How can JsonSerializer.Deserialize ever return null?

[KieranDevvs]

All of the overload methods for Deserialize<T> return T? but in my testing:

• If the JSON is invalid, it throws.
• If the JSON is valid but doesn't match the type, T's constructor is called but no members are populated (thus T is not null).
• If the JSON is string.Empty or null, it throws.
• If the JSON is valid and all fields can be deserialized then T is not null and fully populated.
• If the JSON is valid and some fields can be deseralized then T is not null and is partially populated.

I ask because in my code I'm doing the relevant type checks i.e

if (deserializedObject is null)
{
    // deal with T?...
}

and when unit testing this code, I cant get a scenario where I can meet this condition.

It then begs the question, if T cannot be null (or maybe it can and someone can elaborate for me), then why does the signature return T?

I have a feeling it might be to do with some JSON serialization options that aren't set by default that would return null rather than throwing in some cases, but I cant find anything that resembles this behavior in my own findings.

[gfoidl]

E.g.

using System.Text.Json;

object? obj = JsonSerializer.Deserialize<object>("null");
Console.WriteLine(obj is null);