Impossible cast from Long when Java varargs are involved

[jlprat]

When interacting with some java code involving parametrized methods and varargs of the same type and a Scala Long is used, a ClassCastException is thrown at runtime as primitive long can't be casted to Object.

Compiler version

Scala 3.0.2 and 3.1.0-RC2

Minimized code

You can find a repo with the code here
Given this Java class

public class TypedVarargs<V> {

    public TypedVarargs<V> varArgs(V thing, V... things) {
        return this;
    }
}

And this Scala class:

object ImpossibleCast {
 
    val x = new TypedVarargs[Long]()
    val y = x.varArgs(1L)
    def main(args: Array[String]): Unit = {
        println("runs")
    }
}

Output

When running ImpossibleCast we get the following exception:

[error] java.lang.ExceptionInInitializerError
[error] 	at ImpossibleCast.main(ImpossibleCast.scala)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] 	at java.lang.reflect.Method.invoke(Method.java:498)
[error] Caused by: java.lang.ClassCastException: [J cannot be cast to [Ljava.lang.Object;
[error] 	at ImpossibleCast$.<clinit>(ImpossibleCast.scala:4)
[error] 	at ImpossibleCast.main(ImpossibleCast.scala)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] 	at java.lang.reflect.Method.invoke(Method.java:498)

Expectation

The code should work (it does under Scala 2.13.6) and print:

runs

Workaround

object ImpossibleCast {
 
    val x = new TypedVarargs[java.lang.Long]()
    val y = x.varArgs(1L)
    def main(args: Array[String]): Unit = {
        println("runs")
    }
}

Notice that if we instantiate the TypedVarargs class with java.lang.Long instead of Scala's Long the code works as expected.