Lock json_compatible_type behind ifdef for now

Author: fsandhei

include/nlohmann/detail/meta/type_traits.hpp

@@ -684,11 +684,16 @@ inline constexpr bool value_in_range_of(T val)
 template<bool Value>
 using bool_constant = std::integral_constant<bool, Value>;
 
+// Locking the definition of json_compatible_type behind
+// JSON_HAS_THREE_WAY_COMPARISON because it seems difficult to
+// make use of this type trait well with friend functions. Fails on SFINAE.
+#if JSON_HAS_THREE_WAY_COMPARISON
 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;
 };
+#endif
 
 ///////////////////////////////////////////////////////////////////////////////
 // is_c_string

single_include/nlohmann/json.hpp

@@ -4091,11 +4091,16 @@ inline constexpr bool value_in_range_of(T val)
 template<bool Value>
 using bool_constant = std::integral_constant<bool, Value>;
 
+// Locking the definition of json_compatible_type behind
+// JSON_HAS_THREE_WAY_COMPARISON because it seems difficult to
+// make use of this type trait well with friend functions. Fails on SFINAE.
+#if JSON_HAS_THREE_WAY_COMPARISON
 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;
 };
+#endif
 
 ///////////////////////////////////////////////////////////////////////////////
 // is_c_string