-Yexplicit-nulls does not play nicely with Option.apply

[j-mie6]

Compiler version

3.5.2

Minimized example

//> using options -Yexplicit-nulls

class Foo
Option[Foo](null)

Output

Gives a type error that expected Foo but got Null. Fine, except the point of Option.apply is that it is a null-safe wrapper into Option.

Expectation

Realistically, its type needs to be def apply[A](x: A | Null): Option[A] when that flag is turned on, otherwise it's now useless.

[Gedochao]

cc @noti0na1

[noti0na1]

Since we couldn't update the std library right now, there is a "patch" function for Option defined at library/src/scala/runtime/stdLibPatches/Predef.scala

  extension (opt: Option.type)
    @experimental
    inline def fromNullable[T](t: T | Null): Option[T] = Option(t).asInstanceOf[Option[T]]

[Gedochao]

@noti0na1 potentially could be improved after #22043

[olhotak]

There was a discussion somewhere where @sjrd provided convincing reasons why Option.apply is the wrong name for a method that converts a value of type T|Null to a value of type Option[T], and that any potential future update to the library should add Option.fromNullable instead of changing the type of Option.apply. I can't find that discussion now, but if anyone remembers where it was, it would be good to link it here so we can find it in the future.

[sjrd]

The gist is: List(null) creates a 1-element list where the element is null. It doesn't return Nil. The same is true for every other apply method of "collection-like" things. Option(null) is the odd one. It should return Some(null), not None.

[j-mie6]

Well, yes, though Option's job is to subsume null, so it's not unexpected by any means. At least there is a fromNullable, I suppose

[jducoeur]

I can see the reasoning, although this one's a big deal due to interop. So long as there's a Scalafix to do the rewrite (which seems doable) it's likely okay, but this change needs to be shouted from the rooftops: the use of Option(...) to wrap null-returning Java functions is one of those things that often gets taught early (certainly I do so when teaching folks doing any sort of interop), so it's going to take a lot of habit-changing, and I think it's going to touch a lot of code.

[sjrd]

I mean we're never actually going to change the behavior of Option(null). At worst it's going to get deprecated in favor of fromNullable.