java.util.Collections.min has wrong return type

[molikto]

minimized code

abstract class Test extends java.lang.Comparable[Test] {

  def test: Unit = {
  	java.util.Collections.min(new java.util.ArrayList[Test]())
  }
}

Compilation output

public abstract class Test extends java.lang.Object implements java.lang.Comparable<Test>
  minor version: 0
  major version: 52
  flags: (0x0421) ACC_PUBLIC, ACC_SUPER, ACC_ABSTRACT
  this_class: #2                          // Test
  super_class: #5                         // java/lang/Object
  interfaces: 1, fields: 0, methods: 2, attributes: 4
Constant pool:
   #1 = Utf8               Test
   #2 = Class              #1             // Test
   #3 = Utf8               Ljava/lang/Object;Ljava/lang/Comparable<LTest;>;
   #4 = Utf8               java/lang/Object
   #5 = Class              #4             // java/lang/Object
   #6 = Utf8               java/lang/Comparable
   #7 = Class              #6             // java/lang/Comparable
   #8 = Utf8               test.scala
   #9 = Utf8               <init>
  #10 = Utf8               ()V
  #11 = NameAndType        #9:#10         // "<init>":()V
  #12 = Methodref          #5.#11         // java/lang/Object."<init>":()V
  #13 = Utf8               this
  #14 = Utf8               LTest;
  #15 = Utf8               test
  #16 = Utf8               java/util/ArrayList
  #17 = Class              #16            // java/util/ArrayList
  #18 = Methodref          #17.#11        // java/util/ArrayList."<init>":()V
  #19 = Utf8               java/util/Collections
  #20 = Class              #19            // java/util/Collections
  #21 = Utf8               min
  #22 = Utf8               (Ljava/util/Collection;)Ljava/lang/Comparable;
  #23 = NameAndType        #21:#22        // min:(Ljava/util/Collection;)Ljava/lang/Comparable;
  #24 = Methodref          #20.#23        // java/util/Collections.min:(Ljava/util/Collection;)Ljava/lang/Comparable;
  #25 = Utf8               Code
  #26 = Utf8               LineNumberTable
  #27 = Utf8               LocalVariableTable
  #28 = Utf8               Signature
  #29 = Utf8               SourceFile
  #30 = Utf8               TASTY
  #31 = Utf8               Scala

expectation

the return type should be Object, but instead is Ljava/lang/Comparable

---

See comment bellow for more information where it is wrong.

After trying to fix this problem, I found the root cause is in ClassfileParser, all type bounds is eagerly & in sig2typeBounds, so this is not a bug in erasure or backend.

I cannot think of a proper way to overcame this now. Maybe we should also parse the erasured signature from constant pool, then in Erasure we will always use the ones parsed instead compute ourselves?

[TheElectronWill]

More information about this 👇
Collections.min is declared as follows:

public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T>);

It seems reasonable to assume that T extends Object & SomeInterface is the same as T extends SomeInterface, but it produces a different signature.

For backward-compatibility reasons, T extends Object & SomeInterface is erased to Object, not SomeInterface. The exact rule that javac follows is to take the first type mentioned in the bound.

[sjrd]

When this is fixed, it should be possible to enable the Scala.js test CollectionsOnCollectionsTest.scala, by removing it from the blacklist at
https://github.com/lampepfl/dotty/blob/dfcc0b14c0a08c9933234067b74378f317f5ebc9/project/Build.scala#L1042
It's also possible that other files in the same list could be un-blacklisted at the same time.

[smarter]

Fixed in #9601.