Remove compiler downgrade

[ViktorHofer]

Let's see what's going to crash :)

[ViktorHofer]

@safern would you mind pushing the necessary nullability fixes into this branch?

[safern]

Of course. Working on it.

[ahsonkhan]

I believe this assert is incorrect and will fail with this, so consider just using null-forgiving (!).

[Fact]
public static void WriteDictionaryNullValue()
{
    var input = new Dictionary<string, string> { { "a", null }, {"b", "foo" } };

    string json = JsonSerializer.Serialize(input);
}

Since this is calling the built-in JsonConverter<string?>, null is allowed and will work if passed in. The StringConverter overwrites the nullability, but I suppose it can't be expressed statically.

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/StringConverter.cs

Lines 14 to 17 in 5d8f924

	public override void Write(Utf8JsonWriter writer, string? value, JsonSerializerOptions options)
	{
	writer.WriteStringValue(value);
	}

Is there something we can do here?

[safern]

I don't think we can do anything here. Since the converter here is declared as the abstract type, the compiler will resolve the signature that contains the DisallowNull annotation. So I think we should just use a !. I think we can't declare the converter variable type as JsonConverter<string?>, right?

[ahsonkhan]

No, it has to remain generic. It just happens to be that all the Ts in the particular set of JsonConverter<T> work with null values (because our internal converters know how to deal with it).

I think, ideally, we will re-write parts of this to add special handling for null and honor the DisallowNull even internally.

[safern]

I think that would make sense. Internal usage of these APIs should overall honor the intent and what we tell our customers. It would be great if the usage of ! was lower.

[ahsonkhan]

Thanks for doing this, @safern.

[ViktorHofer]

Thanks Santi, you are my hero :)