Addressing feedback

src/libraries/System.Text.Json/ref/System.Text.Json.cs

@@ -65,7 +65,10 @@ public void Dispose() { }
         public bool MoveNext() { throw null; }
         void System.Collections.IEnumerator.Reset() { }
     }
-    public sealed partial class JsonBoolean : System.Text.Json.JsonNode, System.IEquatable<System.Text.Json.JsonBoolean>
+    public sealed partial class JsonBoolean : System.Text.Json.JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        System.IEquatable<System.Text.Json.JsonBoolean>
+#nullable restore
     {
         public JsonBoolean() { }
         public JsonBoolean(bool value) { }
@@ -220,9 +223,7 @@ public JsonException(string? message, string? path, long? lineNumber, long? byte
         public JsonException(string? message, string? path, long? lineNumber, long? bytePositionInLine, System.Exception? innerException) { }
         public long? BytePositionInLine { get { throw null; } }
         public long? LineNumber { get { throw null; } }
-#pragma warning disable 8609
-        public override string? Message { get { throw null; } }
-#pragma warning restore 8609
+        public override string Message { get { throw null; } }
         public string? Path { get { throw null; } }
         public override void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
     }
@@ -274,7 +275,10 @@ public partial struct JsonNodeOptions
         public System.Text.Json.DuplicatePropertyNameHandlingStrategy DuplicatePropertyNameHandling { readonly get { throw null; } set { } }
         public int MaxDepth { readonly get { throw null; } set { } }
     }
-    public sealed partial class JsonNull : System.Text.Json.JsonNode, System.IEquatable<System.Text.Json.JsonNull>
+    public sealed partial class JsonNull : System.Text.Json.JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        System.IEquatable<System.Text.Json.JsonNull>
+#nullable restore
     {
         public JsonNull() { }
         public override System.Text.Json.JsonValueKind ValueKind { get { throw null; } }
@@ -286,7 +290,10 @@ public JsonNull() { }
         public static bool operator !=(System.Text.Json.JsonNull? left, System.Text.Json.JsonNull? right) { throw null; }
         public override string ToString() { throw null; }
     }
-    public sealed partial class JsonNumber : System.Text.Json.JsonNode, System.IEquatable<System.Text.Json.JsonNumber>
+    public sealed partial class JsonNumber : System.Text.Json.JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        System.IEquatable<System.Text.Json.JsonNumber>
+#nullable restore
     {
         public JsonNumber() { }
         public JsonNumber(byte value) { }
@@ -466,7 +473,10 @@ public JsonSerializerOptions() { }
         public bool WriteIndented { get { throw null; } set { } }
         public System.Text.Json.Serialization.JsonConverter? GetConverter(System.Type typeToConvert) { throw null; }
     }
-    public sealed partial class JsonString : System.Text.Json.JsonNode, System.IEquatable<System.Text.Json.JsonString>
+    public sealed partial class JsonString : System.Text.Json.JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        System.IEquatable<System.Text.Json.JsonString>
+#nullable restore
     {
         public JsonString() { }
         public JsonString(System.DateTime value) { }

src/libraries/System.Text.Json/src/System/Text/Json/JsonException.cs

@@ -130,14 +130,12 @@ public override void GetObjectData(SerializationInfo info, StreamingContext cont
         /// <summary>
         /// Gets a message that describes the current exception.
         /// </summary>
-        public override string? Message
+        public override string Message
         {
-#pragma warning disable 8609
             get
             {
-                return _message;
+                return _message ?? base.Message;
             }
-#pragma warning restore 8609
         }
 
         internal void SetMessage(string? message)

src/libraries/System.Text.Json/src/System/Text/Json/Node/JsonBoolean.cs

@@ -7,7 +7,10 @@ namespace System.Text.Json
     /// <summary>
     ///   Represents a mutable boolean JSON value.
     /// </summary>
-    public sealed class JsonBoolean : JsonNode, IEquatable<JsonBoolean>
+    public sealed class JsonBoolean : JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        IEquatable<JsonBoolean>
+#nullable restore
     {
         /// <summary>
         ///   Initializes a new instance of the <see cref="JsonBoolean"/> class representing the value <see langword="false"/>.

src/libraries/System.Text.Json/src/System/Text/Json/Node/JsonNull.cs

@@ -7,7 +7,10 @@ namespace System.Text.Json
     /// <summary>
     ///   Represents the null JSON value.
     /// </summary>
-    public sealed class JsonNull : JsonNode, IEquatable<JsonNull>
+    public sealed class JsonNull : JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        IEquatable<JsonNull>
+#nullable restore
     {
         /// <summary>
         ///   Initializes a new instance of the <see cref="JsonNull"/> class representing the value <see langword="null"/>.

src/libraries/System.Text.Json/src/System/Text/Json/Node/JsonNumber.cs

@@ -9,7 +9,10 @@ namespace System.Text.Json
     /// <summary>
     ///   Represents a mutable numeric JSON value.
     /// </summary>
-    public sealed class JsonNumber : JsonNode, IEquatable<JsonNumber>
+    public sealed class JsonNumber : JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        IEquatable<JsonNumber>
+#nullable restore
     {
         private string _value = null!;
 

src/libraries/System.Text.Json/src/System/Text/Json/Node/JsonString.cs

@@ -12,7 +12,10 @@ namespace System.Text.Json
     /// <summary>
     ///   Represents a mutable text JSON value.
     /// </summary>
-    public sealed class JsonString : JsonNode, IEquatable<JsonString>
+    public sealed class JsonString : JsonNode,
+#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
+        IEquatable<JsonString>
+#nullable restore
     {
         private string _value = null!;
 

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/JsonConverterEnum.cs

@@ -21,14 +21,14 @@ public override bool CanConvert(Type type)
         [PreserveDependency(
             ".ctor(System.Text.Json.Serialization.Converters.EnumConverterOptions)",
             "System.Text.Json.Serialization.Converters.JsonConverterEnum`1")]
-        public override JsonConverter? CreateConverter(Type type, JsonSerializerOptions? options)
+        public override JsonConverter CreateConverter(Type type, JsonSerializerOptions? options)
         {
-            JsonConverter? converter = (JsonConverter?)Activator.CreateInstance(
+            JsonConverter converter = (JsonConverter)Activator.CreateInstance(
                 typeof(JsonConverterEnum<>).MakeGenericType(type),
                 BindingFlags.Instance | BindingFlags.Public,
                 binder: null,
                 new object[] { EnumConverterOptions.AllowNumbers },
-                culture: null);
+                culture: null)!;
 
             return converter;
         }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/JsonKeyValuePairConverter.cs

@@ -20,17 +20,17 @@ public override bool CanConvert(Type typeToConvert)
         }
 
         [PreserveDependency(".ctor()", "System.Text.Json.Serialization.Converters.JsonKeyValuePairConverter`2")]
-        public override JsonConverter? CreateConverter(Type type, JsonSerializerOptions? options)
+        public override JsonConverter CreateConverter(Type type, JsonSerializerOptions? options)
         {
             Type keyType = type.GetGenericArguments()[0];
             Type valueType = type.GetGenericArguments()[1];
 
-            JsonConverter? converter = (JsonConverter?)Activator.CreateInstance(
+            JsonConverter converter = (JsonConverter)Activator.CreateInstance(
                 typeof(JsonKeyValuePairConverter<,>).MakeGenericType(new Type[] { keyType, valueType }),
                 BindingFlags.Instance | BindingFlags.Public,
                 binder: null,
                 args: null,
-                culture: null);
+                culture: null)!;
 
             return converter;
         }

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

@@ -19,7 +19,7 @@ public abstract class JsonConverterFactory : JsonConverter
         /// </summary>
         protected JsonConverterFactory() { }
 
-        internal JsonConverter? GetConverterInternal(Type typeToConvert, JsonSerializerOptions? options)
+        internal JsonConverter GetConverterInternal(Type typeToConvert, JsonSerializerOptions? options)
         {
             Debug.Assert(CanConvert(typeToConvert));
             return CreateConverter(typeToConvert, options);
@@ -33,6 +33,6 @@ protected JsonConverterFactory() { }
         /// <returns>
         /// An instance of a <see cref="JsonConverter{T}"/> where T is compatible with <paramref name="typeToConvert"/>.
         /// </returns>
-        public abstract JsonConverter? CreateConverter(Type typeToConvert, JsonSerializerOptions? options);
+        public abstract JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions? options);
     }
 }

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

@@ -57,14 +57,14 @@ public override bool CanConvert(Type typeToConvert)
         [PreserveDependency(
             ".ctor(System.Text.Json.Serialization.Converters.EnumConverterOptions, System.Text.Json.JsonNamingPolicy)",
             "System.Text.Json.Serialization.Converters.JsonConverterEnum`1")]
-        public override JsonConverter? CreateConverter(Type typeToConvert, JsonSerializerOptions? options)
+        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions? options)
         {
-            JsonConverter? converter = (JsonConverter?)Activator.CreateInstance(
+            JsonConverter converter = (JsonConverter)Activator.CreateInstance(
                 typeof(JsonConverterEnum<>).MakeGenericType(typeToConvert),
                 BindingFlags.Instance | BindingFlags.Public,
                 binder: null,
                 new object?[] { _converterOptions, _namingPolicy },
-                culture: null);
+                culture: null)!;
 
             return converter;
         }