Dotty: 3.0.0-Mx: Variable argument in constructor corrupts captured variables in super-constructor call.

[j-mie6]

Minimized code

abstract class Foo(x: Any)
class Boom(var x: Unit, y: Unit) extends Foo((x: Int) => x)

The key conditions to trigger this bug seem to be having a variable in the constructor, and an argument after it, and the arguments to the super type must attempt to capture the arguments of the constructor.

Output

This causes a crash about not being able to unify two variables. In fact, it also dumps the desuagared source file:

class Boom(x: Unit, y: Unit) extends Foo(
  Boom#Boom$superArg$1(this.x, this.x_=)
) {
  def x: Unit
  def x_=(x$1: Unit): Unit
  private val y: Unit
  private <static> def Boom$superArg$1(x: Unit, y: Unit): Int => Int =
    {
      def $anonfun(x: Int): Int = x
      closure($anonfun)
    }
}

The problem is really obvious in this snippet: superArg$1(this.x, this.x_=) is clearly not the expected superArg$1(this.x, this.y)
and, clearly, Unit => Unit, expected Unit is the jist of the mangled type-error .

This can be reproduced on scastie targeting 3.0.0-M3, but it appears this bug still exists all the way back to 0.27.0-RC1 and possibly even earlier than that!

It looks as if the compiler insert arguments into these closures in some form of list, and that list is being polluted with the generated setter, pushing out another legitimate argument.

[odersky]

Thanks for the diagnosis! With findings so detailed the bug almost fixed itself. 😄