Case class allows case-field named "productArity", which creates havoc with Product

% scala -version Scala code runner version 3.1.1 – Copyright 2002-2022, LAMP/EPFL

Case class allows case-field with name productArity without any warning/error. This silently breaks functionality of Product. It is not obvious that case class extends Product and field with productArity should not be used. Suggestion: make productArity illegal as a case-field or at least warning.

Example:

object Bug {

  case class Problem(productArity : Int)

  def main(args : Array[String]) : Unit = {
    val cc = Problem(42)
    println("wtf?: "+cc.productArity) //42
  } 
}

[som-snytt]

Same behavior in Scala 2.

Possible messaging: productArity, productElement, and productElementName should be overridden together. Or consider also overriding productElement, etc.

Is it a coincidence that jraty is nearly an anagram of arity?

[som-snytt]

Due to an early experiment, Scala 2 Product#productIterator was still calling into ScalaRunTime.typedProductIterator [sic].

Scala 3 did not inherit that behavior, but does emit _1 component members. That is commented out in Scala 2.

Someone recently asked about fromProduct in Scala 3.

Maybe this ticket can be an umbrella ticket for doc. I guess on Scala 3 it is DOT Product.

There is similar friction around case companions, such as access modifiers for synthetics or whether it extends Function.