SortedMap#empty Fails At Runtime With NoSuchMethodError

[adamgfraser]

Minimized code

import scala.collection.immutable.SortedMap

object Example extends App {
  
  val sortedMap = SortedMap("a" -> 1, "b" -> 2, "c" -> 3)
  val empty     = sortedMap.empty
  println(empty)
}

Output

java.lang.NoSuchMethodError: scala.collection.immutable.SortedMap.empty()Lscala/collection/Map;
	at Example$.<init>(main.scala:6)
	at Example$.<clinit>(main.scala)
	at Example.main(main.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at sbt.Run.invokeMain(Run.scala:115)
	at sbt.Run.execute$1(Run.scala:79)
	at sbt.Run.$anonfun$runWithLoader$4(Run.scala:92)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.util.InterfaceUtil$$anon$1.get(InterfaceUtil.scala:10)
	at sbt.TrapExit$App.run(TrapExit.scala:257)
	at java.lang.Thread.run(Thread.java:748)

Expectation

I would expect that since this method is in the API of the latest Scala collections it would work correctly. At the minimum if it did not work I would expect to get a compilation failure with a helpful error message.

[smarter]

java.lang.NoSuchMethodError: scala.collection.immutable.SortedMap.empty()Lscala/collection/Map;

The correct signature appears to be empty()Lscala/collection/SortedMapOps;

The empty method in question is defined in SortedMapFactoryDefaults:

override def empty: CC[K, V @uncheckedVariance] = sortedMapFactory.empty(ordering)

where CC is defined as:

CC[x, y] <:  Map[x, y] with SortedMapOps[x, y, CC, CC[x, y]] with UnsortedCC[x, y]

Dotty and Scala 2 do not erase intersection types in the same way (scala 2 does some really weird stuff), I'm working on a PR to take that into account when calling into scala 2 code which should hopefully fix that.

[adamgfraser]

Thanks! For what it is worth I also tried to use parentheses like empty() but that resulted in a compilation error missing argument for parameter key of method apply: (key: String): Int @uncheckedVariance. So there doesn't seem to be a way to call this method from Dotty right now.

[smarter]

Indeed, but you should be able to call SortedMap.empty[String, Int]

[adamgfraser]

Yes but if I am working with polymorphic types I may not have an implicit Ordering in scope and need to reuse whatever Ordering is underlying the original map which is why I want to call sortedMap.empty as opposed to SortedMap.empty.