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Align erasure of Array[Nothing] and Array[Null] with Scala 2 #22517

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merged 3 commits into from
Feb 5, 2025
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Align erasure of Array[Nothing] and Array[Null] with Scala 2 #22517

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c197afb
fix: align erasure of Array[Nothing] and Array[Null] with Scala 2
hamzaremmal Feb 4, 2025
da06de5
fix: reliably run sbt-test scala2-compat/erasure
hamzaremmal Feb 5, 2025
f77b1f9
fix: align Scala 2's erasure of Array[? <: Null] and Array[? <: Nothi…
hamzaremmal Feb 5, 2025
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20 changes: 11 additions & 9 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 @@ -326,7 +326,7 @@ object TypeErasure {
val sym = t.symbol
// Only a few classes have both primitives and references as subclasses.
if (sym eq defn.AnyClass) || (sym eq defn.AnyValClass) || (sym eq defn.MatchableClass) || (sym eq defn.SingletonClass)
|| isScala2 && !(t.derivesFrom(defn.ObjectClass) || t.isNullType) then
|| isScala2 && !(t.derivesFrom(defn.ObjectClass) || t.isNullType | t.isNothingType) then
NoSymbol
// We only need to check for primitives because derived value classes in arrays are always boxed.
else if sym.isPrimitiveValueClass then
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -781,12 +781,14 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst

private def eraseArray(tp: Type)(using Context) = {
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)
if isGenericArrayElement(elemtp, isScala2 = sourceLanguage.isScala2) then
defn.ObjectType
else if sourceLanguage.isScala2 && (elemtp.hiBound.isNullType || elemtp.hiBound.isNothingType) then
JavaArrayType(defn.ObjectType)
else
try 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)
}
Expand Down
2 changes: 2 additions & 0 deletions sbt-test/scala2-compat/erasure/build.sbt
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@@ -1,3 +1,5 @@
ThisBuild / fork := true
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Workaround until #22524 is resolved


lazy val scala2Lib = project.in(file("scala2Lib"))
.settings(
scalaVersion := sys.props("plugin.scala2Version")
Expand Down
6 changes: 6 additions & 0 deletions sbt-test/scala2-compat/erasure/dottyApp/Api.scala
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Expand Up @@ -195,4 +195,10 @@ class Z {
def objectARRAY_88(x: Array[Any]): Unit = {}
def objectARRAY_89(x: Array[AnyRef]): Unit = {}
def objectARRAY_90(x: Array[AnyVal]): Unit = {}

def nothing$ARRAY_91(x: Array[Nothing]): Unit = {}
def null$ARRAY_92(x: Array[Null]): Unit = {}
def nothing$ARRAY_93(x: Array[_ <: Nothing]): Unit = {}
def null$ARRAY_94(x: Array[_ <: Null]): Unit = {}

}
8 changes: 6 additions & 2 deletions sbt-test/scala2-compat/erasure/dottyApp/Main.scala
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Expand Up @@ -53,10 +53,10 @@ object Main {
z.c_40(dummy)
z.c_41(dummy)
z.c_42(dummy)
z.b_43(dummy)
//z.b_43(dummy)
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TODO: Inspect why both these methods' erasures are wrong, or why were they considered correct.

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@hamzaremmal hamzaremmal Feb 5, 2025 •
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I've extracted this to its issue (#22525). I would like to merge this one so I can make progress in #22480 @smarter @sjrd, is that okay with you?

z.c_44(dummy)
z.c_45(dummy)
z.b_46(dummy)
//z.b_46(dummy)
z.c_47(dummy)
// z.a_48(dummy)
// z.c_49(dummy)
Expand Down Expand Up @@ -101,6 +101,10 @@ object Main {
z.objectARRAY_88(dummy)
z.objectARRAY_89(dummy)
z.objectARRAY_90(dummy)
z.objectARRAY_91(dummy)
z.objectARRAY_92(dummy)
z.objectARRAY_93(dummy)
z.objectARRAY_94(dummy)

val methods = classOf[scala2Lib.Z].getDeclaredMethods.toList ++ classOf[dottyApp.Z].getDeclaredMethods.toList
methods.foreach { m =>
Expand Down
6 changes: 6 additions & 0 deletions sbt-test/scala2-compat/erasure/scala2Lib/Api.scala
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Expand Up @@ -186,4 +186,10 @@ class Z {
def objectARRAY_88(x: Array[Any]): Unit = {}
def objectARRAY_89(x: Array[AnyRef]): Unit = {}
def objectARRAY_90(x: Array[AnyVal]): Unit = {}

def objectARRAY_91(x: Array[Nothing]): Unit = {}
def objectARRAY_92(x: Array[Null]): Unit = {}
def objectARRAY_93(x: Array[_ <: Nothing]): Unit = {}
def objectARRAY_94(x: Array[_ <: Null]): Unit = {}
Comment on lines +192 to +193
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This should actually be:

Suggested change
def objectARRAY_93(x: Array[_ <: Nothing]): Unit = {}
def objectARRAY_94(x: Array[_ <: Null]): Unit = {}
def nothing$ARRAY_93(x: Array[_ <: Nothing]): Unit = {}
def null$ARRAY_94(x: Array[_ <: Null]): Unit = {}

but for some reason, it passes the test locally, I will have to test it better, but my machine keeps crashing... so checking with the CI :)

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So this is weird, the CI passes while Array[_ <: Nothing] erasure is wrong. I've tested with the following:

// In Scala 2
class Foo {
    def a: Array[Nothing] = null
    def b: Array[Null] = null
    def c: Array[_ <: Nothing] = null
    def s: Array[_ <: Null] = null
}
// In Scala 3
@main def Test =
    (new Foo).a
    (new Foo).b
    (new Foo).c
    (new Foo).s

The generated bytecode from Scala 2 is the following:

Compiled from "s2.scala"
public class Foo {
  public java.lang.Object[] a();
  public java.lang.Object[] b();
  public scala.runtime.Nothing$[] c();
  public scala.runtime.Null$[] s();
  public Foo();
}

We can clearly see Scala 3 (this PR) emitting a call to the wrong method for Foo.c:

public final class s3$package$ implements java.io.Serializable {
  public static final s3$package$ MODULE$;

  public static {};
    Code:
       0: new           #2                  // class s3$package$
       3: dup
       4: invokespecial #16                 // Method "<init>":()V
       7: putstatic     #18                 // Field MODULE$:Ls3$package$;
      10: return

  public scala.runtime.Null$[] Test();
    Code:
       0: new           #29                 // class Foo
       3: dup
       4: invokespecial #30                 // Method Foo."<init>":()V
       7: invokevirtual #34                 // Method Foo.a:()[Ljava/lang/Object;
      10: pop
      11: new           #29                 // class Foo
      14: dup
      15: invokespecial #30                 // Method Foo."<init>":()V
      18: invokevirtual #37                 // Method Foo.b:()[Ljava/lang/Object;
      21: pop
      22: new           #29                 // class Foo
      25: dup
      26: invokespecial #30                 // Method Foo."<init>":()V
      29: invokevirtual #40                 // Method Foo.c:()Ljava/lang/Object;
      32: pop
      33: new           #29                 // class Foo
      36: dup
      37: invokespecial #30                 // Method Foo."<init>":()V
      40: invokevirtual #43                 // Method Foo.s:()[Lscala/runtime/Null$;
      43: areturn
}

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I've checked a bit and throwing a LinkageError will not be considered a failure... but the problem is that there is a linkage error before the code I've added (I still need to find which one).

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There is already an issue opened on sbt (sbt/sbt#6852). The proposed workaround (fork := true) worked locally


}
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