Allow assigning `null` to `Nullable`

[GrimMaple]

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[dkorpel]

Assigning null is different than setting the Nullable state to null, see: The problem with Nullable: nullable(null).isNull is false.

I'm not sure this is a good idea. @atilaneves

[pbackus]

I think the real issue here is the name Nullable.

If it were called Optional or Maybe, nobody would expect assigning null to work like this in the first place, and nobody would be surprised to find out that it doesn't.

Unfortunately, we must keep the name Nullable for compatibility, but perhaps we could rename it and make Nullable an alias?

[GrimMaple]

Seeing how this PR isn't a particular hit with the D community, there is another solution I can propose: a compile-time error when trying to assign null to nullable saying something like "Use nullify instead"

@pbackus just renaming it to optional wouldn't be enough -- we would need to invent nullopt or something

[atilaneves]

I'm not convinced of the utility of this either.

[GrimMaple]

I don't see a lot of movement on this, so I'm going to reask: would making a error on assigning null be more of an accepted solution?

[atilaneves]

I don't see a lot of movement on this, so I'm going to reask: would making a error on assigning null be more of an accepted solution?

It's already a compile-time error, isn't it?

[GrimMaple]

I don't see a lot of movement on this, so I'm going to reask: would making a error on assigning null be more of an accepted solution?

It's already a compile-time error, isn't it?

Well it's not a particularly great compile-time error. I was suggesting explicitly erroring "Use nullify instead of assigning null"

[atilaneves]

I don't see a lot of movement on this, so I'm going to reask: would making a error on assigning null be more of an accepted solution?

It's already a compile-time error, isn't it?

Well it's not a particularly great compile-time error. I was suggesting explicitly erroring "Use nullify instead of assigning null"

That sounds like a good idea.

[dkorpel]

If the null is a valid value, (e.g. Nullable!string), you should still be able to assign that. Otherwise (e.g. Nullable!int) it can give an error.