Don't use concept. Causes compilation error on GCC 10

Opting out of using concept and rather defining a structure while still
being able to not use SFINAE and instead `requires` expression.

Compiling with C++23 on GCC 13.2.1 fails on overload resolution on
calling operator== for `std::nullptr_t`. The solution for this was to
simply add another overload for nullptr, on par with the equivalent
`nullptr_t` constructor.

Author: fsandhei

include/nlohmann/detail/meta/type_traits.hpp

@@ -684,10 +684,11 @@ inline constexpr bool value_in_range_of(T val)
 template<bool Value>
 using bool_constant = std::integral_constant<bool, Value>;
 
-#ifdef JSON_HAS_CPP_20
-    template <typename T, typename BasicJsonType>
-    concept CompatibleType = !is_basic_json<uncvref_t<T>>::value && is_compatible_type<BasicJsonType, uncvref_t<T>>::value;
-#endif
+template <typename T, typename BasicJsonType, typename U = uncvref_t<T>>
+struct json_compatible_type
+{
+    static constexpr auto value = !is_basic_json<U>::value && is_compatible_type<BasicJsonType, U>::value;
+};
 
 ///////////////////////////////////////////////////////////////////////////////
 // is_c_string

include/nlohmann/json.hpp

@@ -3694,12 +3694,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief comparison: equal
     /// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
-    template <detail::CompatibleType<basic_json_t> T>
+    template <typename T>
+    requires detail::json_compatible_type<T, basic_json_t>::value
     bool operator==(T rhs) const noexcept
     {
         return *this == basic_json(rhs);
     }
 
+    /// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
+    bool operator==(std::nullptr_t rhs) const noexcept
+    {
+        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
@@ -3726,9 +3733,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief comparison: 3-way
     /// @sa https://json.nlohmann.me/api/basic_json/operator_spaceship/
-    template<typename ScalarType>
-    requires std::is_scalar_v<ScalarType>
-    std::partial_ordering operator<=>(ScalarType rhs) const noexcept // *NOPAD*
+    template <typename T>
+    requires detail::json_compatible_type<T, basic_json_t>::value
+    std::partial_ordering operator<=>(T rhs) const noexcept // *NOPAD*
     {
         return *this <=> basic_json(rhs); // *NOPAD*
     }

single_include/nlohmann/json.hpp

@@ -4091,10 +4091,11 @@ inline constexpr bool value_in_range_of(T val)
 template<bool Value>
 using bool_constant = std::integral_constant<bool, Value>;
 
-#ifdef JSON_HAS_CPP_20
-    template <typename T, typename BasicJsonType>
-    concept CompatibleType = !is_basic_json<uncvref_t<T>>::value && is_compatible_type<BasicJsonType, uncvref_t<T>>::value;
-#endif
+template <typename T, typename BasicJsonType, typename U = uncvref_t<T>>
+struct json_compatible_type
+{
+    static constexpr auto value = !is_basic_json<U>::value && is_compatible_type<BasicJsonType, U>::value;
+};
 
 ///////////////////////////////////////////////////////////////////////////////
 // is_c_string
@@ -22913,12 +22914,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief comparison: equal
     /// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
-    template <detail::CompatibleType<basic_json_t> T>
+    template <typename T>
+    requires detail::json_compatible_type<T, basic_json_t>::value
     bool operator==(T rhs) const noexcept
     {
         return *this == basic_json(rhs);
     }
 
+    /// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
+    bool operator==(std::nullptr_t rhs) const noexcept
+    {
+        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
@@ -22945,9 +22953,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief comparison: 3-way
     /// @sa https://json.nlohmann.me/api/basic_json/operator_spaceship/
-    template<typename ScalarType>
-    requires std::is_scalar_v<ScalarType>
-    std::partial_ordering operator<=>(ScalarType rhs) const noexcept // *NOPAD*
+    template <typename T>
+    requires detail::json_compatible_type<T, basic_json_t>::value
+    std::partial_ordering operator<=>(T rhs) const noexcept // *NOPAD*
     {
         return *this <=> basic_json(rhs); // *NOPAD*
     }