Define less_general() only without three-way comparison (#5859)

less_general() (#5840) calls operator<(value_t, value_t), which exists in
C++20 only for GCC. Explicit instantiation of basic_json compiles every
member, so clang-cl and MSVC failed on unit-explicit_instantiation and
unit-regression2 in C++20 mode. Its only caller, operator<, is defined
only without three-way comparison, as it was before #5840.

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

include/nlohmann/json.hpp

@@ -5518,16 +5518,6 @@ public:
 #endif
     }
 
-    JSON_HEDLEY_NEVER_INLINE
-    static bool less_general(const_reference lhs, const_reference rhs) noexcept
-    {
-        // default_result is used if we cannot compare values. In that case,
-        // we compare types. Note we have to call the operator explicitly,
-        // because MSVC has problems otherwise.
-        JSON_IMPLEMENT_OPERATOR( <, false, false, operator<(lhs_type, rhs_type),
-                                 compare_iteratively<true>(lhs, rhs, true) == compare_result::less, false)
-    }
-
 #if JSON_HAS_THREE_WAY_COMPARISON
     JSON_HEDLEY_NEVER_INLINE
     static std::partial_ordering three_way_general(const_reference lhs, const_reference rhs) noexcept // *NOPAD*
@@ -5540,6 +5530,16 @@ public:
                                 lhs_type <=> rhs_type, // *NOPAD*
                                 to_partial_ordering(compare_iteratively<true>(lhs, rhs, false)), true)
     }
+#else
+    JSON_HEDLEY_NEVER_INLINE
+    static bool less_general(const_reference lhs, const_reference rhs) noexcept
+    {
+        // default_result is used if we cannot compare values. In that case,
+        // we compare types. Note we have to call the operator explicitly,
+        // because MSVC has problems otherwise.
+        JSON_IMPLEMENT_OPERATOR( <, false, false, operator<(lhs_type, rhs_type),
+                                 compare_iteratively<true>(lhs, rhs, true) == compare_result::less, false)
+    }
 #endif
 
     /// @brief compare two leaves of the same type inside equal_structured()