fix: align erasure of Array[Nothing] and Array[Null] with Scala 2

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -596,8 +596,8 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
    *   will be returned.
    *
    *  In all other situations, |T| will be computed as follow:
-   *   - For a refined type scala.Array+[T]:
-   *      - if T is Nothing or Null, []Object
+   *   - For a refined type scala.Array[T]:
+   *      - {Scala 2} if T is Nothing or Null, []Object
    *      - otherwise, if T <: Object, []|T|
    *      - otherwise, if T is a type parameter coming from Java, []Object
    *      - otherwise, Object
@@ -783,10 +783,12 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
     val defn.ArrayOf(elemtp) = tp: @unchecked
     if (isGenericArrayElement(elemtp, isScala2 = sourceLanguage.isScala2)) defn.ObjectType
     else
-      try
-        val eElem = erasureFn(sourceLanguage, semiEraseVCs = false, isConstructor, isSymbol, inSigName)(elemtp)
-        if eElem.isInstanceOf[WildcardType] then WildcardType
-        else JavaArrayType(eElem)
+      try 
+        if sourceLanguage.isScala2 && (elemtp.isNullType || elemtp.isNothingType) then
+          JavaArrayType(defn.ObjectType)
+        else erasureFn(sourceLanguage, semiEraseVCs = false, isConstructor, isSymbol, inSigName)(elemtp) match
+          case _: WildcardType => WildcardType
+          case elem => JavaArrayType(elem)
       catch case ex: Throwable =>
         handleRecursive("erase array type", tp.show, ex)
   }

sbt-test/scala2-compat/i22515/build.sbt

@@ -0,0 +1,13 @@
+val scala3Version = sys.props("plugin.scalaVersion")
+val scala2Version = sys.props("plugin.scala2Version")
+
+lazy val s2 = project.in(file("scala-2"))
+  .settings(
+    scalaVersion := scala2Version
+  )
+
+lazy val s3 = project.in(file("scala-3"))
+  .dependsOn(s2)
+  .settings(
+    scalaVersion := scala3Version
+  )

sbt-test/scala2-compat/i22515/scala-3/Test.scala

@@ -0,0 +1,3 @@
+@main def Test = 
+    Foo.nothing
+    Foo.`null`

sbt-test/scala2-compat/i22515/test

@@ -0,0 +1 @@
+> s3/run