Bad symbol is selected leading to ClassCastException. Regression from scala 2.13

[jtjeferreira]

Compiler version

Scala 3.3.2 (3.4.0 also affected)

Minimized code

https://scastie.scala-lang.org/YdGomTp2S0Sc48M47mXLEA

case class Config(_config: String)

abstract class Foo(val config: Config) {
  def this(config: String) = {
    this(Config(config))
  }
}

class Bar(config: String) extends Foo(config) {
  def foo(): Unit = {
    config.getClass()
  }
}

new Bar("").foo() //This line will crash in prod

Output

Caused by: java.lang.ClassCastException: class Playground$Config cannot be cast to class java.lang.String (Playground$Config is in unnamed module of loader sbt.internal.BottomClassLoader @11415fc3; java.lang.String is in module java.base of loader 'bootstrap')
at Playground$Bar.config$accessor(main.scala:11)
	at Playground$Bar.foo(main.scala:13)
	at Playground$.<clinit>(main.scala:17)

Expectation

That code compiles and runs without crashing (like in scala 2.13)

[SethTisue]

minimized a bit further,

class Foo(val s: Any):
  def this(s: String) =
    this(0)
class Bar(s: String) extends Foo(s):
  def foo = s
Bar("").foo

java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String

[odersky]

How I fixed it:
I suspected from the bug description that something in the scheme of super parameter aliases went wrong. This scheme optimizes away duplicates of parameter fields in subclasses if they can be shown to be always aliases of a val parameter in a superclass. But I did not know anymore where that scheme was implemented. So I looked at the output at various phases. Typer and PostTyper and FirstTransform all looked OK, but at Erasure I saw that an accessor s$accessor was generated where it should not have been. I then searched for the $accessor in the codebase and found that it was the suffix of the semantic name category ParamAccessorName. I then searched for occurrences of that name and found phase ParamForwarding which implemented the faulty transform. ParamForwarding relied on info provided by method forwardParamAccessors in PostTyper. That method was clearly missing the condition that forwarders have to go through the primary constructor. Secondary constructors can call super with anything they choose, so without looking at their code we cannot assume anything.