Warning when assigning null to Unit

[kavedaa]

Compiler version

3.3.0

Minimized example

scala> val a: Unit = null

Output Error/Warning message

1 warning found
-- [E129] Potential Issue Warning: ---------------------------------------------
1 |val a: Unit = null
  |              ^^^^
  |A pure expression does nothing in statement position; you may be omitting necessary parentheses
  |
  | longer explanation available when compiling with `-explain`
val a: Unit = ()

Why this Error/Warning was not helpful

The message was unhelpful because...

...it's hard to understand why it is relevant in this scenario. I initially thought that it's perhaps not allowed to assign null to a value of Unit type, but since it's just a warning and not an error, then I guess it is. So I don't really understand at all why this warning appears here.

Suggested improvement

It could be made more helpful by...

...not giving a warning at all, or at least changing the explanation if it's actually a legitimate warning.

[kavedaa]

Duh...I was clearly not thinking clearly when wrote this. In the sense that I probably don't really understand how Unit works. Closing.

[som-snytt]

Warnings are designed for when we are not thinking clearly!

The error here is that it does not warn with -Wvalue-discard.

Scala 2 emits both warnings:

scala> val x: Unit = null
                     ^
       warning: discarded non-Unit value of type Null(null)
                     ^
       warning: a pure expression does nothing in statement position
val x: Unit = ()

I'll leave the ticket closed, but I think it is a bug.

[som-snytt]

Welcome to Scala 3.4.0-RC1-bin-SNAPSHOT-git-8c1cdc2 (21, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> val x: Unit = null
1 warning found
-- [E129] Potential Issue Warning: -------------------------------------------------------------------------------------
1 |val x: Unit = null
  |              ^^^^
  |              A pure expression does nothing in statement position; you may be omitting necessary parentheses
  |
  | longer explanation available when compiling with `-explain`
val x: Unit = ()

scala> def i = 42
def i: Int

scala> val x: Unit = i
1 warning found
-- [E175] Potential Issue Warning: -------------------------------------------------------------------------------------
1 |val x: Unit = i
  |              ^
  |              discarded non-Unit value of type (i : => Int)
val x: Unit = ()

Also it drives me nuts that I expand the edit window, then github UI re-minimizes it. Thanks for listening.

[kavedaa]

Thanks for taking a look. I agree, it would be clearer with the "discarded" warning.

[bishabosha]

Thanks for opening the issue, I agree that it can be confusing, as assigning a value to Unit type acts in a special way, looks like #18723 will make the situation clearer