False positive unused pattern var warning

[philwalk]

reproduction steps

using Scala 2.13.3

package vastblue.apps

object FalsePositiveWarning extends App {
  val rows = Seq.empty[Map[String,Any]]
  for {
    row:Map[String,Any] <- rows
    item = ColItem(row)
  } yield item

  case class ColItem(row:Map[String,Any])
}

problem

The compiler issue this incorrect warning:

[info] Compiling 1 Scala source to C:\opt\ue\apps\target\scala-2.13\classes ...
[warn] C:\opt\ue\apps\src\main\scala\vastblue\apps\FalsePositiveWarning.scala:6:5: pattern var row in value $anonfun is never used: use a wildcard `_` or suppress this warning with `row@_`
[warn]     row:Map[String,Any] <- rows
[warn]     ^
[warn] one warning found

[SethTisue]

duplicate of #10287, I think?

[philwalk]

If we remove the type from row, there is no warning:

package vastblue.apps

object FalsePositiveWarning extends App {
  val rows = Seq.empty[Map[String,Any]]
  for {
    row /* : Map[String,Any] */ <- rows
    item = ColItem(row)
  } yield item

  case class ColItem(row:Map[String,Any])
}

Perhaps it has the same underlying cause as #10287, although it's not obvious to me ...

[SethTisue]

adding a type ascription changes the desugaring