2.13.9 Good code error: x in variable x is never used.

[He-Pin]

Reproduction steps

akka/akka#31615

Scala version: 2.13.9

Problem

[09-22 03:19:18.235] [error] /home/runner/work/akka/akka/akka-actor/src/main/scala/akka/actor/Actor.scala:331:15: parameter value _log_= in variable _log is never used
[09-22 03:19:18.235] [error]   private var _log: LoggingAdapter = _
[09-22 03:19:18.236] [error]        

I think the code is fine, but not ok when compile it with 2.13.9

Other issues:

1. the variable is _log not _log_.
2. it's local variable not parameter

[som-snytt]

Reproduction is trivial:

trait T {
  private var x: String = _

  def y: String = {
    if (x eq null) x = "hello, world"
    x
  }
}

and

➜  skalac -version
Scala compiler version 2.13.10-20220920-001308-98972e5 -- Copyright 2002-2022, LAMP/EPFL and Lightbend, Inc.

➜  skalac -d /tmp -Wunused -Vprint:typer t12646.scala
t12646.scala:3: warning: parameter value x_= in variable x is never used
  private var x: String = _
              ^
[[syntax trees at end of                     typer]] // t12646.scala
package <empty> {
  abstract trait T extends scala.AnyRef {
    def /*T*/$init$(): Unit = {
      ()
    };
    <accessor> private val x: String = _;
    <accessor> private def x_=(x$1: String): Unit;
    def y: String = {
      if (T.this.x.eq(null))
        T.this.x_=("hello, world")
      else
        ();
      T.this.x
    }
  }
}

1 warning

[jxnu-liguobin]

Is this as expected for the private accessor method?

It seems that this PR has changed this behavior.

[som-snytt]

The unused thing is the synthetic parameter to the setter. What?

[som-snytt]

Fix in linked PR. The strangeness has to do with fields in traits. I'm not sure why the parameter symbol is the setter (x_=) as shown in the confusing error message.