[2.13.9] `productIterator` change to case classes breaks bincompat if the class is also a value class

[adamw]

MiMA reports the following error when trying to update to 2.13.9, for value classes:

[error] core: Failed binary compatibility check against com.softwaremill.sttp.model:core_2.13:1.5.2! Found 2 potential problems
[error]  * extension static method productIterator$extension(java.lang.String)scala.collection.Iterator in class sttp.model.Method does not have a correspondent in current version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("sttp.model.Method.productIterator$extension")
[error]  * extension method productIterator$extension(java.lang.String)scala.collection.Iterator in object sttp.model.Method does not have a correspondent in current version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("sttp.model.Method.productIterator$extension")

this is for a value case class:

case class Method(method: String) extends AnyVal

is this something that can be safely ignored?

[som-snytt]

I think it is not safe.

Reported at lightbend-labs/mima#723

[adamw]

Oh thanks! I'll close this one as it is clearly a duplicate :)

[som-snytt]

I'll probably need a ticket over here...

[adamw]

Ah sure, sorry, reopening then ;)

[som-snytt]

➜ scalac -d /tmp value.scala
➜ scalac -d /tmp -cp /tmp valuation.scala
➜ scala -cp /tmp Test
hello, world
➜ skalac -d /tmp value.scala  # recompile the case class
➜ scala -cp /tmp Test
Exception in thread "main" java.lang.NoSuchMethodError: 'scala.collection.Iterator Value$.productIterator$extension(java.lang.String)'
        at C.f(valuation.scala:4)

where scalac is 2.13.8 and skalac is 2.13.9 and

➜ cat value.scala

case class Value(s: String) extends AnyVal
➜ cat valuation.scala

class C {
  def f() =
    Value("hello, world").productIterator.foreach(println)
}

object Test extends App {
  new C().f()
}

[SethTisue]

The PR in question is scala/scala#10002 .

In addition figuring out how to address the issue itself, we should also give a little thought to whether our overall testing regimen could have or should have caught this.

(As Som noted on the MiMa ticket, apparently the Scala standard library doesn't have any case classes which are also value classes — if it did, this issue would have shown up.)

[som-snytt]

For convenience, copying my comment

A case class extending AnyVal had product methods:

  public static java.lang.String productElementName$extension(java.lang.String, int);
  public static scala.collection.Iterator<java.lang.Object> productIterator$extension(java.lang.String);
  public static java.lang.Object productElement$extension(java.lang.String, int);
  public static int productArity$extension(java.lang.String);
  public static java.lang.String productPrefix$extension(java.lang.String);
  public scala.collection.Iterator<java.lang.String> productElementNames();
  public java.lang.String productPrefix();
  public int productArity();
  public java.lang.Object productElement(int);
  public scala.collection.Iterator<java.lang.Object> productIterator();
  public java.lang.String productElementName(int);

where productElementNames lacks an extension method and would therefore always box.

Now productIterator also lacks an $extension method for some reason. (Presumably, the reason is that methods inherited from a trait don't get the value class treatment.)

Product extends Any is a universal trait.