Desugared guard (withFilter) in for comp fails position check

[som-snytt]

Reproduction steps

Scala version: 2.13.15

//> using options -Yvalidate-pos:typer
object A {
  def f(list: List[String]) = for (string <- list if string.length > 5)
    println(string)
}

Problem

def f(list: List[String]) = list.withFilter(((string) => string.length.$greater(5))).foreach(((string) => println(string)))

The problem is with the position of the string parameter.

For patvar usage tracking scala/scala#10812, the pattern tree and its duplicate in the construction of a guard expression were reversed at https://github.com/scala/scala/blob/v2.13.15/src/reflect/scala/reflect/internal/TreeGen.scala#L728 because one patvar is now tracked as an "alias" of the original other. The result is that the tree positions, which are subtly different, are reversed. In particular, one pos is an offset and the other is a range.

In 2.13.14, the offset range of the parameter to the withFilter function ([111]) is also out of range of the enclosing trees ([121:146]), but by some quirk, it doesn't fail validation. (The quirky feature is that "range" means "non-synthetic", so the enclosed tree must have a range to be checkable.)

Noticed at scoverage/scalac-scoverage-plugin#641

[som-snytt]

Although -Yvalidate-pos is a "private" option, labelling as a "blocker" because it will block me from getting a good night's rest.