ClassCastException when omitting a by-name argument that has a default value in enum or superclass constructors

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Compiler version

3.7.2-RC1-bin-20250519-d36e423-NIGHTLY, 3.7.0-RC1, 3.6.4, 3.3.5

Minimized code

1. Enum variant:

enum Bar(
  first: => Int = 0,
  second: Option[String]
) {

  case B extends Bar(
    second = None
  )
}

Bar.B     // throws ClassCastException

Enum Scastie

2. Super class variant:

class Bar(
  first: => Int = 0,
  second: Option[String]
)

object B extends Bar(
  second = None
)

B     // throws VerifyError or ClassCastException depending on environment

Class Scastie

Output

Runtime error when evaluating Bar.B:

Enum variant on scala-cli 3.7.2-...-NIGHTLY:

java.lang.ClassCastException: class java.lang.Integer cannot be cast to class scala.Function0 (java.lang.Integer is in module java.base of loader 'bootstrap'; scala.Function0 is in unnamed module of loader java.net.URLClassLoader @6ef81f31

Super class variant on scala**.js** 3.6.4:

Uncaught org.scalajs.linker.runtime.UndefinedBehaviorError: java.lang.ClassCastException: number(0) cannot be cast to scala.Function0
    at $throwClassCastException (internal-....js?t=1747788060853:51:9)
    at Module.$as_F0 (Function0.scala:38:7)
    at new $Example$B$ (Example.scala:...)

Super class variant on scala-cli 3.7.2-...-NIGHTLY and also on 3.6.4:

java.lang.VerifyError: Bad type on operand stack
Exception Details:
  Location:
    rs$line$1$B$.<init>()V @10: invokespecial
  Reason:
    Type uninitializedThis (current frame, stack[3]) is not assignable to 'rs$line$1$B$'
  Current Frame:
    bci: @10
    flags: { flagThisUninit }
    locals: { uninitializedThis, 'scala/None$' }
    stack: { uninitializedThis, 'rs$line$1$', 'scala/None$', uninitializedThis }
  Bytecode:
    0000000: b200 164c 2ab2 001b 2b2a b700 1fb8 0025
    0000010: c000 27b6 002b 2bb7 002e b1

  ... 30 elided

Expectation

Should not throw, or at the very least, should not compile, if the code is illegal.

Details

• For classes, bug seems to be specific to extends, e.g. val B = Bar(second = None) does not trigger the bug.
• Requires by-name argument for the first argument (: =>). Making it by-value avoids the bug.
• Requires a certain type for the second argument. Option, Either, and case classes trigger the bug. String, regular classes do not trigger the bug. Perhaps needs a Product to trigger the bug? Not sure.

[som-snytt]

After erasure, the "superarg" function is confused about how to box the by-name int.

    {
      val second$1: None = None
      def first$1(): Int = Bar.$lessinit$greater$default$1()
      super(
        B.B$$superArg$1(second$1, Int.box(first$1()).asInstanceOf[Function0]),
        second$1)
    }
    private <static> def B$$superArg$1(second$1: None, first$1: Function0):
      Function0 = () => Int.unbox(first$1)

The finicky detail is because it must decide the default first is also a function of the second. It's like it consulted chatgpt.