fix: correctly require a ClassTag when building a multidimensional Array

[Cherry-picked 9da1fcb]

Author: hamzaremmal

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

@@ -386,6 +386,12 @@ object TypeErasure {
     case _ => false
   }
 
+  /** Is `tp` of the form `Array^N[T]` where T is generic? */
+  def isGenericArrayArg(tp: Type)(using Context): Boolean = tp.dealias match
+    case defn.ArrayOf(elem) => isGenericArrayArg(elem)
+    case _ => isGeneric(tp)
+  end isGenericArrayArg
+
   /** The erased least upper bound of two erased types is computed as follows
    *  - if both argument are arrays of objects, an array of the erased lub of the element types
    *  - if both arguments are arrays of same primitives, an array of this primitive

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -1282,14 +1282,11 @@ trait Applications extends Compatibility {
   def convertNewGenericArray(tree: Tree)(using Context): Tree = tree match {
     case Apply(TypeApply(tycon, targs@(targ :: Nil)), args) if tycon.symbol == defn.ArrayConstructor =>
       fullyDefinedType(tree.tpe, "array", tree.srcPos)
-
-      def newGenericArrayCall =
+      if TypeErasure.isGenericArrayArg(targ.tpe) then
         ref(defn.DottyArraysModule)
-          .select(defn.newGenericArrayMethod).withSpan(tree.span)
-          .appliedToTypeTrees(targs).appliedToTermArgs(args)
-
-      if (TypeErasure.isGeneric(targ.tpe))
-        newGenericArrayCall
+            .select(defn.newGenericArrayMethod).withSpan(tree.span)
+            .appliedToTypeTrees(targs)
+            .appliedToTermArgs(args)
       else tree
     case _ =>
       tree

tests/run/i23901.scala

@@ -0,0 +1,8 @@
+import scala.reflect.ClassTag
+
+object MyArray:
+  def empty[T: ClassTag]: Array[Array[T]] = new Array[Array[T]](0)
+
+@main def Test =
+  val arr: Array[Array[String]] = MyArray.empty[String]
+  assert(arr.length == 0)