System.Text.Json: .NET 5.0 perf regression serializing Nullable<T>

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/NullableConverter.cs

@@ -70,5 +70,7 @@ internal override void WriteNumberWithCustomHandling(Utf8JsonWriter writer, T? v
                 _converter.WriteNumberWithCustomHandling(writer, value.Value, handling);
             }
         }
+
+        internal override bool IsNull(T? value) => !value.HasValue;
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs

@@ -294,6 +294,8 @@ internal override sealed bool TryReadAsObject(ref Utf8JsonReader reader, JsonSer
             return success;
         }
 
+        internal virtual bool IsNull(T value) => value == null;
+
         internal bool TryWrite(Utf8JsonWriter writer, in T value, JsonSerializerOptions options, ref WriteStack state)
         {
             if (writer.CurrentDepth >= options.EffectiveMaxDepth)
@@ -342,7 +344,7 @@ internal bool TryWrite(Utf8JsonWriter writer, in T value, JsonSerializerOptions
                     }
                 }
             }
-            else if (value == null && !HandleNullOnWrite)
+            else if (IsNull(value) && !HandleNullOnWrite)
             {
                 // We do not pass null values to converters unless HandleNullOnWrite is true. Null values for properties were
                 // already handled in GetMemberAndWriteJson() so we don't need to check for IgnoreNullValues here.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.Helpers.cs

@@ -21,7 +21,7 @@ private static void WriteCore<TValue>(
             Debug.Assert(writer != null);
 
             //  We treat typeof(object) special and allow polymorphic behavior.
-            if (value != null && inputType == JsonClassInfo.ObjectType)
+            if (inputType == JsonClassInfo.ObjectType && value != null)
             {
                 inputType = value!.GetType();
             }