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Scala 2.12 fails to compile reflective calls working in 2.11 #10047

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scabug opened this issue Nov 12, 2016 · 4 comments
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Scala 2.12 fails to compile reflective calls working in 2.11 #10047

scabug opened this issue Nov 12, 2016 · 4 comments
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@scabug
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scabug commented Nov 12, 2016

Consider this minimal example:

import scala.language.reflectiveCalls

class A

trait MyTrait {
  def obj: A
}

object Main extends MyTrait {
  lazy val obj = new A {
    def x = 42
  }

  println(obj.x)
}

While this compiles correctly in Scala 2.11.8, I'm getting the following error in Scala 2.12.0:

[error] /scala-test/test.scala:14: value x is not a member of A
[error]   println(obj.x)

I read the breaking changes in Scala 2.12 release notes and I couldn't find anything related to this. I tried adding more fields and methods to MyTrait to be sure SAM conversions wouldn't apply, but the error persists.

Is this an intended breaking change or a bug? Is there an workaround?
@scabug
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scabug commented Nov 12, 2016

Imported From: https://issues.scala-lang.org/browse/SI-10047?orig=1
Reporter: @ruippeixotog
Affected Versions: 2.12.0

@scabug
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scabug commented Nov 12, 2016

@SethTisue said (edited on Nov 12, 2016 4:31:28 PM UTC):
"Concretely, when computing the type of an overriding field, the type of the overridden field is used as the expected type. As a result, the inferred type of a val or lazy val may change in Scala 2.12." http://scala-lang.org/news/2.12.0

In your example, the type of the lazy val is simply A. If you want the structural type, you can explicitly declare it:

lazy val obj: A { def x: Int } = ...

@scabug scabug closed this as completed Nov 12, 2016
@scabug
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scabug commented Nov 12, 2016

@ruippeixotog said:
Oh, you're right, I overlooked that part... Thank you and sorry for the trouble!

@scabug
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scabug commented Nov 12, 2016

@SethTisue said:
No worries!

@jackkoenig jackkoenig mentioned this issue Aug 17, 2017
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@ucbjrl ucbjrl mentioned this issue Aug 24, 2017
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