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fix: correctly require a ClassTag when building a multidimensional Array #23902

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Merged
merged 1 commit into from
Sep 12, 2025
+18 −7
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fix: correctly require a ClassTag when building a multidimensional Array #23902

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6 changes: 6 additions & 0 deletions compiler/src/dotty/tools/dotc/core/TypeErasure.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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11 changes: 4 additions & 7 deletions compiler/src/dotty/tools/dotc/typer/Applications.scala
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Expand Up @@ -1427,14 +1427,11 @@ 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
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8 changes: 8 additions & 0 deletions tests/run/i23901.scala
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Original file line number Diff line number Diff line change
@@ -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)
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maybe test the runtime class?

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What do you mean?

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well to assert that it is specialised based on the classtag

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Before this fix, we couldn't even create the array in Line 7, this is a run test witnessing that we are indeed creating the array correctly. Also, dotty will add a call to asInstanceOf[Array[Array[String]]] here which is enough to assert that the right array is created. Also note that asInstanceOf was what indicated that something was really wrong it the generated code.

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