Usage of `@unchecked` annotation with `val` is inconsistent between Scala 2 and 3

[SethTisue]

in Scala 2, one annotates the scrutinee:

scala 2.13.4> val Some(x) = Option(3)
                                  ^
              warning: match may not be exhaustive.
              It would fail on the following input: None
val x: Int = 3

scala 2.13.4> val Some(x) = Option(3): @unchecked
val x: Int = 3

whereas in Scala 3.0.0-M2 under -source:3.1 one must annotate the pattern, while annotating the scrutinee is ignored:

scala> val Some(x) = Option(3)
1 |val Some(x) = Option(3)
  |    ^^^^^^^
  |pattern's type Some[Int] is more specialized than the right hand side expression's type Option[Int]
  |
  |If the narrowing is intentional, this can be communicated by writing `: @unchecked` after the full pattern.

scala> val Some(x) = Option(3): @unchecked                                                                              
1 |val Some(x) = Option(3): @unchecked
  |    ^^^^^^^
  |pattern's type Some[Int] is more specialized than the right hand side expression's type Option[Int] @unchecked
  |
  |If the narrowing is intentional, this can be communicated by writing `: @unchecked` after the full pattern.

scala> val Some(x): @unchecked = Option(3)
val x: Int = 3

Scala 3's behavior is as documented at https://dotty.epfl.ch/docs/reference/changed-features/pattern-bindings.html :

Sometimes one wants to decompose data anyway, even though the pattern is refutable. For instance, if at some point one knows that a list elems is non-empty one might want to decompose it like this:

val first :: rest = elems   // error

This works in Scala 2. In fact it is a typical use case for Scala 2's rules. But in Scala 3.1 it will give a type error. One can avoid the error by marking the pattern with an @unchecked annotation:

val first :: rest : @unchecked = elems   // OK

This will make the compiler accept the pattern binding. It might give an error at runtime instead, if the underlying assumption that elems can never be empty is wrong.

but I suggest that Scala 3 should adopt the Scala 2 convention, because:

• it is more consistent with match; one also annotates the scrutinee with match, like (foo: @unchecked) match ...
• we want to support cross-building between 2 and 3, and 2's style is the established precedent

(this came up at scala/bug#12245)

This became more important with the release of Scala 2.13.4, where improved exhaustivity checking has sent many users scurrying to add @unchecked to their matches.

[liufengyun]

During the meeting, it is agreed that Scala 3 should align with Scala 2.

[liufengyun]

To calrify: in Scala 2.13.4, we need to enable scala -Xlint:valpattern to get the warnings:

scala> val List(x, y) = List(1, 2)
                            ^
       warning: match may not be exhaustive.
       It would fail on the following inputs: List(_), Nil
val x: Int = 1
val y: Int = 2

And it can be suppressed with : @unchecked:

scala> val List(x, y) = List(1, 2): @unchecked
val x: Int = 1
val y: Int = 2

Dotty needs to follow the Scala 2 behavior assuming -Xlint:valpattern in -source 3.1 mode.

[som-snytt]

To calcify, in Scala 2 @unchecked is automatically supplied and -Xlint suppresses it, at which point @unchecked can be supplied manually.

The change was prompted by lihaoyi's list of warts, but I see there was also an old ticket requesting relief from MatchError.

Even as a friendly lint, it didn't seem the PR would make it in, so it's nice that it landed on the right side of history, and that the @unchecked has also landed on the right side.

[liufengyun]

Thanks @som-snytt . In #10793 , -source 3.1 in Dotty has the same behavior as -Xlint:valpattern in Scala 2.13.4. Only syntax has changed in that PR.

There is a minor difference in behavior. Scala 2.13.4 with -Xlint:valpattern only warns for the 2nd case:

scala> val Some(a) = Some(5): Any
val a: Any = 5

scala> val Some(a) = Some(5): Option[Int]
                            ^
       warning: match may not be exhaustive.
       It would fail on the following input: None
val a: Int = 5

Dotty with -source 3.1 warns for both cases.