Fix scala#22226: Use classOf[BoxedUnit] for Unit array in ArrayConstructors.

The `ArrayConstructors` phase rewrites array constructors to calls
to `scala.runtime.Arrays.newArray`. When it does that, it must pass
the run-time `jl.Class` of the element type. Previously, it used
`classOf[Unit]` when creating an `Array[Unit]` (or nested). That
is not correct, as from the Java perspective, we need to create
`Array[BoxedUnit]`.

We now identify `elemType <: Unit` and replace it with `BoxedUnit`.

---

This highlights a limitation of the Scala.js backend. We should
rewrite calls to `newArray` in the backend to use direct array
creation instead.

[Cherry-picked 442e6a0]

Author: sjrd

compiler/src/dotty/tools/dotc/transform/ArrayConstructors.scala

@@ -27,7 +27,10 @@ class ArrayConstructors extends MiniPhase {
 
   override def transformApply(tree: tpd.Apply)(using Context): tpd.Tree = {
     def expand(elemType: Type, dims: List[Tree]) =
-      tpd.newArray(elemType, tree.tpe, tree.span, JavaSeqLiteral(dims, TypeTree(defn.IntClass.typeRef)))
+      val elemTypeNonVoid =
+        if elemType.isValueSubType(defn.UnitType) then defn.BoxedUnitClass.typeRef
+        else elemType
+      tpd.newArray(elemTypeNonVoid, tree.tpe, tree.span, JavaSeqLiteral(dims, TypeTree(defn.IntClass.typeRef)))
 
     if (tree.fun.symbol eq defn.ArrayConstructor) {
       val TypeApply(tycon, targ :: Nil) = tree.fun: @unchecked

tests/sjs-junit/test/org/scalajs/testsuite/compiler/RegressionTestScala3.scala

@@ -145,6 +145,17 @@ class RegressionTestScala3 {
     assertEquals(5, Issue14289.Container.b())
     assertEquals(true, Issue14289.Container.c())
   }
+
+  @Test def createArrayOfUnitIssue22226(): Unit = {
+    val a = Array.ofDim[Unit](0)
+    assertSame(classOf[Array[Unit]], a.getClass())
+
+    val b = new Array[Unit](0)
+    assertSame(classOf[Array[Unit]], b.getClass())
+
+    val c = Array.ofDim[Unit](0, 0)
+    assertSame(classOf[Array[Array[Unit]]], c.getClass())
+  }
 }
 
 object RegressionTestScala3 {