs/integration/cmake.md

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

docs/mkdocs/docs/integration/package_managers.md

@@ -121,23 +121,10 @@ meson wrap install nlohmann_json
 Please see the Meson project for any issues regarding the packaging.
 
 The provided `meson.build` can also be used as an alternative to CMake for installing `nlohmann_json` system-wide in
-which case a pkg-config file and the CMake package config files are installed. To use it, have your build system require
-the `nlohmann_json` pkg-config dependency, or use [`find_package(nlohmann_json)`](cmake.md#external) in CMake. In Meson,
-it is preferred to use the [`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a
-subproject fallback, rather than using the subproject directly.
-
-The options that change the library's configuration are available in Meson as well, named like the
-[CMake options](cmake.md#cmake-options) without the `JSON_` prefix: `MultipleHeaders`, `GlobalUDLs`,
-`ImplicitConversions`, `DisableEnumSerialization`, `Diagnostics`, `Diagnostic_Positions`,
-`LegacyDiscardedValueComparison`, and `StrictNulHandling`. They have the same defaults as in CMake, except that
-`MultipleHeaders` is `false`. Set them with `-D` when setting up the build, or with the subproject name as prefix when
-the library is used as a subproject:
-
-```shell
-meson setup build -Dnlohmann_json:Diagnostics=true
-```
-
-The resulting compile definitions are part of the Meson dependency, the pkg-config file, and the CMake target.
+which case a pkg-config file is installed. To use it, have your build system require the `nlohmann_json`
+pkg-config dependency. In Meson, it is preferred to use the
+[`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a subproject fallback, rather than
+using the subproject directly.
 
 ??? example "Example: Wrap"
 

docs/mkdocs/mkdocs.yml

@@ -287,6 +287,7 @@ nav:
           - 'JSON_DIAGNOSTICS': api/macros/json_diagnostics.md
           - 'JSON_DIAGNOSTIC_POSITIONS': api/macros/json_diagnostic_positions.md
           - 'JSON_DISABLE_ENUM_SERIALIZATION': api/macros/json_disable_enum_serialization.md
+          - 'JSON_DISABLE_TUPLE_REFERENCE_CONVERSION': api/macros/json_disable_tuple_reference_conversion.md
           - 'JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20': api/macros/json_has_cpp_11.md
           - 'JSON_HAS_EXPERIMENTAL_FILESYSTEM, JSON_HAS_FILESYSTEM': api/macros/json_has_filesystem.md
           - 'JSON_HAS_RANGES': api/macros/json_has_ranges.md

include/nlohmann/detail/conversions/to_json.hpp

@@ -471,11 +471,13 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
     j = { std::get<Idx>(t)... };
 }
 
-#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.
+// 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.
 template<typename BasicJsonType, typename Tuple>
 inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<0> /*unused*/)
 {
@@ -493,7 +495,6 @@ 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

@@ -916,6 +916,10 @@ 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

@@ -25,6 +25,7 @@
 #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

@@ -636,6 +636,18 @@ 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

@@ -1595,7 +1595,12 @@ 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, int > = 0 >
+                   !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 >
     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))))

meson.build

@@ -2,111 +2,23 @@ project('nlohmann_json',
     'cpp',
     version : '3.12.0',
     license : 'MIT',
-    meson_version : '>= 0.64',
-    default_options: ['cpp_std=c++11'],
 )
 
-if get_option('MultipleHeaders')
-    incdir = 'include'
-else
-    incdir = 'single_include'
-endif
-
-# The same compile definitions as the CMake target (see target_compile_definitions
-# in CMakeLists.txt): only an option that differs from its default adds one.
-json_defines = []
-if not get_option('GlobalUDLs')
-    json_defines += 'JSON_USE_GLOBAL_UDLS=0'
-endif
-if not get_option('ImplicitConversions')
-    json_defines += 'JSON_USE_IMPLICIT_CONVERSIONS=0'
-endif
-if get_option('DisableEnumSerialization')
-    json_defines += 'JSON_DISABLE_ENUM_SERIALIZATION=1'
-endif
-if get_option('Diagnostics')
-    json_defines += 'JSON_DIAGNOSTICS=1'
-endif
-if get_option('Diagnostic_Positions')
-    json_defines += 'JSON_DIAGNOSTIC_POSITIONS=1'
-endif
-if get_option('LegacyDiscardedValueComparison')
-    json_defines += 'JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1'
-endif
-if get_option('StrictNulHandling')
-    json_defines += 'JSON_STRICT_NUL_HANDLING=1'
-endif
-
-cpp_args = []
-foreach define : json_defines
-    cpp_args += '-D' + define
-endforeach
-
 nlohmann_json_dep = declare_dependency(
-    compile_args: cpp_args,
-    include_directories: include_directories(incdir)
+    include_directories: include_directories('single_include')
 )
-meson.override_dependency('nlohmann_json', nlohmann_json_dep)
 
-# The multi-header version under the name earlier versions of this file used
 nlohmann_json_multiple_headers = declare_dependency(
-    compile_args: cpp_args,
     include_directories: include_directories('include')
 )
 
 if not meson.is_subproject()
-    install_subdir(
-        incdir / 'nlohmann',
-        install_dir: get_option('includedir'),
-        install_tag: 'devel',
-    )
+install_headers('single_include/nlohmann/json.hpp', subdir: 'nlohmann')
+install_headers('single_include/nlohmann/json_fwd.hpp', subdir: 'nlohmann')
 
-    pkgc = import('pkgconfig')
-    pkgc.generate(name: 'nlohmann_json',
-        version: meson.project_version(),
-        description: 'JSON for Modern C++',
-        extra_cflags: cpp_args,
-        install_dir: get_option('datadir') / 'pkgconfig',
-    )
-
-    # CMake package config files, so that find_package(nlohmann_json) works. The
-    # include directory is given relative to the config files, so that the
-    # installation can be relocated. This is not possible if includedir or datadir
-    # is an absolute path outside the prefix (e.g., with the separate outputs of
-    # Nix); then the absolute include directory is used, as CMake does.
-    fs = import('fs')
-    cmake_install_dir = get_option('datadir') / 'cmake' / meson.project_name()
-    cmake_to_prefix = []
-    if fs.is_absolute(get_option('includedir')) or fs.is_absolute(cmake_install_dir)
-        cmake_include_dir = (get_option('prefix') / get_option('includedir')).replace('\\', '/')
-    else
-        foreach component : cmake_install_dir.split('/')
-            cmake_to_prefix += '..'
-        endforeach
-        cmake_include_dir = '${_IMPORT_PREFIX}/' + get_option('includedir')
-    endif
-
-    cmake_conf = configuration_data()
-    cmake_conf.set('PROJECT_NAME', meson.project_name())
-    cmake_conf.set('PROJECT_VERSION', meson.project_version())
-    cmake_conf.set('PROJECT_VERSION_MAJOR', meson.project_version().split('.')[0])
-    cmake_conf.set('NLOHMANN_JSON_TARGET_NAME', meson.project_name())
-    cmake_conf.set('NLOHMANN_JSON_TARGETS_EXPORT_NAME', meson.project_name() + 'Targets')
-    cmake_conf.set('NLOHMANN_JSON_INCLUDE_DIR', cmake_include_dir)
-    cmake_conf.set('NLOHMANN_JSON_CONFIG_TO_PREFIX', '/'.join(cmake_to_prefix))
-    cmake_conf.set('NLOHMANN_JSON_COMPILE_DEFINITIONS', ';'.join(json_defines))
-
-    foreach cmake_file : [
-        ['cmake/config.cmake.in', 'nlohmann_jsonConfig.cmake'],
-        ['cmake/nlohmann_jsonConfigVersion.cmake.in', 'nlohmann_jsonConfigVersion.cmake'],
-        ['cmake/nlohmann_jsonTargets.cmake.in', 'nlohmann_jsonTargets.cmake'],
-    ]
-        configure_file(
-            input: cmake_file[0],
-            output: cmake_file[1],
-            configuration: cmake_conf,
-            format: 'cmake@',
-            install_dir: cmake_install_dir,
-        )
-    endforeach
+pkgc = import('pkgconfig')
+pkgc.generate(name: 'nlohmann_json',
+    version: meson.project_version(),
+    description: 'JSON for Modern C++'
+)
 endif

meson_options.txt

@@ -1,48 +0,0 @@
-option(
-    'MultipleHeaders',
-    type: 'boolean',
-    value: false,
-    description: 'Use non-amalgamated version of the library',
-)
-option(
-    'GlobalUDLs',
-    type: 'boolean',
-    value: true,
-    description: 'Place user-defined string literals in the global namespace',
-)
-option(
-    'ImplicitConversions',
-    type: 'boolean',
-    value: true,
-    description: 'Enable implicit conversions',
-)
-option(
-    'DisableEnumSerialization',
-    type: 'boolean',
-    value: false,
-    description: 'Disable default integer enum serialization',
-)
-option(
-    'Diagnostics',
-    type: 'boolean',
-    value: false,
-    description: 'Use extended diagnostic messages',
-)
-option(
-    'Diagnostic_Positions',
-    type: 'boolean',
-    value: false,
-    description: 'Enable diagnostic positions',
-)
-option(
-    'LegacyDiscardedValueComparison',
-    type: 'boolean',
-    value: false,
-    description: 'Enable legacy discarded value comparison',
-)
-option(
-    'StrictNulHandling',
-    type: 'boolean',
-    value: false,
-    description: 'Enable strict NUL-byte handling',
-)

single_include/nlohmann/json.hpp

@@ -3327,6 +3327,10 @@ 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
@@ -4646,6 +4650,18 @@ 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
 {
@@ -7000,11 +7016,13 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
     j = { std::get<Idx>(t)... };
 }
 
-#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.
+// 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.
 template<typename BasicJsonType, typename Tuple>
 inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<0> /*unused*/)
 {
@@ -7022,7 +7040,6 @@ 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*/)
@@ -27478,7 +27495,12 @@ 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, int > = 0 >
+                   !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 >
     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))))
@@ -32528,6 +32550,7 @@ 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

@@ -0,0 +1,79 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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

@@ -18,6 +18,11 @@
 // 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;
@@ -28,6 +33,7 @@ using ordered_json = nlohmann::ordered_json;
 
 #include <cstdio>
 #include <list>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 
@@ -542,6 +548,18 @@ 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