method overloading type inference regression

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When preparing twitter util for cross-publishing to scala 2.12, we found that a method that we had previously used without having to specify types now required type annotations.

The diff is here:

https://github.com/twitter/util/pull/163/files

The method is Witness#apply, which is overloaded.

https://github.com/twitter/util/blob/develop/util-core/src/main/scala/com/twitter/util/Event.scala#L416-L442

We did a little more thinking about what might be going on here:

twitter/util#162 (comment)

We were able to construct a reproduction case which succeeds on 2.11, but fails on 2.12.

trait Updatable[T] {
  def update(t: T): Unit
}

object A {
  def apply[T](f: T => Unit): T => Unit = f

  def apply[T](u: Updatable[T]): T => Unit = { t: T =>
    u() = t
  }
}

object C {
  val x: Any => Unit = A(println(_)) // fails
  val y: Int => Unit = A({ num => num + 2 }) // fails
}

The output on 2.12 is:

[error] /Users/moses/projects/reproduce/src/main/scala/com/mosesn/reproduce/Driver.scala:14: missing parameter type for expanded function ((x$1: <error>) => println(x$1))
[error]   val x: Any => Unit = A(println(_)) // fails
[error]                                  ^
[error] /Users/moses/projects/reproduce/src/main/scala/com/mosesn/reproduce/Driver.scala:15: missing parameter type
[error]   val y: Int => Unit = A({ num => num + 2 }) // fails
[error]                            ^
[error] two errors found
[error] (compile:compile) Compilation failed
[error] Total time: 4 s, completed May 29, 2016 8:06:20 AM

Under 2.11, it compiles successfully.

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Imported From: https://issues.scala-lang.org/browse/SI-9796?orig=1
Reporter: Moses Nakamura (mosesn)
Affected Versions: 2.12.0-M4

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Moses Nakamura (mosesn) said (edited on May 29, 2016 3:26:15 PM UTC):
Update: adding another method to Updatable makes it work ¯\_(ツ)_/¯

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@som-snytt said (edited on May 29, 2016 6:39:55 PM UTC):
Updatable is a SAM type. You'll see the new behavior in 2.11 under -Xexperimental. You'll see the old behavior under -Xsource:2.11.

When each A.apply takes a function parameter, then the args are typed with no expected type during overload resolution.

In the working case, the shape of the args selects only the alternative that takes a function.

This is discussed in the announcement under breaking changes.

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Moses Nakamura (mosesn) said:
Aha! I understand now, I didn't get what a SAM type was. Thanks!