-Ywarn-unused:patvars false positive

[oscar-broman]

reproduction steps

Scala 2.13.2

scala -Ywarn-unused:patvars
scala> for (k @ (a, _) <- Option((0, 0)) if a == 0) yield k

warning: pattern var k in value $anonfun is never used: use a wildcard `_` or suppress this warning with `k@_`

problem

k is used and the code won't compile without it. The warning appears to happen due to withFilter not using k.

[SethTisue]

looks like one that @som-snytt will want to 👀

[som-snytt]

Duplicates #11400
Duplicates #10287