Overriding with an inline method and inline match is unsound

[ch1nq]

Compiler version

3.7.4

3.8.1-RC1-bin-20251215-a227106-NIGHTLY

Minimized code

import scala.compiletime.erasedValue

object Test {
  type MatchType[T] = T match {
    case EmptyTuple => String
    case _          => Int
  }

  trait MyTrait[T] {
    def f: MatchType[T]
  }

  class MyClass[T] extends MyTrait[T] {
    override inline def f: MatchType[T] = inline erasedValue[T] match {
      case _: EmptyTuple => "empty"
      case _             => 42
    }
  }

  @main def main(): Unit = {
    val a: MyTrait[EmptyTuple] = new MyClass[EmptyTuple]
    val str: String = a.f
  }
}

Output

Exception in thread "main" java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')
        at Test$.main(overrideInline.scala:22)
        at main.main(overrideInline.scala:20)

Expectation

Should not compile

[sjrd]

Ouch, thanks for the report.

I don't think overriding is the core issue. An inline match should not typecheck against a match type in the first place. They don't have the same semantics. Match types correspond to the semantics of regular matches at the term level.

[hamzaremmal]

Isn't the root issue here the fact that we need to retain the body of the overriden method so we just reduce the inline match with the assumption that T <: Any and by omitting the fact that T is a type variable and it can be instantiated to anything, therefore changing the reduction of the match type? This is a case where this would be problemetatic. If it was a simple inline function, I don't think it would have been problematic, no?