Why would you ever allow an Optional<T?>

[jezzsantos]

I am wondering why you would intend anyone to define an Optional<DateTime?> or Optional<string?> or Optional<MyClass?> or Optional<MyEnum>, and forbid nullable types.
Then interpret new Optional<DateTime>(deafult(DateTime)) as as actually being None`.

Obviously, I know that value types cannot be null (and reference type can be null), but the class offer whatever contract it likes.
So, maybe this is a design philosophy question? rather than a strictly csharp question.

I would have thought, from a usability perspective the contract of this class would be better if it did not allow nullable types as T.
Perhaps C# cannot enforce/prevent that, so we have to deal with it regardless? or we use reflection to verify that.

Can anyone offer insight?

[sakno]

Optional<T?> is allowed for some corner cases. For instance, if you want to distinguish undefined and null just like in JavaScript. Take a look at this article.

Perhaps C# cannot enforce/prevent that

It can. notnull constraint can be applied to T. This generates a warning even for nullable value type such as int?. But not doing this was a design decision.