Unused warning occurred even if the variable was used

[tkrs]

Given the following code:

object Hello {
  for {
    a <- Option(1)
    b <- Option(2) if a == 1
  } yield a + b
}

Compiling with -Ywarn-unused:params on 2.12.7 gives the following warning:

[warn] /Users/tkrs/code/github.com/tkrs/hello/src/main/scala/Hello.scala:7:5: parameter value b in value$anonfun is never used
[warn]     b <- Option(2) if a == 1
[warn]     ^
[warn] one warning found

However, no warning occurred on 2.12.6.

[SethTisue]

/cc @som-snytt

[som-snytt]

Thanks, I'm already taking a look, sigh. Someone brought up another issue, not sure if it's a ticket, but my response was that the current approach will remain brittle, short of a rewrite.

[som-snytt]

It snags the filtering expression:

Replaying: for (x <- Option(1); y <- Option(2) if x > 1) yield x + y
[[syntax trees at end of                    parser]] // <console>
        val res2 = Option(1).flatMap(((x) => Option(2).withFilter(((y) => x.$greater(1))).map(((y) => x.$plus(y)))))

The related umbrella issue #10287 is still open.

[adriaanm]

Ok, I'm rescheduling for 2.13.x, since we shouldn't be rewriting things in 2.12 anymore, and it sounds like that's what's required for a robust fix.

[manuchadha1979]

Hi

In which release this issue has been or is planned to be resolved?

regards

[dwijnand]

@manuchadha1979 this is currently targeting the 2.13.1 milestone, but it looks like no one has picked up fixing it yet.

[manuchadha1979]

Thanks. Would the current work around be either not to use the compiler option or not to use use the for comprehension?

[dwijnand]

It's your choice.

An alternative to not using the compiler option or not using the for comprehension is this:

 object Hello {
   for {
     a <- Option(1)
     b <- Option(2) if a == 1
+    useB = b // WORKAROUND scala/bug#11175 -Ywarn-unused:params false positive
   } yield a + b
 }

[som-snytt]

Modulo side-effects:

$ scala -Wunused:params

     ________ ___   / /  ___
    / __/ __// _ | / /  / _ |
  __\ \/ /__/ __ |/ /__/ __ |
 /____/\___/_/ |_/____/_/ | |
                          |/  version 2.13.0

scala> for (x <- Option(1); y <- Option(2) if x > 1) yield x + y
                            ^
       warning: parameter value y in anonymous function is never used
res0: Option[Int] = None

scala> for (x <- Option(1) if x > 1; y <- Option(2)) yield x + y
res1: Option[Int] = None

[joan38]

It's your choice.

An alternative to not using the compiler option or not using the for comprehension is this:

 object Hello {
   for {
     a <- Option(1)
     b <- Option(2) if a == 1
+    useB = b // WORKAROUND scala/bug#11175 -Ywarn-unused:params false positive
   } yield a + b
 }

I used a _ instead:

_ = b // Workaround scala/bug#11175 -Ywarn-unused:params false positive

[som-snytt]

Apologies to the 22 thumbs-up voters, who probably meant thumbs-down.

The technology was pushed to its limit.

Is it a coincidence that it topped out at 22, or does github use a Tuple?

[SethTisue]

consolidating at #10287