Compiler regression in type inferrence

[felix-hedenstrom]

Compiler version

The problem exists in 3.8.x, it does not exist in 3.7.4 or 3.3.7

Minimized code

//> using scala 3.8.3

enum Foo(val id: Int) {
  case Bar(override val id: Int, name: Option[String]) extends Foo(id)
}

def y[A](a: A): A = a

val bar: Foo.Bar = Foo.Bar(1, Some("x"))
val result = y(bar.copy(name = None))
val check: Foo.Bar = result

Output

[error] ./copy-issue2.scala:11:22
[error] Found:    (result : Foo)
[error] Required: Foo.Bar
[error] val check: Foo.Bar = result
[error]                      ^^^^^^

Expectation

That it works like in scala versions before 3.8.x and compiles

[som-snytt]

Workaround:

def none: Option[String] = None
val result = y(bar.copy(name = none))

The difference is in how the named args are handled. (The example above was previously desugared as below.)

    val result: Foo.Bar =
      y[Foo.Bar](
        {
          val name$1: Option[String] = none
          val id$1: Int @uncheckedVariance = bar.copy$default$1
          bar.copy(id$1, name = name$1)
        }
      )

That will probably be deemed a bug.

[som-snytt]

I was looking for enum at https://github.com/scala/scala3/releases/tag/3.8.0 but maybe the difference is

#24104

as a guess. (Edit: maybe not.)