Disable std::swap specialization in C++20 and added a friend swap function

include/nlohmann/detail/macro_scope.hpp

@@ -20,7 +20,11 @@
 #endif
 
 // C++ language standard detection
-#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
+#if (defined(__cplusplus) && __cplusplus >= 202002L) || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
+    #define JSON_HAS_CPP_20
+    #define JSON_HAS_CPP_17
+    #define JSON_HAS_CPP_14
+#elif (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
     #define JSON_HAS_CPP_17
     #define JSON_HAS_CPP_14
 #elif (defined(__cplusplus) && __cplusplus >= 201402L) || (defined(_HAS_CXX14) && _HAS_CXX14 == 1)

include/nlohmann/json.hpp

@@ -5865,6 +5865,34 @@ class basic_json
     /*!
     @brief exchanges the values
 
+    Exchanges the contents of the JSON value from @a left with those of @a right. Does not
+    invoke any move, copy, or swap operations on individual elements. All
+    iterators and references remain valid. The past-the-end iterator is
+    invalidated. implemented as a friend function callable via ADL.
+
+    @param[in,out] left JSON value to exchange the contents with
+    @param[in,out] right JSON value to exchange the contents with
+
+    @complexity Constant.
+
+    @liveexample{The example below shows how JSON values can be swapped with
+    `swap()`.,swap__reference}
+
+    @since version 1.0.0
+    */
+    friend void swap(reference left, reference right) noexcept (
+        std::is_nothrow_move_constructible<value_t>::value and
+        std::is_nothrow_move_assignable<value_t>::value and
+        std::is_nothrow_move_constructible<json_value>::value and
+        std::is_nothrow_move_assignable<json_value>::value
+    )
+    {
+        left.swap(right);
+    }
+
+    /*!
+    @brief exchanges the values
+
     Exchanges the contents of a JSON array with those of @a other. Does not
     invoke any move, copy, or swap operations on individual elements. All
     iterators and references remain valid. The past-the-end iterator is
@@ -8635,6 +8663,9 @@ struct less<::nlohmann::detail::value_t>
     }
 };
 
+// C++20 prohibit function specialization in the std namespace.
+#ifndef JSON_HAS_CPP_20
+
 /*!
 @brief exchanges the values of two JSON objects
 
@@ -8649,6 +8680,8 @@ inline void swap<nlohmann::json>(nlohmann::json& j1, nlohmann::json& j2) noexcep
     j1.swap(j2);
 }
 
+#endif
+
 } // namespace std
 
 /*!

single_include/nlohmann/json.hpp

@@ -2048,7 +2048,11 @@ JSON_HEDLEY_DIAGNOSTIC_POP
 #endif
 
 // C++ language standard detection
-#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
+#if (defined(__cplusplus) && __cplusplus >= 202002L) || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
+    #define JSON_HAS_CPP_20
+    #define JSON_HAS_CPP_17
+    #define JSON_HAS_CPP_14
+#elif (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
     #define JSON_HAS_CPP_17
     #define JSON_HAS_CPP_14
 #elif (defined(__cplusplus) && __cplusplus >= 201402L) || (defined(_HAS_CXX14) && _HAS_CXX14 == 1)
@@ -21628,6 +21632,34 @@ class basic_json
     /*!
     @brief exchanges the values
 
+    Exchanges the contents of the JSON value from @a left with those of @a right. Does not
+    invoke any move, copy, or swap operations on individual elements. All
+    iterators and references remain valid. The past-the-end iterator is
+    invalidated. implemented as a friend function callable via ADL.
+
+    @param[in,out] left JSON value to exchange the contents with
+    @param[in,out] right JSON value to exchange the contents with
+
+    @complexity Constant.
+
+    @liveexample{The example below shows how JSON values can be swapped with
+    `swap()`.,swap__reference}
+
+    @since version 1.0.0
+    */
+    friend void swap(reference left, reference right) noexcept (
+        std::is_nothrow_move_constructible<value_t>::value and
+        std::is_nothrow_move_assignable<value_t>::value and
+        std::is_nothrow_move_constructible<json_value>::value and
+        std::is_nothrow_move_assignable<json_value>::value
+    )
+    {
+        left.swap(right);
+    }
+
+    /*!
+    @brief exchanges the values
+
     Exchanges the contents of a JSON array with those of @a other. Does not
     invoke any move, copy, or swap operations on individual elements. All
     iterators and references remain valid. The past-the-end iterator is
@@ -24398,6 +24430,9 @@ struct less<::nlohmann::detail::value_t>
     }
 };
 
+// C++20 prohibit function specialization in the std namespace.
+#ifndef JSON_HAS_CPP_20
+
 /*!
 @brief exchanges the values of two JSON objects
 
@@ -24412,6 +24447,8 @@ inline void swap<nlohmann::json>(nlohmann::json& j1, nlohmann::json& j2) noexcep
     j1.swap(j2);
 }
 
+#endif
+
 } // namespace std
 
 /*!