For-comprehension with Option - incorrectly marked parameter as unused

[kgadek]

Reproduction steps

Scala version: 2.13.9 executed with -Ywarn-unused -Werror.

[info] Starting scala interpreter...
Welcome to Scala 2.13.9 (OpenJDK 64-Bit Server VM, Java 11.0.18).
Type in expressions for evaluation. Or try :help.

scala> for { a1 <- Some(1); a2 <- Some(0); if !(a1 > 10); res = a1 - a2 } yield res
                            ^
       warning: parameter value a2 in anonymous function is never used
error: No warnings can be incurred under -Werror.


scala> for { a2 <- Some(0); a1 <- Some(1); if !(a1 > 10); res = a1 - a2 } yield res
val res8: Option[Int] = Some(1)

Problem

This looks somewhat similar to #11343. The above statement should compile without any warning. Reordering unrelated statements fixes the error.

I've been able to reproduce it on scastie: https://scastie.scala-lang.org/fY4Fp3sAQ0unBDkf1uibdg — it seems 2.13.10 is also affected

[SethTisue]

Minimizes to for { a2 <- Some(0); if true } yield a2

-Vprint:parser shows us that the for desugars to Some(0).withFilter(a2 => true).map(a2 => a2), which of course gives the same warning.

The desugared form makes it clear why the warning is emitted; in withFilter(a2 => true), the a2 parameter is in fact not used.

@som-snytt this seems familiar — surely there's a ticket on this already?

[som-snytt]

yes, this is #10287

I will "consolidate" this example on that ticket.

Maybe I will take a swing at it when I take a last look at related warnings. Especially now that Dotty has unused warnings and we are motivated if not obligated to maintain parity.

That ticket is only 2500 tickets ago, but it feels like half a decade.