Add C++20 3-way comparison operator and fix broken comparisons

Fixes nlohmann#3207.
Fixes nlohmann#3409.

cmake/test.cmake

@@ -156,6 +156,7 @@ endfunction()
 #############################################################################
 # json_test_add_test_for(
 #     <file>
+#     [NAME <name>]
 #     MAIN <main>
 #     [CXX_STANDARDS <version_number>...] [FORCE])
 #
@@ -165,22 +166,30 @@ endfunction()
 #
 # if C++ standard <version_number> is supported by the compiler and the
 # source file contains JSON_HAS_CPP_<version_number>.
+# Use NAME <name> to override the filename-derived test name.
 # Use FORCE to create the test regardless of the file containing
 # JSON_HAS_CPP_<version_number>.
 # Test targets are linked against <main>.
 # CXX_STANDARDS defaults to "11".
 #############################################################################
 
 function(json_test_add_test_for file)
-    cmake_parse_arguments(args "FORCE" "MAIN" "CXX_STANDARDS" ${ARGN})
-
-    get_filename_component(file_basename ${file} NAME_WE)
-    string(REGEX REPLACE "unit-([^$]+)" "test-\\1" test_name ${file_basename})
+    cmake_parse_arguments(args "FORCE" "MAIN;NAME" "CXX_STANDARDS" ${ARGN})
 
     if("${args_MAIN}" STREQUAL "")
         message(FATAL_ERROR "Required argument MAIN <main> missing.")
     endif()
 
+    if("${args_NAME}" STREQUAL "")
+        get_filename_component(file_basename ${file} NAME_WE)
+        string(REGEX REPLACE "unit-([^$]+)" "test-\\1" test_name ${file_basename})
+    else()
+        set(test_name ${args_NAME})
+        if(NOT test_name MATCHES "test-[^$]+")
+            message(FATAL_ERROR "Test name must start with 'test-'.")
+        endif()
+    endif()
+
     if("${args_CXX_STANDARDS}" STREQUAL "")
         set(args_CXX_STANDARDS 11)
     endif()

doc/mkdocs/docs/api/basic_json/operator_spaceship.md

@@ -0,0 +1,5 @@
+# <small>nlohmann::basic_json::</small>operator<=>
+
+## Version history
+
+- Added in version 3.11.0.

doc/mkdocs/mkdocs.yml

@@ -151,6 +151,7 @@ nav:
         - 'operator value_t': api/basic_json/operator_value_t.md
         - 'operator[]': api/basic_json/operator[].md
         - 'operator=': api/basic_json/operator=.md
+        - 'operator<=>': api/basic_json/operator_spaceship.md
         - 'operator==': api/basic_json/operator_eq.md
         - 'operator!=': api/basic_json/operator_ne.md
         - 'operator<': api/basic_json/operator_lt.md

include/nlohmann/detail/macro_scope.hpp

@@ -98,14 +98,22 @@
 #endif
 
 #ifndef JSON_HAS_THREE_WAY_COMPARISON
-    #if defined(__cpp_lib_three_way_comparison) && __cpp_lib_three_way_comparison >= 201907L \
-        && defined(__cpp_impl_three_way_comparison)&& __cpp_impl_three_way_comparison >= 201907L
-        #define JSON_HAS_THREE_WAY_COMPARISON 1
-    #else
-        #define JSON_HAS_THREE_WAY_COMPARISON 0
+    #if defined(__cpp_impl_three_way_comparison) && __cpp_impl_three_way_comparison >= 201907L
+        #if !defined(__cpp_lib_three_way_comparison) && defined(__has_include)
+            #if __has_include(<compare>)
+                #include <compare>
+            #endif
+        #endif
+        #if defined(__cpp_lib_three_way_comparison) && __cpp_lib_three_way_comparison >= 201907L
+            #define JSON_HAS_THREE_WAY_COMPARISON 1
+        #endif
     #endif
 #endif
 
+#ifndef JSON_HAS_THREE_WAY_COMPARISON
+    #define JSON_HAS_THREE_WAY_COMPARISON 0
+#endif
+
 // disable documentation warnings on clang
 #if defined(__clang__)
     #pragma clang diagnostic push
@@ -423,3 +431,7 @@
 #ifndef JSON_DIAGNOSTICS
     #define JSON_DIAGNOSTICS 0
 #endif
+
+#ifndef JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
+    #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0
+#endif

include/nlohmann/detail/macro_unscope.hpp

@@ -25,6 +25,8 @@
     #undef JSON_HAS_FILESYSTEM
     #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
     #undef JSON_HAS_THREE_WAY_COMPARISON
+    #undef JSON_HAS_RANGES
+    #undef JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
 #endif
 
 #include <nlohmann/thirdparty/hedley/hedley_undef.hpp>

include/nlohmann/detail/value_t.hpp

@@ -5,6 +5,11 @@
 #include <cstdint> // uint8_t
 #include <string> // string
 
+#include <nlohmann/detail/macro_scope.hpp>
+#if JSON_HAS_THREE_WAY_COMPARISON
+    #include <compare> // partial_ordering
+#endif
+
 namespace nlohmann
 {
 namespace detail
@@ -64,7 +69,11 @@ Returns an ordering that is similar to Python:
 
 @since version 1.0.0
 */
-inline bool operator<(const value_t lhs, const value_t rhs) noexcept
+#if JSON_HAS_THREE_WAY_COMPARISON
+    inline std::partial_ordering operator<=>(const value_t lhs, const value_t rhs) noexcept // *NOPAD*
+#else
+    inline bool operator<(const value_t lhs, const value_t rhs) noexcept
+#endif
 {
     static constexpr std::array<std::uint8_t, 9> order = {{
             0 /* null */, 3 /* object */, 4 /* array */, 5 /* string */,
@@ -75,7 +84,25 @@ inline bool operator<(const value_t lhs, const value_t rhs) noexcept
 
     const auto l_index = static_cast<std::size_t>(lhs);
     const auto r_index = static_cast<std::size_t>(rhs);
+#if JSON_HAS_THREE_WAY_COMPARISON
+    if (l_index < order.size() && r_index < order.size())
+    {
+        return order[l_index] <=> order[r_index]; // *NOPAD*
+    }
+    return std::partial_ordering::unordered;
+#else
     return l_index < order.size() && r_index < order.size() && order[l_index] < order[r_index];
+#endif
+}
+
+// GCC disagrees with Clang, MSVC, and ICC; selects builtin operator over user-defined
+// (see GCC bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105200)
+// ICC may produce incorrect result; test and add if needed
+#if JSON_HAS_THREE_WAY_COMPARISON && defined(__GNUC__)
+inline bool operator<(const value_t lhs, const value_t rhs) noexcept
+{
+    return std::is_lt(lhs <=> rhs); // *NOPAD*
 }
+#endif
 }  // namespace detail
 }  // namespace nlohmann