Regression for unpacking var-arg splices when using Scala 2.13.17

[WojciechMazur]

It's no longer possible to guard against null var arg splice in the receiver method. The vararg == null is ignored.
The problem exists when using Scala 3 (using any version) with Scala 2.13.17 stdlib.
Effectively there is no way to detect null splice:

• _ != null is ignored
• _.length > 0 results in java.lang.NullPointerException: Cannot read the array length because the return value of "scala.collection.immutable.ArraySeq$ofRef.unsafeArray()" is null
• Option(_).isDefined evaluates to true (becouse of the null check)

Based on failures in scalatest/scalatest in #24190

Compiler version

All Scala versions, tested: 3.3.7 LTS, 3.7.4-RC1, 3.8.0-RC1-bin-20251017-96aff06-NIGHTLY

Minimized code

class NullArgumentException(msg: String) extends IllegalArgumentException(msg)

trait Suite
class Suites(suitesToNest: Suite*) {
  if suitesToNest == null then
    throw NullArgumentException("The passed suites splice is null")

  for
    s <- suitesToNest // triggers NullPointerException
    if s == null
  do
    throw new NullArgumentException("One of the passed suite was null")
}

@main def Test = {
  val aNull: Array[Suite] = null
  try {
    new Suites(aNull*)
    throw IllegalStateException("Should not be reached")
  } catch {
    case ex: NullArgumentException => println(s"ok, expected: $ex")
  }
}

Output

// scala test.scala  -S 3.7.4-RC1 --dep org.scala-lang:scala-library:2.13.17 
Exception in thread "main" java.lang.NullPointerException
        at java.base/java.lang.reflect.Array.getLength(Native Method)
        at scala.collection.ArrayOps$ArrayIterator.<init>(ArrayOps.scala:126)
        at scala.collection.immutable.ArraySeq$ofRef.iterator(ArraySeq.scala:348)
        at scala.collection.View$Filter.iterator(View.scala:144)
        at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:629)
        at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:628)
        at scala.collection.AbstractIterable.foreach(Iterable.scala:936)
        at scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:906)
        at Suites.<init>(test.scala:9)
        at test$package$.Test(test.scala:18)
        at Test.main(test.scala:15)

Same problem in nightly which ships it's own stdlib

// scala test.scala  -S 3.nightly
Compiled project (Scala 3.8.0-RC1-bin-20251018-96aff06-NIGHTLY, JVM (21))
Exception in thread "main" java.lang.NullPointerException
        at java.base/java.lang.reflect.Array.getLength(Native Method)
        at scala.runtime.ScalaRunTime$.array_length(ScalaRunTime.scala:89)
        at scala.collection.ArrayOps$ArrayIterator.<init>(ArrayOps.scala:129)
        at scala.collection.immutable.ArraySeq$ofRef.iterator(ArraySeq.scala:351)
        at scala.collection.View$Filter.iterator(View.scala:149)
        at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:632)
        at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:337)
        at scala.collection.AbstractIterable.foreach(Iterable.scala:946)
        at scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:916)
        at Suites.<init>(test.scala:9)
        at test$package$.Test(test.scala:18)
        at Test.main(test.scala:15)

Expectation

Same behaviour as when using 2.13.16 stdlib

// scala test.scala  -S 3.7.4-RC1 --dep org.scala-lang:scala-library:2.13.16
Compiled project (Scala 3.7.4-RC1, JVM (21))
[hint]  "scala-library is outdated, update to 2.13.17"
     scala-library 2.13.16 -> org.scala-lang:scala-library:2.13.17
ok, expected: NullArgumentException: The passed suites splice is null

[WojciechMazur]

cc @hamzaremmal @lrytz

[lrytz]

I didn't look in detail, but a quick search turns up this 2.13.17 PR that looks related: scala/scala#11021

cc @sjrd

[sjrd]

Looks like Scala 3 uses ScalaRunTime.wrapXArray when passing an array* to a varargs:

new Suites(scala.runtime.ScalaRunTime.wrapRefArray(a()))

Scala 2.13, however, uses Predef.copyArrayToImmutableIndexedSeq:

new Suites($line4.$read.INSTANCE().$iw(), scala.Predef.copyArrayToImmutableIndexedSeq($iw.this.$outer().$line5$read().$iw().a()))

The latter handles null, as it is meant for arbitrary, user-controlled values. The former was indeed changed by my PR.

IMO Scala 3 should use copyArrayToImmutableIndexedSeq as well. It's generally not OK to directly wrap a user-controlled array into an immutable.ArraySeq anyway, since the user code could still modify the underlying array, breaking the ArraySeq immutability.

[hamzaremmal]

It's generally not OK to directly wrap a user-controlled array into an immutable.ArraySeq anyway, since the user code could still modify the underlying array, breaking the ArraySeq immutability

In the implementation of immutable.ArraySeq, why don't we copy the array instead of having a reference to an external array?

[sjrd]

Because there are performance-sensitive applications in which one wants to build the array mutably first, and then export it as an ArraySeq.