1.9.8 regression: copy with different signature than constructor produces error

[OndrejSpanel]

Following code fails with a compile error with 1.9.8 and 1.9.9 in Scala 3. It works fine with quicklens 1.9.7 or with Scala 2.

The error is:

Multiple methods named copy found in Frozen

import com.softwaremill.quicklens.*

object Main {
  case class Frozen(state: String, ext: Int) {
    def copy(state: String): Frozen = Frozen(state, ext)
  }

  def main(args: Array[String]): Unit = {
    val f = Frozen("A", 0)
    f.modify(_.state).setTo("B")
  }
}

My guess is the regression was caused by #236

[KacperFKorban]

Hmmm, we might need a heuristic to pick the correct copy method. I'll have to check what Scala 2 does.
Should we always choose the synthetic one, if there are multiple alternatives? Or filter by the ones with the desired name first?

[OndrejSpanel]

Should we always choose the synthetic one,

When there is a user provided copy, there should be no synthetic one available:

https://www.scala-lang.org/files/archive/spec/3.4/05-classes-and-objects.html?utm_source=chatgpt.com

A method named copy is implicitly added to every case class unless the class already has a member (directly defined or inherited) with that name,

[KacperFKorban]

Hmmm, so this looks like a dotty or spec issue, because this compiles:

//> using scala 3.nightly
//> using dep "com.softwaremill.quicklens::quicklens:1.9.9"
//> using options "-Xprint:typer"

import com.softwaremill.quicklens.*

object Main {
  case class Frozen(state: String, ext: Int) {
    def copy(state: Char): Frozen = Frozen(state.toString, ext)
  }

  def main(args: Array[String]): Unit = {
    val f = Frozen("A", 0)
    // f.modify(_.state).setTo("B")
    f.copy('B')
    f.copy("B", 2)
  }
}

Scala-cli one-liner: scala-cli https://gist.github.com/KacperFKorban/b808cfa80ac20dbb4b85b4d1b6a253a2

[OndrejSpanel]

Interesting. Do you want to post this on scala3, or should I? Meanwhile, picking some particular copy when multiple are available sounds like a good choice. It would be best to mimic what Scala 3 compiler does, but as this seems not to be well specified, picking any should do.

[KacperFKorban]

You can post it if it's not a problem 😄

[OndrejSpanel]

They decided it is a spec issue, the behaviour is correct.

Now as for quicklens, here is a bit different repro, with no synthetic methods at all. Again, having two copy variants prevents the code from compiling on Scala 3:

object Main {
  class Frozen(val state: String, val ext: Int) {
    def copy(state: String = state, ext: Int = ext): Frozen = new Frozen(state, ext)
    def copy(state: String): Frozen = new Frozen(state, ext)
  }

  def main(args: Array[String]): Unit = {
    val f = new Frozen("A", 0)
    f.modify(_.state).setTo("B")
  }
}

The error is a bit confusing:

Unsupported source object: must be a case class or sealed trait, but got: val of type Main.Frozen (f)

If you remove the 2nd copy, it compiles fine.