Spurious warning in for expression

[lambdista]

reproduction steps

using Scala 2.13.2,

$ scala -Wunused
Welcome to Scala 2.13.2 (OpenJDK 64-Bit Server VM, Java 1.8.0_252).
Type in expressions for evaluation. Or try :help.

scala> :paste
// Entering paste mode (ctrl-D to finish)

val optA: Option[Int] = Option(42)
val optB: Option[Int] = Option(0)

val x1: Option[Int] = for {
  a <- optA
  b <- optB
  if a != b
} yield a


// Exiting paste mode, now interpreting.


  b <- optB
  ^
<pastie>:6: warning: parameter value b in anonymous function is never used
val optA: Option[Int] = Some(42)
val optB: Option[Int] = Some(0)
val x1: Option[Int] = Some(42)

problem

It gives the warning because b is not used in yield. Still, it is used in if a != b. I guess that for comprehension is desugared more or less in:

optA.flatMap { a =>
  optB.withFilter(b => a != b).map(b => a)
}

Instead it should be desugared in the following one in order not to get the warning:

optA.flatMap { a =>
  optB.withFilter(b => a != b).map(_ => a)
}

[som-snytt]

The umbrella issue #10287

Similar to #11175 (different part of the desugared expression) and #11400 (in the map but refutable pattern).

Probably it's time to consolidate at #10287

[SethTisue]

agreed, duplicate (or close enough) of #10287, feel free to add the code in question as a comment there