From abc370bf80d506e41182d0fd8344c9a925588893 Mon Sep 17 00:00:00 2001
From: Hamza Remmal <hamza@remmal.net>
Date: Thu, 11 Sep 2025 15:37:35 +0200
Subject: [PATCH] fix: correctly require a `ClassTag` when building a
 multidimensional `Array`

[Cherry-picked 9da1fcbeef4e9dc868b0f6121708f7736cc81c94]
---
 compiler/src/dotty/tools/dotc/core/TypeErasure.scala  |  6 ++++++
 .../src/dotty/tools/dotc/typer/Applications.scala     | 11 ++++-------
 tests/run/i23901.scala                                |  8 ++++++++
 3 files changed, 18 insertions(+), 7 deletions(-)
 create mode 100644 tests/run/i23901.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeErasure.scala b/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
index f5ce7a6b4698..83fca3908836 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
+++ b/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
diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
index 38d5bcf3fe9b..90cc7c9a3d84 100644
--- a/compiler/src/dotty/tools/dotc/typer/Applications.scala
+++ b/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
diff --git a/tests/run/i23901.scala b/tests/run/i23901.scala
new file mode 100644
index 000000000000..7bfcfff23551
--- /dev/null
+++ b/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)