Nullable with reference types

[Steven Schveighoffer via Digitalmars-d-learn]

On 6/30/15 11:16 AM, Jonathan M Davis wrote:
> On Tuesday, 30 June 2015 at 00:02:38 UTC, Meta wrote:
>> On Monday, 29 June 2015 at 19:29:37 UTC, sigod wrote:
>>> Hi, everyone.
>>>
>>> ```
>>> import std.typecons : Nullable;
>>>
>>> class Test {}
>>>
>>> Nullable!Test test;
>>> assert(test.isNull);
>>> ```
>>>
>>> Why does `Nullable` allowed to be used with reference types (e.g.
>>> classes)?
>>>
>>> P.S. I have experience with C#, where `Nullable<T>` cannot be used
>>> with reference types. And it sounds logical to me.
>>
>> It's a design mistake in Nullable. I would suggest that either never
>> use Nullable with a type that already has a null value, or use the
>> "overload" of Nullable that takes a null value, and set it to null.
>> Example:
>>
>> Class Test {}
>> alias NullableTest = Nullable!(Test, null);
>
> I tend to think that it's incredibly stupid to use something like
> Nullable for a type that's already Nullable. It's just silly. If a type
> is already nullable, then just use that and stop being adding extra
> overhead for no good reason. However, it _is_ true that if you need to
> have a nullable variable in generic code where the type that you need to
> be nullable could be any type, then having Nullable work with all types
> - and work with them all in the same way - is useful. Without that,
> you'd have to special case your code for types which were naturally
> nullable (and thus used null) and those which required Nullable. So, I
> can see why it could be useful to have Nullable work with classes, but I
> also question how common such a use case is.

I know this is just back-of-envelope, but what's wrong with:

alias Nullable(T) if(is(T == class)) = T;

bool isNull(T)(T t) if(is(T == class)) { return t is null;}

?

-Steve