java interop: not auto box

[ohze]

minimized code

• ConfigFactory.java

import java.util.Map;
public class ConfigFactory {
    public static void parseMap(Map<String, ? extends Object> values) { }
}

• Test.scala

import scala.jdk.CollectionConverters._
object Test {
  ConfigFactory.parseMap(Map("a" -> 1).asJava)
  ConfigFactory.parseMap(Map("a" -> "", "b" -> true).asJava)
}

Compilation output

[error] -- [E007] Type Mismatch Error: /Users/thanhbv/ohze/dotty-example/src/main/scala/Test.scala:3:39 
[error] 3 |  ConfigFactory.parseMap(Map("a" -> 1).asJava)
[error]   |                         ^^^^^^^^^^^^^^^^^^^^
[error]   |                         Found:    java.util.Map[String, Int]
[error]   |                         Required: java.util.Map[String, ? <: Object]
[error] -- [E007] Type Mismatch Error: /Users/thanhbv/ohze/dotty-example/src/main/scala/Test.scala:4:53 
[error] 4 |  ConfigFactory.parseMap(Map("a" -> "", "b" -> true).asJava)
[error]   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]   |                         Found:    java.util.Map[String, Any]
[error]   |                         Required: java.util.Map[String, ? <: Object]

expectation

Compile successfully as in scala 2
Use case: When using "com.typesafe" % "config" library

[ohze]

Note: Call ConfigFactory.parseMap(m.asJava) with:

• val m = Map("a" -> true) // => success in scala 2, fail in dotty
• val m: Map[String, AnyRef] = Map("a" -> true) // => fail in scala 2, success in dotty
• val m = Map("a" -> (true: java.lang.Boolean)) // => success in both cases but too verbose :(

[odersky]

@lrytz @SethTisue @szeiger Do you know how this is done in Scala 2? If I look at the signature for asJava

def asJava[K, V](m: Map[K, V]): java.util.Map[K, V]

it's not clear to me by what magic that turns a Map[String, Int] to a Java Map<String, Integer>.

[smarter]

I think the problem is that we get Required: java.util.Map[String, ? <: Object] when we want java.util.Map[String, ? <: Any]. I'm working on doing something similar to scala/scala#7966 in dotty which might fix this sort of things.

[odersky]

@smarter Yes, that makes sense. I'll assign this one to you for tracking then.

[smarter]

Fixed in #9601.