New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Two variance checking holes: constructors and default getters #8737

scabug opened this Issue Jul 20, 2014 · 3 comments


None yet
3 participants
Copy link

scabug commented Jul 20, 2014

The variance validator does not check constructors or default methods. The test case here shows that both of these decisions are unsound (and I suspect the decision not to check case class apply/unapply methods also falls into this category).

The VarianceChecker class in dotty PR #147 detects both variance violations.

As far as default getters are concerned we need to be careful with default getters for case class copy methods. These are allowed to violate variance because of the particular way copy works. The Dotty PR achieves this by
annotating the default getters of copy methods with @uncheckedVariance.


This comment has been minimized.

Copy link

scabug commented Jul 20, 2014

Imported From:
Reporter: @odersky
Affected Versions: 2.11.1


This comment has been minimized.

Copy link

scabug commented Nov 8, 2015

@retronym said:
From a re-report, #9549:

class C[+A] {
  private[this] var y: A = _
  def getY: A = y
  class Inner(x: A) {
    y = x
object Test {
  def main(args: Array[String]) = {
    val x = new C[String]
    val y: C[Any] = x
    val i = new y.Inner(1)
    val s: String = x.getY

This comment has been minimized.

Copy link

scabug commented Sep 26, 2016

Chung-Kil Hur (Gil) said (edited on Sep 26, 2016 4:16:14 AM UTC):
Dear developers,

The following code also seems to show a related unsoundness.

sealed abstract class MyADT[+A]
case object MyNone extends MyADT[Nothing]
case class MyFun[A](fun: A=>A) extends MyADT[A]

def data : MyADT[Any] = MyFun((x:Int)=>x+1)

val foo : Any =
  data match {
    case MyFun(f) => f("a")
    case _ => 0

In this code, I simply try to encode the following polymorphic ADT in such a way that MyNone does not require unnecessary parentheses ().

MyADT[A] = MyNone | MyFun of A->A

However, as the code shows, Scala unsoundly accepts the wrong code and produce a type error at run time.
So, could you let me know whether there is a sound way to encode MyADT without requiring unnecessary parentheses in Scala?


Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment