Imported From: https://issues.scala-lang.org/browse/SI-10206?orig=1
Reporter: Yuval Itzchakov (Yuval.Itzchakov)
Affected Versions: 2.11.8, 2.12.0, 2.12.1
See #6760

@adriaanm said:
This seems to be specific to the "clone" method. Could you use a different name as a workaround?

@adriaanm said (edited by @SethTisue on Feb 23, 2017 10:32:28 PM UTC):
Hah, nice one. This is because implicit conversions that target a superclass of AnyRef are ruled out:

        pt match {
          case Function1(_, out) =>
            // must inline to avoid capturing result
            def prohibit(sym: Symbol) = (sym.tpe <:< out) && { // BUG: when looking for implicit conversion to `?{def clone: ?}`, this triggers because AnyRef has that method
              maybeInvalidConversionError(s"the result type of an implicit conversion must be more specific than ${sym.name}")
              true
            }
            if (prohibit(AnyRefClass) || (settings.isScala211 && prohibit(AnyValClass)))
              result = SearchFailure
          case _                 => false
        }

@adriaanm said:
scala/scala#5736

Yuval Itzchakov (Yuval.Itzchakov) said (edited on Feb 24, 2017 8:42:41 AM UTC):
Thanks a lot for the quick fix Adriaan!

Would you mind elaborating how/why a structural type supertyping AnyRef is created here?

Jasper-M said (edited on Feb 24, 2017 9:44:52 AM UTC):
Consider this REPL transcript:

scala> class Foo { def apply(a: Int, b: Int) = (a,b) }
defined class Foo

scala> implicit class IntFoo(i: Int) { def foo = new Foo }
defined class IntFoo

scala> 1.foo(2,3)
res1: (Int, Int) = (2,3)

So when you call foo.clone(1, 2) an implicit conversion to structural type

?{ def clone: ? }

has to be considered. And that is a supertype of {{AnyRef}}.

Yuval Itzchakov (Yuval.Itzchakov) said (edited on Feb 24, 2017 10:42:57 AM UTC):
Jasper, perhaps I'm missing something here.

When you write

1.foo(2,3)

Why does an implicit conversion to a structural have to be considered? I would expect that an implicit conversion on Int to be looked up and searched for a method called clone, but I do not understand where structural comes in.

To add to the question, even if there is a structural type to has to be looked up, why is it a supertype of AnyRef and not a subtype?

Jasper-M said:
To be clear, I don't know the internals of the compiler. But it is my understanding that when I call 1.foo and the compiler sees that Int doesn't have a foo method, it will look for an implicit value that conforms to

Int => ?{def foo: ?}

because it doesn't know anything about the required result type other than that it should have a method called foo.
And in the clone case, AnyRef had a method def clone(): AnyRef and also a bunch of other methods, so that makes AnyRef a subtype of the structural type that contains only a method def clone: ?.

Now aside of that, I think the compiler uses some extra internal mechanisms other than pure basic userfacing implicit search, because

scala> implicitly[AnyRef <:< Any{def clone: Any}]
<console>:8: error: Cannot prove that AnyRef <:< Any{def clone: Any}.
              implicitly[AnyRef <:< Any{def clone: Any}]
                        ^

scala> implicitly[AnyRef <:< Any{def clone(): Any}]
res1: <:<[AnyRef,Any{def clone(): Any}] = <function1>

Yuval Itzchakov (Yuval.Itzchakov) said (edited on Feb 24, 2017 3:55:36 PM UTC):
@Jasper Thanks for that explanation, that makes sense.

Just to clear my mind, when we say "structural type", we're talking about some structure of a type and not explicitly about the Scala feature called structural types, right? We just need a way to describe a shape of a type we don't know yet.

Regarding your example in the code, what is that trying to convey? Not sure I follow. I would imagine the first one doesn't work is it doesn't match the structure of Object.clone.