ClassCastException: class cannot be cast to class scala.runtime.Nothing$

[jtjeferreira]

Compiler version

Scala 3.6.2, 3.5.2, 3.3.4

Minimized code

import com.fasterxml.jackson.databind.JsonNode
import com.fasterxml.jackson.databind.node.{
  JsonNodeFactory,
  ObjectNode,
  TextNode
}

val json: ObjectNode = JsonNodeFactory.instance.objectNode()

def foo(a: String): Unit = {
  a match {
    case "a" =>
      json.set("type", TextNode.valueOf("value"))
    case _ =>
      json.set("type", TextNode.valueOf("value"))

  }
}
foo("")

method set is a java method:

public <T extends JsonNode> T set(String propertyName, JsonNode value)

https://scastie.scala-lang.org/i507EyzXTdO1BM362YaOhA

Output

Caused by: java.lang.ClassCastException: class com.fasterxml.jackson.databind.node.ObjectNode cannot be cast to class scala.runtime.Nothing$

Expectation

That it does not crash like in scala2 https://scastie.scala-lang.org/gvqImCf7QeKBXahfClCZHA

[Gedochao]

Scala CLI repro:

//> using dep com.fasterxml.jackson.module::jackson-module-scala:2.15.4
import com.fasterxml.jackson.databind.JsonNode
import com.fasterxml.jackson.databind.node.{
  JsonNodeFactory,
  ObjectNode,
  TextNode
}

val json: ObjectNode = JsonNodeFactory.instance.objectNode()

def foo(a: String): Unit = {
  a match {
    case "a" =>
      json.set("type", TextNode.valueOf("value"))
    case _ =>
      json.set("type", TextNode.valueOf("value"))

  }
}
@main def main = foo("")

Fails at runtime with:

Exception in thread "main" java.lang.ClassCastException: class com.fasterxml.jackson.databind.node.ObjectNode cannot be cast to class scala.runtime.Nothing$ (com.fasterxml.jackson.databind.node.ObjectNode and scala.runtime.Nothing$ are in unnamed module of loader 'app')
        at repro$package$.foo(repro.scala:16)
        at repro$package$.main(repro.scala:20)
        at main.main(repro.scala:20)

Optimally, we'd like to minimize without the fasterxml dependency.

[Gedochao]

cc @sjrd

[He-Pin]

seems like the return type is convert to nothing instead insert a ()

[He-Pin]

a quic fix :

import com.fasterxml.jackson.databind.JsonNode
import com.fasterxml.jackson.databind.node.{
  JsonNodeFactory,
  ObjectNode,
  TextNode
}

val json: ObjectNode = JsonNodeFactory.instance.objectNode()

def foo(a: String): Unit = {
  a match {
    case "a" =>
      json.set("type", TextNode.valueOf("value"))
    case _ =>
      json.set("type", TextNode.valueOf("value"))
      ()
  }
}
foo("")

This is a bug I think even I insert the () at the end of the method block will not work.

[sjrd]

The signature of set makes no sense. It is not possible for such a method to return a valid T for all T extending JsonNode. The Scala compiler chooses to instantiate T = Nothing, and as expected, set is not able to fulfill its promise, so you get a CCE.

There's no language bug, here.

Explicitly write the type parameter to something that set actually returns. Presumably set[JsonNode](...).

[He-Pin]

That's seems true

abstract class BaseJsonNode
    extends JsonNode
abstract class ContainerNode<T extends ContainerNode<T>>
    extends BaseJsonNode
class ObjectNode
    extends ContainerNode<ObjectNode>

and In ObjectNode:

    public <T extends JsonNode> T set(String propertyName, JsonNode value)
    {
        if (value == null) {
            value = nullNode();
        }
        _children.put(propertyName, value);
        return (T) this;
    }

I think the code should be written to :

    public ObjectNode set(String propertyName, JsonNode value)
    {
        if (value == null) {
            value = nullNode();
        }
        _children.put(propertyName, value);
        return (T) this;
    }

@pjfanning cc

[He-Pin]

The current implementation in Jackson will cause error anyway, eg:

ArrayNode arrayNode = objectNode.set(..)

which will throw ClassCustException too, but the compiler will not complain.

[pjfanning]

If anyone is suggesting that it is Jackson that gets changed, I honestly don't think the Jackson team (which includes me) will be changing the Java code in Jackson to suit the Scala 3 compiler.

Scala 3 users form a tiny minority of Jackson users and it's hard to justify discommoding other Jackson users by changing Jackson APIs.

Jackson 3 might be a place where we can make a change but there is no date for a GA release of Jackson 3.

[pjfanning]

Interestingly, Jackson 3 code has been changed so that ObjectNode.set and setAll returns ObjectNode explicitly.

https://github.com/FasterXML/jackson-databind/blob/9b93072efe469d7bb9c3281e90a022f4e83d4443/src/main/java/tools/jackson/databind/node/ObjectNode.java#L537

There are snapshots published.
Note the s01.oss.sonatype.org and the groupId has changed to tools.jackson (and the package names too).

https://s01.oss.sonatype.org/content/repositories/snapshots/tools/jackson/core/jackson-databind/3.0.0-SNAPSHOT/