Can't resolve to overloaded apply (regression 2.11.9 ?)

[xuwei-k]

https://github.com/xuwei-k/apply-overload-2.11.9

import org.scalacheck.{Arbitrary, Gen}

case class Foo[A](x: Option[Int], y: A)

object Foo {
  // error
  implicit def arbitrary[A](implicit a: Arbitrary[A]): Arbitrary[Foo[A]] =
    Arbitrary(Gen.resultOf[Option[Int], A, Foo[A]](Foo.apply[A]))

  def apply[A](x: Int, y: A): Foo[A] = Foo(Some(x), y)
}

---

[error]  found   : (Int, A) => example.Foo[A]
[error]  required: (Option[Int], A) => example.Foo[A]
[error]     Arbitrary(Gen.resultOf[Option[Int], A, Foo[A]](Foo.apply[A]))
[error]                                                             ^

---

but this is ok (change method definition order)

object Foo {
  def apply[A](x: Int, y: A): Foo[A] = Foo(Some(x), y)

  // ok
  implicit def arbitrary[A](implicit a: Arbitrary[A]): Arbitrary[Foo[A]] =
    Arbitrary(Gen.resultOf[Option[Int], A, Foo[A]](Foo.apply[A]))
}

[sjrd]

If it's a 2.11.9 regression, it's most certainly caused by scala/scala#5730. I have the impression that that PR was a very risky move for a last minor version of 2.11 ...

[adriaanm]

Thanks for the report and the detailed analysis! Since this is also broken in 2.12.1, it may not be scala/scala#5730, since those changes aren't in there

[adriaanm]

I'll report back when I figure out what's common between 2.11.9 and 2.12.1 that's causing this.

[adriaanm]

Now that we'll have a 2.11.10, I'm working on diagnosing this today before we cut 2.11.10.

[adriaanm]

I confirmed this compiles on 2.11.9 with scala/scala#5730 reverted. Looking into what I can do to support this use case. Still suspicious that it also fails on 2.12.1.

Minimal repro:

object Foo {
  // spurious error if:
  //   - this definition precedes that of apply (which is overloaded with the synthetic one derived from the case class)
  //   - AND `Foo.apply` is explicitly applied to `[A]` (no error if `[A]` is inferred)
  //
  def referToOverloadedApply[A]: Foo[A] = Foo.apply[A]("bla")
  //                                                   ^
  //  found   : String("bla")
  //  required: Int

  def apply[A](x: Int): Foo[A] = ???
}
case class Foo[A](x: String) // must be polymorphic

[ashawley]

It looks like it worked fine in the milestones, just not the release candidates:

scala	scalacheck	compiles
2.12.0-M1	1.12.4	✅
2.12.0-M5	1.13.2	✅
2.12.0-RC1	1.13.2	❌
2.12.0-RC2	1.13.2	❌

$ sbt ++2.12.0-M1 compile
[success] Total time: 100 s, completed Apr 4, 2017 3:21:31 PM
$ sbt ++2.12.0-M5 compile
[success] Total time: 165 s, completed Apr 4, 2017 3:34:24 PM
$ sbt ++2.12.0-RC1 compile
[error] apply-overload-2.11.9/Test.scala:10: type mismatch;
[error]  found   : (Int, A) => example.Foo[A]
[error]  required: (Option[Int], A) => example.Foo[A]
[error]     Arbitrary(Gen.resultOf[Option[Int], A, Foo[A]](Foo.apply[A]))
[error]                                                             ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 46 s, completed Apr 4, 2017 3:47:30 PM
$ sbt ++2.12.0-RC2 compile
[error] apply-overload-2.11.9/Test.scala:10: type mismatch;
[error]  found   : (Int, A) => example.Foo[A]
[error]  required: (Option[Int], A) => example.Foo[A]
[error]     Arbitrary(Gen.resultOf[Option[Int], A, Foo[A]](Foo.apply[A]))
[error]                                                             ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 279 s, completed Apr 4, 2017 3:53:25 PM

Dunno if that helps, or not.

[adriaanm]

It does, thank you!

[adriaanm]

(It's probably the fields refactoring, but will have to look in detail to be sure.)