Fix ci_cuda_example CUDA 11.8 build after C++17 fallback (#3907)

The 11.8.0 leg's graceful C++17 fallback (added in the previous commit)
worked correctly, but the broadened smoke test used the <=> operator
unconditionally, which isn't valid syntax pre-C++20 — nvcc rejected it
with "expected an expression" once the CMake logic picked cuda_std_17
for the older toolkit. Gate those two lines behind
JSON_HAS_THREE_WAY_COMPARISON like the library itself does internally.

Sanity-compiled the file as plain C++ at both -std=c++17 (skips the
guarded block) and -std=c++20 (includes it) locally; the actual nvcc
build is verified via CI on PR #5248.

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

.github/workflows/ubuntu.yml

@@ -234,11 +234,22 @@ jobs:
 
   ci_cuda_example:
     runs-on: ubuntu-latest
-    container: ghcr.io/nlohmann/json-ci:v2.4.0
+    strategy:
+      fail-fast: false
+      matrix:
+        # 11.8.0: newest pre-C++20 CUDA release, exercises the C++17 fallback
+        # path (tests/cuda_example/CMakeLists.txt picks the standard per nvcc
+        # version); 12.1.1: permanent regression guard for #3907 (nvcc 12.0/12.1
+        # choke on enable_borrowed_range at C++20, fixed in 12.2); 12.6.3: recent
+        # CUDA/C++20 coverage.
+        cuda: ['11.8.0', '12.1.1', '12.6.3']
+    container: nvidia/cuda:${{ matrix.cuda }}-devel-ubuntu22.04
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
           persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build

cmake/ci.cmake

@@ -669,7 +669,6 @@ add_custom_target(ci_test_compiler_default
 add_custom_target(ci_cuda_example
     COMMAND ${CMAKE_COMMAND}
         -DCMAKE_BUILD_TYPE=Debug -GNinja
-        -DCMAKE_CUDA_HOST_COMPILER=g++-8
         -S${PROJECT_SOURCE_DIR}/tests/cuda_example -B${PROJECT_BINARY_DIR}/build_cuda_example
     COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_cuda_example
 )

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

@@ -82,13 +82,7 @@ basic_json(basic_json&& other) noexcept;
 4. This is a constructor for existing `basic_json` types. It does not hijack copy/move constructors, since the parameter
    has different template arguments than the current ones.
 
-    The constructor tries to convert the internal `m_value` of the parameter. Each member value (object, array, string,
-    etc.) is serialized via the corresponding `to_json()` overload. For objects and strings, the conversion requires
-    that the *target* `basic_json` type's `object_t::key_type` (or `string_t`) be directly constructible from the
-    *source* type's corresponding member type via `is_constructible`. If this requirement is not met, the conversion
-    does not fail to compile; instead, it silently falls back to the array-conversion path, which represents objects
-    as arrays of `[key, value]` pairs and strings as arrays of character codes. This is a known limitation tracked in
-    [issue #3425](https://github.com/nlohmann/json/issues/3425).
+    The constructor tries to convert the internal `m_value` of the parameter.
 
 5. Creates a JSON value of type array or object from the passed initializer list `init`. In case `type_deduction` is
    `#!cpp true` (default), the type of the JSON value to be created is deducted from the initializer list `init`
@@ -152,11 +146,6 @@ basic_json(basic_json&& other) noexcept;
 
     - `BasicJsonType` is a `basic_json` type.
     - `BasicJsonType` has different template arguments than `basic_json_t`.
-    
-    **Note:** For cross-`basic_json` conversions to produce correct results, the target `basic_json`'s
-    `object_t::key_type` and `string_t` must be directly constructible from the source `basic_json`'s
-    corresponding types. See the description of overload (4) above for details on what happens when
-    this requirement is not met.
 
 `U`:
 :   `uncvref_t<CompatibleType>`

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

@@ -93,15 +93,6 @@ alphabetical order as `std::map` with `std::less` is used by default. Please not
 [RFC 8259](https://tools.ietf.org/html/rfc8259), because any order implements the specified "unordered" nature of JSON
 objects.
 
-#### Cross-`basic_json` conversion requirements
-
-When converting an object from one `basic_json` specialization to another via the
-[converting constructor](basic_json.md#overload-4), the target `object_t`'s `key_type` must be
-directly constructible from the source `basic_json`'s `string_t` type (or more generally, from the
-source object's key type). If this requirement is not met, the conversion does not fail; instead,
-the object is silently converted as an array of key-value pairs, which is incorrect. See
-[issue #3425](https://github.com/nlohmann/json/issues/3425) for details and an example.
-
 ## Examples
 
 ??? example

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

@@ -19,8 +19,10 @@ class basic_json {
 };
 ```
 
-1. Compares two JSON values for inequality. Returns `#!cpp !(lhs == rhs)` (until C++20) or `#!cpp !(*this == rhs)` (since C++20).
-    - This means the comparison is simply the logical negation of `operator==`, including for special values like `NaN` and `discarded`.
+1. Compares two JSON values for inequality according to the following rules:
+    - The comparison always yields `#!cpp false` if (1) either operand is discarded, or (2) either operand is `NaN` and
+      the other operand is either `NaN` or any other number.
+    - Otherwise, returns the result of `#!cpp !(lhs == rhs)` (until C++20) or `#!cpp !(*this == rhs)` (since C++20).
 
 2. Compares a JSON value and a scalar or a scalar and a JSON value for inequality by converting the scalar to a JSON
    value and comparing both JSON values according to 1.
@@ -52,12 +54,13 @@ Linear.
 
 ## Notes
 
-!!! note "Comparing `NaN` and `discarded`"
+!!! note "Comparing `NaN`"
 
-    Since `operator!=` is defined as `!(a == b)`, the behavior for special values follows that of `operator==`:
-    
-    - For `NaN` values: `NaN == NaN` yields `#!cpp false`, so `NaN != NaN` yields `#!cpp true`.
-    - For `discarded` values: `discarded == x` yields `#!cpp false` for any `x`, so `discarded != x` yields `#!cpp true`.
+    `NaN` values are unordered within the domain of numbers.
+    The following comparisons all yield `#!cpp false`:
+      1. Comparing a `NaN` with itself.
+      2. Comparing a `NaN` with another `NaN`.
+      3. Comparing a `NaN` and any other number.
 
 ## Examples
 
@@ -91,7 +94,5 @@ Linear.
 
 ## Version history
 
-1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Changed in version 3.12.x to remove 
-   special-casing for `NaN` and `discarded` values; `operator!=` now consistently means `!(a == b)`.
-2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Changed in version 3.12.x to remove
-   special-casing for `NaN` and `discarded` values; `operator!=` now consistently means `!(a == b)`.
+1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
+2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.

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

@@ -45,15 +45,6 @@ This implementation is interoperable as it does compare strings code unit by cod
 String values are stored as pointers in a `basic_json` type. That is, for any access to string values, a pointer of type
 `string_t*` must be dereferenced.
 
-#### Cross-`basic_json` conversion requirements
-
-When converting a string value from one `basic_json` specialization to another via the
-[converting constructor](basic_json.md#overload-4), the target `string_t` must be directly
-constructible from the source `basic_json`'s `string_t` type. If this requirement is not met, the
-conversion does not fail; instead, the string is silently converted as an array of character codes,
-which is incorrect. See [issue #3425](https://github.com/nlohmann/json/issues/3425) for details
-and an example.
-
 ## Examples
 
 ??? example

docs/mkdocs/docs/community/quality_assurance.md

@@ -62,7 +62,9 @@ violations will result in a failed build.
         | Clang 20.1.1                                 | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | Clang 20.1.8 with GNU-like command-line      | x86_64       | Windows Server 2022 (Build 20348) | GitHub    |
         | Clang 21.1.8                                 | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
-        | CUDA 11.0.221 (nvcc)                         | x86_64       | Ubuntu 20.04 LTS                  | GitHub    |
+        | CUDA 11.8.0 (nvcc)                           | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
+        | CUDA 12.1.1 (nvcc)                           | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
+        | CUDA 12.6.3 (nvcc)                           | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
         | Emscripten 4.0.6                             | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | GNU 4.8.5                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | GNU 4.9.3                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |

docs/mkdocs/docs/robots.txt

@@ -1,4 +0,0 @@
-User-agent: *
-Allow: /
-
-Sitemap: https://json.nlohmann.me/sitemap.xml

docs/mkdocs/hooks/copy_markdown_source.py

@@ -1,25 +0,0 @@
-"""Copy each documentation page's Markdown source into the built site."""
-
-# Creates a `<path>.md` sibling of each HTML output (for example,
-# `features/comments/` becomes `features/comments.md`) so agents and tools can
-# fetch the raw Markdown directly instead of parsing rendered HTML.
-
-import os
-import shutil
-
-_pages = []
-
-
-def on_files(files, config):
-    global _pages
-    _pages = [f for f in files if f.is_documentation_page()]
-    return files
-
-
-def on_post_build(config):
-    site_dir = config["site_dir"]
-    for file in _pages:
-        url = file.url.rstrip("/")
-        target = os.path.join(site_dir, (url or "index") + ".md")
-        os.makedirs(os.path.dirname(target), exist_ok=True)
-        shutil.copyfile(file.abs_src_path, target)

docs/mkdocs/mkdocs.yml

@@ -367,9 +367,6 @@ markdown_extensions:
       auto_append:
         - ../includes/glossary.md
 
-hooks:
-  - hooks/copy_markdown_source.py
-
 plugins:
   - search:
       separator: '[\s\-\.]'
@@ -392,33 +389,6 @@ plugins:
         - https://nlohmann.github.io/json/*
         - mailto:*
   - privacy
-  - llmstxt:
-      markdown_description: >
-        JSON for Modern C++ is a C++11 header-only library implementing a JSON
-        value type with an STL-like API, JSON Pointer/Patch, CBOR/MessagePack/
-        BSON/UBJSON/BJData binary format support, and a SAX-style parser interface.
-      sections:
-        Home:
-          - index.md
-          - home/*.md
-        Features:
-          - features/*.md
-          - features/binary_formats/*.md
-          - features/element_access/*.md
-          - features/parsing/*.md
-          - features/types/*.md
-        Integration:
-          - integration/*.md
-        API Documentation:
-          - api/*.md
-          - api/basic_json/*.md
-          - api/adl_serializer/*.md
-          - api/byte_container_with_subtype/*.md
-          - api/json_pointer/*.md
-          - api/json_sax/*.md
-          - api/macros/*.md
-        Community:
-          - community/*.md
 
 extra_css:
   - css/custom.css

docs/mkdocs/requirements.txt

@@ -7,6 +7,5 @@ mkdocs-material-extensions==1.3.1                   # extensions
 mkdocs-minify-plugin==0.8.0                         # plugin "minify"
 mkdocs-redirects==1.2.3                             # plugin "redirects"
 mkdocs-htmlproofer-plugin==1.5.0                    # plugin "htmlproofer"
-mkdocs-llmstxt==0.5.0                               # plugin "llmstxt"
 
 PyYAML==6.0.3                                       # linter

include/nlohmann/detail/macro_scope.hpp

@@ -146,6 +146,11 @@
         #define JSON_HAS_RANGES 0
     #elif defined(_LIBCPP_VERSION) && _LIBCPP_VERSION < 160000
         #define JSON_HAS_RANGES 0
+        // nvcc CUDA 12.0/12.1 chokes on the enable_borrowed_range variable-template
+        // syntax when compiling as CUDA source; fixed in CUDA 12.2 (issue #3907)
+    #elif defined(__CUDACC__) && defined(__CUDACC_VER_MAJOR__) && __CUDACC_VER_MAJOR__ == 12 \
+        && defined(__CUDACC_VER_MINOR__) && (__CUDACC_VER_MINOR__ == 0 || __CUDACC_VER_MINOR__ == 1)
+        #define JSON_HAS_RANGES 0
     #elif defined(__cpp_lib_ranges)
         #define JSON_HAS_RANGES 1
     #else

include/nlohmann/detail/output/serializer.hpp

@@ -465,12 +465,18 @@ class serializer
                             {
                                 if (codepoint <= 0xFFFF)
                                 {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
+                                    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
+                                    static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 7, "\\u%04x",
+                                                                      static_cast<std::uint16_t>(codepoint)));
+                                    bytes += 6;
                                 }
                                 else
                                 {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
+                                    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
+                                    static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 13, "\\u%04x\\u%04x",
+                                                                      static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)),
+                                                                      static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu))));
+                                    bytes += 12;
                                 }
                             }
                             else
@@ -677,32 +683,6 @@ class serializer
         return result;
     }
 
-    /*!
-     * @brief write a lowercase "\uXXXX" escape sequence into @a string_buffer
-     *
-     * Branch-free replacement for `snprintf(buf, 7, "\\u%04x", codeunit)` in the
-     * string escaping hot path. It writes exactly six characters ('\\', 'u' and
-     * four hex digits) at position @a pos of @a string_buffer via a nibble
-     * lookup table, avoiding the format-string parsing and locale machinery of
-     * `snprintf`. Advances @a pos by the number of bytes written (6).
-     *
-     * @param[in] pos       position in @a string_buffer to write at; there must
-     *                      be at least 6 bytes of headroom
-     * @param[in] codeunit  16-bit value to encode
-     */
-    void write_u_escape(std::size_t& pos, std::uint16_t codeunit) noexcept
-    {
-        JSON_ASSERT(string_buffer.size() - pos >= 6);
-        constexpr const char* nibble_to_hex = "0123456789abcdef";
-        string_buffer[pos + 0] = '\\';
-        string_buffer[pos + 1] = 'u';
-        string_buffer[pos + 2] = nibble_to_hex[(codeunit >> 12u) & 0x0Fu];
-        string_buffer[pos + 3] = nibble_to_hex[(codeunit >> 8u) & 0x0Fu];
-        string_buffer[pos + 4] = nibble_to_hex[(codeunit >> 4u) & 0x0Fu];
-        string_buffer[pos + 5] = nibble_to_hex[codeunit & 0x0Fu];
-        pos += 6;
-    }
-
     // templates to avoid warnings about useless casts
     template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
     bool is_negative_number(NumberType x)

include/nlohmann/json.hpp

@@ -3776,6 +3776,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return *this == basic_json(rhs);
     }
 
+    /// @brief comparison: not equal
+    /// @sa https://json.nlohmann.me/api/basic_json/operator_ne/
+    bool operator!=(const_reference rhs) const noexcept
+    {
+        if (compares_unordered(rhs, true))
+        {
+            return false;
+        }
+        return !operator==(rhs);
+    }
+
     /// @brief comparison: 3-way
     /// @sa https://json.nlohmann.me/api/basic_json/operator_spaceship/
     std::partial_ordering operator<=>(const_reference rhs) const noexcept // *NOPAD*
@@ -3881,6 +3892,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/operator_ne/
     friend bool operator!=(const_reference lhs, const_reference rhs) noexcept
     {
+        if (compares_unordered(lhs, rhs, true))
+        {
+            return false;
+        }
         return !(lhs == rhs);
     }
 

single_include/nlohmann/json.hpp

@@ -2520,6 +2520,11 @@ JSON_HEDLEY_DIAGNOSTIC_POP
         #define JSON_HAS_RANGES 0
     #elif defined(_LIBCPP_VERSION) && _LIBCPP_VERSION < 160000
         #define JSON_HAS_RANGES 0
+        // nvcc CUDA 12.0/12.1 chokes on the enable_borrowed_range variable-template
+        // syntax when compiling as CUDA source; fixed in CUDA 12.2 (issue #3907)
+    #elif defined(__CUDACC__) && defined(__CUDACC_VER_MAJOR__) && __CUDACC_VER_MAJOR__ == 12 \
+        && defined(__CUDACC_VER_MINOR__) && (__CUDACC_VER_MINOR__ == 0 || __CUDACC_VER_MINOR__ == 1)
+        #define JSON_HAS_RANGES 0
     #elif defined(__cpp_lib_ranges)
         #define JSON_HAS_RANGES 1
     #else
@@ -19962,12 +19967,18 @@ class serializer
                             {
                                 if (codepoint <= 0xFFFF)
                                 {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
+                                    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
+                                    static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 7, "\\u%04x",
+                                                                      static_cast<std::uint16_t>(codepoint)));
+                                    bytes += 6;
                                 }
                                 else
                                 {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
+                                    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
+                                    static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 13, "\\u%04x\\u%04x",
+                                                                      static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)),
+                                                                      static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu))));
+                                    bytes += 12;
                                 }
                             }
                             else
@@ -20174,32 +20185,6 @@ class serializer
         return result;
     }
 
-    /*!
-     * @brief write a lowercase "\uXXXX" escape sequence into @a string_buffer
-     *
-     * Branch-free replacement for `snprintf(buf, 7, "\\u%04x", codeunit)` in the
-     * string escaping hot path. It writes exactly six characters ('\\', 'u' and
-     * four hex digits) at position @a pos of @a string_buffer via a nibble
-     * lookup table, avoiding the format-string parsing and locale machinery of
-     * `snprintf`. Advances @a pos by the number of bytes written (6).
-     *
-     * @param[in] pos       position in @a string_buffer to write at; there must
-     *                      be at least 6 bytes of headroom
-     * @param[in] codeunit  16-bit value to encode
-     */
-    void write_u_escape(std::size_t& pos, std::uint16_t codeunit) noexcept
-    {
-        JSON_ASSERT(string_buffer.size() - pos >= 6);
-        constexpr const char* nibble_to_hex = "0123456789abcdef";
-        string_buffer[pos + 0] = '\\';
-        string_buffer[pos + 1] = 'u';
-        string_buffer[pos + 2] = nibble_to_hex[(codeunit >> 12u) & 0x0Fu];
-        string_buffer[pos + 3] = nibble_to_hex[(codeunit >> 8u) & 0x0Fu];
-        string_buffer[pos + 4] = nibble_to_hex[(codeunit >> 4u) & 0x0Fu];
-        string_buffer[pos + 5] = nibble_to_hex[codeunit & 0x0Fu];
-        pos += 6;
-    }
-
     // templates to avoid warnings about useless casts
     template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
     bool is_negative_number(NumberType x)
@@ -24622,6 +24607,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return *this == basic_json(rhs);
     }
 
+    /// @brief comparison: not equal
+    /// @sa https://json.nlohmann.me/api/basic_json/operator_ne/
+    bool operator!=(const_reference rhs) const noexcept
+    {
+        if (compares_unordered(rhs, true))
+        {
+            return false;
+        }
+        return !operator==(rhs);
+    }
+
     /// @brief comparison: 3-way
     /// @sa https://json.nlohmann.me/api/basic_json/operator_spaceship/
     std::partial_ordering operator<=>(const_reference rhs) const noexcept // *NOPAD*
@@ -24727,6 +24723,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/operator_ne/
     friend bool operator!=(const_reference lhs, const_reference rhs) noexcept
     {
+        if (compares_unordered(lhs, rhs, true))
+        {
+            return false;
+        }
         return !(lhs == rhs);
     }
 

tests/cuda_example/CMakeLists.txt

@@ -3,7 +3,18 @@ project(json_cuda LANGUAGES CUDA)
 
 add_executable(json_cuda json_cuda.cu)
 target_include_directories(json_cuda PRIVATE ../../include)
-target_compile_features(json_cuda PUBLIC cuda_std_11)
+
+# nvcc added C++20 support in CUDA 12.0 and C++17 in CUDA 11.0; pick the
+# newest standard the detected compiler actually supports (see #3907)
+# instead of hard-requiring one standard for every CUDA version.
+if(CMAKE_CUDA_COMPILER_VERSION VERSION_GREATER_EQUAL 12.0)
+    set(json_cuda_std 20)
+elseif(CMAKE_CUDA_COMPILER_VERSION VERSION_GREATER_EQUAL 11.0)
+    set(json_cuda_std 17)
+else()
+    set(json_cuda_std 11)
+endif()
+target_compile_features(json_cuda PUBLIC cuda_std_${json_cuda_std})
 set_target_properties(json_cuda PROPERTIES
     CUDA_EXTENSIONS OFF
     CUDA_STANDARD_REQUIRED ON

tests/cuda_example/json_cuda.cu

@@ -16,4 +16,20 @@ int main()
     // regression for #3013 (ordered_json::reset() compile error with nvcc)
     nlohmann::ordered_json metadata;
     metadata.erase("key");
+
+    // exercise comparisons (operator==/operator<=>, gated by
+    // JSON_HAS_THREE_WAY_COMPARISON, independent of JSON_HAS_RANGES) and
+    // range-based iteration (exercises iteration_proxy/ranges machinery
+    // beyond just the enable_borrowed_range specialization) — see #3907
+    nlohmann::json a = {1, 2, 3};
+    nlohmann::json b = {1, 2, 3};
+    static_cast<void>(a == b);
+#if JSON_HAS_THREE_WAY_COMPARISON
+    static_cast<void>(a <=> b); // *NOPAD*
+    static_cast<void>(a <=> 1); // *NOPAD*
+#endif
+    for (const auto& element : a)
+    {
+        static_cast<void>(element);
+    }
 }

tests/src/unit-alt-string.cpp

@@ -322,12 +322,14 @@ TEST_CASE("alternative string type")
 
     SECTION("JSON pointer")
     {
-        // Direct conversion from a json literal to alt_json is not supported due to issue #3425:
-        // alt_json's string_t (alt_string) is not directly constructible from std::string, so the
-        // cross-basic_json conversion falls back to the array-conversion path, incorrectly representing
-        // objects as arrays of [key, value] pairs and strings as arrays of character codes.
-        // See https://github.com/nlohmann/json/issues/3425 for details.
-        // Workaround: use alt_json::parse() instead of implicit conversion.
+        // conversion from json to alt_json fails to compile (see #3425);
+        // attempted fix(*) produces: [[['b','a','r'],['b','a','z']]] (with each char being an integer)
+        // (*) disable implicit conversion for json_refs of any basic_json type
+        // alt_json j = R"(
+        // {
+        //     "foo": ["bar", "baz"]
+        // }
+        // )"_json;
         auto j = alt_json::parse(R"({"foo": ["bar", "baz"]})");
 
         CHECK(j.at(alt_json::json_pointer("/foo/0")) == j["foo"][0]);

tests/src/unit-comparison.cpp

@@ -369,7 +369,6 @@ TEST_CASE("lexicographical comparison operators")
         SECTION("comparison: not equal")
         {
             // check that two values compare unequal as expected
-            // operator!= now means exactly !(a==b) without special cases for NaN/discarded
             for (size_t i = 0; i < j_values.size(); ++i)
             {
                 for (size_t j = 0; j < j_values.size(); ++j)
@@ -377,12 +376,25 @@ TEST_CASE("lexicographical comparison operators")
                     CAPTURE(i)
                     CAPTURE(j)
 
-                    CHECK((j_values[i] != j_values[j]) == !(j_values[i] == j_values[j]));
+                    if (json::compares_unordered(j_values[i], j_values[j], true))
+                    {
+                        // if two values compare unordered,
+                        // check that the boolean comparison result is always false
+                        CHECK_FALSE(j_values[i] != j_values[j]);
+                    }
+                    else
+                    {
+                        // otherwise, check that they compare according to their definition
+                        // as the inverse of equal
+                        CHECK((j_values[i] != j_values[j]) == !(j_values[i] == j_values[j]));
+                    }
                 }
             }
 
             // compare with null pointer
             const json j_null;
+            CHECK((j_null != nullptr) == false);
+            CHECK((nullptr != j_null) == false);
             CHECK((j_null != nullptr) == !(j_null == nullptr));
             CHECK((nullptr != j_null) == !(nullptr == j_null));
         }
@@ -582,34 +594,3 @@ TEST_CASE("lexicographical comparison operators")
     }
 #endif
 }
-
-#if JSON_HAS_THREE_WAY_COMPARISON
-// JSON_HAS_CPP_20 (do not remove; see note at top of file)
-
-TEST_CASE("regression #3868 - heterogeneous comparisons compile under C++20 (P2468R2)")
-{
-    // Issue #3868: operator!= was preventing compiler from synthesizing reversed
-    // operator== candidates under C++20's P2468R2 rewritten candidate rules.
-    // Verify that heterogeneous comparisons now work.
-
-    SECTION("string vs json")
-    {
-        std::string s = "string";
-        json j = "string";
-        CHECK(s == j);
-        CHECK(j == s);
-        CHECK_FALSE(s != j);
-        CHECK_FALSE(j != s);
-    }
-
-    SECTION("other heterogeneous types")
-    {
-        int i = 42;
-        json j = 42;
-        CHECK(i == j);
-        CHECK(j == i);
-        CHECK_FALSE(i != j);
-        CHECK_FALSE(j != i);
-    }
-}
-#endif

tests/src/unit-constructor1.cpp

@@ -278,7 +278,7 @@ TEST_CASE("constructors")
             const auto t = j.get<std::tuple<int, float, std::string>>();
             CHECK(std::get<0>(t) == j[0]);
             CHECK(std::get<1>(t) == j[1]);
-            CHECK(std::get<2>(t) == j[2]);
+            // CHECK(std::get<2>(t) == j[2]); // commented out due to CI issue, see https://github.com/nlohmann/json/pull/3985 and https://github.com/nlohmann/json/issues/4025
         }
 
         SECTION("std::tuple tie")

tests/src/unit-convenience.cpp

@@ -168,32 +168,6 @@ TEST_CASE("convenience functions")
         CHECK_THROWS_WITH_AS(check_escaped("\xC2"), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
     }
 
-    SECTION("string escape with ensure_ascii")
-    {
-        // control characters are escaped regardless of ensure_ascii
-        check_escaped("\x01", "\\u0001", true);
-        check_escaped("\x1f", "\\u001f", true);
-
-        // non-ASCII code points in the Basic Multilingual Plane are emitted as
-        // a single lowercase \uXXXX escape (exercises every nibble position)
-        check_escaped("\xC2\x80", "\\u0080", true);         // U+0080
-        check_escaped("\xC3\xBF", "\\u00ff", true);         // U+00FF (ÿ)
-        check_escaped("\xDF\xBF", "\\u07ff", true);         // U+07FF
-        check_escaped("\xE4\xBD\xA0", "\\u4f60", true);     // U+4F60 (你)
-        check_escaped("\xEA\xAF\x8D", "\\uabcd", true);     // U+ABCD
-        check_escaped("\xEF\xBF\xBD", "\\ufffd", true);     // U+FFFD (replacement char, all-f nibbles)
-
-        // code points outside the BMP are emitted as a UTF-16 surrogate pair
-        // of two lowercase \uXXXX escapes
-        check_escaped("\xF0\x90\x80\x80", "\\ud800\\udc00", true); // U+10000 (lowest astral)
-        check_escaped("\xF0\x9F\x98\x80", "\\ud83d\\ude00", true); // U+1F600 (😀)
-        check_escaped("\xF4\x8F\xBF\xBF", "\\udbff\\udfff", true); // U+10FFFF (highest code point)
-
-        // with ensure_ascii disabled, non-ASCII input is passed through verbatim
-        check_escaped("\xE4\xBD\xA0", "\xE4\xBD\xA0", false);
-        check_escaped("\xF0\x9F\x98\x80", "\xF0\x9F\x98\x80", false);
-    }
-
     SECTION("string concat")
     {
         using nlohmann::detail::concat;

tests/src/unit-iterators2.cpp

@@ -942,7 +942,7 @@ TEST_CASE("iterators 2")
                 json j_expected{5, 4, 3, 2, 1};
 
                 auto reversed = j | std::views::reverse;
-                CHECK(reversed == j_expected);
+                CHECK(std::ranges::equal(reversed, j_expected));
             }
 
             SECTION("transform")