JarCastingBackground on SI-9871
Scala 2.12.0 introduces support for SAM types, which is one of the key features for Java 8 support in Scala. Unfortunately, this new feature has the potential to break existing code.
For example, the code snippet below compiles just fine in Scala 2.11.8 (because the overload that takes MyFunction1[Int, Int] is more specific that the overload that takes Function1[Int, Int]):
abstract class MyFunction1[T, U] extends Function1[T, U]
object Test {
def foo(x: Function1[Int, Int]) = ???
def foo(x: MyFunction1[Int, Int]) = ???
val myf: MyFunction1[Int, Int] = ???
foo(myf)
}
However, the same code snippet doesn't compile in Scala 2.12.0:
Test.scala:8: error: ambiguous reference to overloaded definition,
both method foo in object Test of type (x: MyFunction1[Int,Int])Nothing
and method foo in object Test of type (x: Int => Int)Nothing
match argument types (MyFunction1[Int,Int])
foo(myf)
^
We have submitted this issue to the Scala compiler issue tracker under SI-9871.
In the discussion that ensued, @adriaanm from the Scala team at Lightbend confirmed that this behavior is intentional. According to the type compatibility rules introduced in Scala 2.12.0, Function1[Int, Int] is compatible with MyFunction1[Int, Int] (because MyFunction1 is a SAM), which means that none of the two overloads is more specific than the other.
This is an unfortunate issue that blocks our migration from Scala 2.11 to Scala 2.12.
Working around SI-9871
One idea suggested by @adriaanm to work around the issue is to introduce @notASam, a new annotation that would let programmers opt out from automatic promotion of eligible types to SAM types.
Inspired by this idea, I hacked up a small compiler plugin that implements @notASam. With the compiler plugin enabled, the following code compiles equally well in both Scala 2.11.8 and Scala 2.12.0:
package scala.annotation {
class notASam extends StaticAnnotation
}
@scala.annotation.notASam
abstract class MyFunction1[T, U] extends Function1[T, U]
object Test {
def foo(x: Function1[Int, Int]) = ???
def foo(x: MyFunction1[Int, Int]) = ???
val myf: MyFunction1[Int, Int] = ???
foo(myf)
}
Enjoy!
resolvers += Resolver.sonatypeRepo("releases")
addCompilerPlugin("com.github.xenoby" % "workaround9871" % "1.0.0" cross CrossVersion.full)