Wrong overload resolution: picks Any over Long with an Int argument

[sjrd]

minimized code

object Asserts {
  def assertEquals(expected: Any, actual: Any): Unit = {
    println(1)
    assert(expected.equals(actual), s"expected $expected but got $actual")
  }

  def assertEquals(expected: Long, actual: Long): Unit = {
    println(2)
    assert(expected == actual, s"expected $expected but got $actual")
  }
}

object Test {
  def main(args: Array[String]): Unit = {
    def foo(): Long = 42L

    Asserts.assertEquals(42, foo()) // an Int and a Long
  }
}

observed behavior

The first overload, taking Anys, is selected over the second one.

1
Exception in thread "main" java.lang.AssertionError: assertion failed: expected 42 but got 42
        at dotty.DottyPredef$.assertFail(DottyPredef.scala:17)
        at Asserts$.assertEquals(hello.scala:4)
        at Test$.main(hello.scala:17)
        at Test.main(hello.scala)

expectation

Like in Scala 2, the second overload, taking Longs, should be selected, so that it prints:

2

note

The API of Asserts above is defined in JUnit; it cannot be changed.

workaround

We can work around the issue by making sure that both arguments truly are Longs:

object Test {
  def main(args: Array[String]): Unit = {
    def foo(): Long = 42L

    Asserts.assertEquals(42L, foo()) // Long and Long
  }
}

[odersky]

I believe this is a consequence that weak conformance is gone. In Dotty an Int is converted to a Long by a standard implicit conversion. If I translate the examples to classes and conversions:

object X {
  case class Int1()
  case class Long1()
  implicit def i2l(x: Int1): Long1 = Long1()

  def ae(expected: Any, actual: Any): Unit = {
    println(1)
    assert(expected.equals(actual), s"expected $expected but got $actual")
  }

  def ae(expected: Long1, actual: Long1): Unit = {
    println(2)
    assert(expected == actual, s"expected $expected but got $actual")
  }
}

object Test {
  def main(args: Array[String]): Unit = {
    import X._
    ae(Int1(), Long1())
  }
}

I get the same output for both scalac and dotc:

1
Exception in thread "main" java.lang.AssertionError: assertion failed: expected Int1() but got Long1()

[odersky]

Since I guess we do not want to bring weak conformance back, I see no way to fix this. Close?

[sjrd]

Can it not be fixed directly in overload resolution?

If not fixed, we risk breaking quite a lot of testing code using JUnit, with runtime assertions. That's not a good migration story. :(

[smarter]

Yeah, isn't Scala overloading resolution supposed to always match Java ?

[odersky]

Yeah, isn't Scala overloading resolution supposed to always match Java ?

There's no such requirement. But we try to come close, for compatibility's sake.

So, we'd have to bring weak conformance back? That would be unfortunate.