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[WIP] Fix override checking for Java covariant array return types #24377

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[WIP] Fix override checking for Java covariant array return types #24377

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b2452cd
Add test
tanishiking Nov 11, 2025
4fffd1c
WIP
tanishiking Nov 11, 2025
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24 changes: 23 additions & 1 deletion compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -993,7 +993,29 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
(sym1 eq NullClass) && isNullable(tp2)
}
case tp1 @ AppliedType(tycon1, args1) =>
compareAppliedType1(tp1, tycon1, args1)
// Special case: Java arrays are covariant.
// When checking overrides (frozenConstraint), allow B[] <: A[] if B <: A
// for Java-defined types.
def checkJavaArrayCovariance: Boolean = tp2 match {
case AppliedType(tycon2, arg2 :: Nil)
if frozenConstraint // for override checking (frozen_<:)
&& tycon1.typeSymbol == defn.ArrayClass
&& tycon2.typeSymbol == defn.ArrayClass
&& args1.length == 1 =>
// Check if element types are Java-defined to detect Java arrays
val elem1Sym = args1.head.typeSymbol
val elem2Sym = arg2.typeSymbol
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@tanishiking tanishiking Nov 11, 2025 •
edited
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Ah, it doesn't work if the elemnt type is defined in Scala :/
We need to somehow check AppliedType(ArrayClass, args)s are JavaArray or not 🤔

if elem1Sym.exists && elem2Sym.exists
&& elem1Sym.is(JavaDefined) && elem2Sym.is(JavaDefined)
then
// Arrays are covariant in Java: B[] <: A[] if B <: A
isSubType(args1.head, arg2)
else
false
case _ => false
}

checkJavaArrayCovariance || compareAppliedType1(tp1, tycon1, args1)
case tp1: SingletonType =>
def comparePaths = tp2 match
case tp2: (TermRef | ThisType) =>
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17 changes: 17 additions & 0 deletions tests/pos/i24074/JavaPart.java
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Original file line number Diff line number Diff line change
@@ -0,0 +1,17 @@
public class JavaPart {
public interface A { }
public interface B extends A {
int onlyInB();
}

public interface Lvl1 {
A[] getData();
}

public interface Lvl2 extends Lvl1 {
@Override
B[] getData();
}

public interface Lvl3 extends Lvl2, Lvl1 { }
}
6 changes: 6 additions & 0 deletions tests/pos/i24074/Test.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
// trait Test extends JavaPart.Lvl3
// object TestImpl extends Test:
// def getData(): Array[JavaPart.B] = ???
object Test:
def test(lvl3: JavaPart.Lvl3): Unit =
lvl3.getData.head.onlyInB()
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