Returning default(T?) shows null warning even though T is nullable

[AustinBryan]

Was just playing around on SharpLab and found this:

public static T? Foo<T>() => default;    // Cannot convert null literal to non-nullable reference
public static string? Bar() => default;    // Perfectly fine

This seems counter intuitive to me. Maybe I'm not understanding this right.

Both defaults are returning null and both return types are nullable, so neither should give an error, right?

I saying T? or (T)? By that I mean, if I did Foo<int> would T be int? or int? And I did Foo<int?> is T int?? or int?? (not like that would make any meaningful sense).

I would expect that if I passed either int or int? to T?, T would be an int?.
Is this a bug or by intent because of the implicit casting that happens from T? to T?

But if it is the latter, why is it there no warning for string??

Also Foo<T?> is an error, so that doesn't solve this.

[stakx]

if I did Foo<int> would T be int? or int?

It would be int.

And I did Foo<int?> is T int?? or int??

There's no such thing as int??. Using T? requires you to add a generic parameter type constraint where T : struct to your method, because Nullable<T> likewise constraints T to be struct and that constraint is "propagated" to your method. It appears that a where T : struct explicitly disallows T to be a nullable value type (my guess is to prevent nested or multiply-nullable types), so once you add the constraint, you'll no longer be able to use e.g. int? as a generic type argument.

[Mteheran]

Try to use T as structure:

public static T? Foo() where T:struct => default(T?);

[AustinBryan]

@stakx I know there isn't, that's why I said:

not like that would make any meaningful sense.

If I did that though, wouldn't I not be able to use string??

[Mteheran]

unfortunately, "string?" is not available in the current version. It is gonna be included in C# 8