CollectionConverters conversion between java.lang.Long and scala.Long

[DieBauer]

Compiler version

Scala compiler version 3.2.1 -- Copyright 2002-2022, LAMP/EPFL

Minimized code

create java file in ./test/L.java

package test;

import java.util.Map;

public class L {

    public static Long y() {
        return 123L;
    }

    public static Map<String, Long> maps() {
        return Map.of("A", 1L);
    }
}

create following scala file in ./Call.scala

import test._

class Call {

  def x(): Long = L.y()

  import scala.jdk.CollectionConverters._

//  def mmap(): Map[String, Long] = L.maps().asScala.toMap
  def maps(): Map[String, Long] = L.maps().asScala.toMap.collect { case (k, v) if v > 0L => (k, v)}

}

Run the following:

javac -d classes test/L.java
scalac -classpath ./classes Call.scala

Output

-- [E007] Type Mismatch Error: Call.scala:9:65 ---------------------------------
9 |  def maps(): Map[String, Long] = L.maps().asScala.toMap.collect { case (k, v) if v > 0L => (k, v)}
  |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                Found:    PartialFunction[(String, Long), (String, Long)]
  |                Required: PartialFunction[(String, Long), (String, Long²)]
  |
  |                where:    Long  is a class in package java.lang
  |                          Long² is a class in package scala
  |
  | longer explanation available when compiling with `-explain`
2 errors found

Expectation

Scala compiler can convert java.lang.Long to scala.Long, just as the case with the single statement in the example file.

[odersky]

Implicit conversions are only applied at the top-level. If there is an implicit conversion scala.Long to java.lang.Long this does not mean there is also an implicit conversion from PartialFunction[X, (String, scala.Long)] to PartialFunction[X, (String, java.lang.Long)].

[DieBauer]

Thanks for the reply!
My main issue is that this line

  def maps(): Map[String, Long] = L.maps().asScala.toMap.collect { case (k, v) if v > 0L => (k, v)}

does compile with Scala 2.13.10, but fails with the above error in Scala 3.x.
So in my codebase I have to call v.longValue to make it compile now.

I created this issue to understand if 3.x is more correct, of if 2.13 is wrong to apply the conversion.

[lrytz]

@DieBauer I get the same error with Scala 2.13.10, can you check again?

Test.scala:9: error: Cannot prove that (String, Long) <:< (String, Long).
  def mmap(): Map[String, Long] = L.maps().asScala.toMap
                                                   ^

[DieBauer]

Correct! That was a mistake from my side to include that statement. The fact is that the second approach, with collect ensures that this can be converted.

import test._

class Call {

  def x(): Long = L.y()

  import scala.jdk.CollectionConverters._

  def maps(): Map[String, Long] = L.maps().asScala.toMap.collect { case (k, v) if v > 0L => (k, v)}

}

Could you see that this does compile with 2.13.10 and fails with 3.2.1?

[lrytz]

Ah, thanks. Yes, that example compiles with 2.13.10, and compilation fails with 3.2.1.

-- [E007] Type Mismatch Error: A.scala:9:65 ------------------------------------
9 |  def maps(): Map[String, Long] = L.maps().asScala.toMap.collect { case (k, v) if v > 0L => (k, v)}
  |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                Found:    PartialFunction[(String, Long), (String, Long)]
  |                Required: PartialFunction[(String, Long), (String, Long²)]
  |
  |                where:    Long  is a class in package java.lang
  |                          Long² is a class in package scala

[DieBauer]

I diffed the output after the typer phase for both scala 2 and 3, and in scala 2 the value type (java Long) gets an implicit conversion applied scala.Tuple2.apply[String, Long](k, scala.Predef.Long2long(v)). While in Scala 3 that doesn't happen Tuple2.apply[String, Long](k, v), even though the guard on the value 0L does get the conversion applied.

Scala 3 output

package <empty> {
  class B() extends Object() {
    import scala.jdk.CollectionConverters.*
    def maps(): Map[String, Long] = 
      jdk.CollectionConverters.MapHasAsScala[String, Long](L.maps()).asScala.
        toMap
      [String, Long](<:<.refl[(String, Long)]).collect[String, Long](
        {
          def $anonfun(x$1: (String, Long)): (String, Long) = 
            x$1:(x$1 : (String, Long)) @unchecked match 
              {
                case Tuple2.unapply[String, Long](k @ _, v @ _) if 
                  Long2long(v).>(0L)
                 => 
                  Tuple2.apply[String, Long](k, v)
              }
          closure($anonfun:PartialFunction[(String, Long), (String, Long)])
        }
      )
  }
}

Scala 2 output

package <empty> {
  class B extends scala.AnyRef {
    def <init>(): B = {
      B.super.<init>();
      ()
    };
    import scala.jdk.CollectionConverters._;
    def maps(): Map[String,Long] = scala.jdk.CollectionConverters.MapHasAsScala[String, Long](L.maps()).asScala.toMap[String, Long](scala.this.<:<.refl[(String, Long)]).collect[String, Long](({
      @SerialVersionUID(value = 0) final <synthetic> class $anonfun extends scala.runtime.AbstractPartialFunction[(String, Long),(String, Long)] with java.io.Serializable {
        def <init>(): <$anon: ((String, Long)) => (String, Long)> = {
          $anonfun.super.<init>();
          ()
        };
        final override def applyOrElse[A1 <: (String, Long), B1 >: (String, Long)](x1: A1, default: A1 => B1): B1 = ((x1.asInstanceOf[(String, Long)]: (String, Long)): (String, Long) @unchecked) match {
          case (_1: String, _2: Long): (String, Long)((k @ _), (v @ _)) if scala.Predef.Long2long(v).>(0L) => scala.Tuple2.apply[String, Long](k, scala.Predef.Long2long(v))
          case (defaultCase$ @ _) => default.apply(x1)
        };
        final def isDefinedAt(x1: (String, Long)): Boolean = ((x1.asInstanceOf[(String, Long)]: (String, Long)): (String, Long) @unchecked) match {
          case (_1: String, _2: Long): (String, Long)((k @ _), (v @ _)) if scala.Predef.Long2long(v).>(0L) => true
          case (defaultCase$ @ _) => false
        }
      };
      new $anonfun()
    }: PartialFunction[(String, Long),(String, Long)]))
  }
}