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scalac sometimes ignores companion implicits #7291

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scabug opened this issue Mar 23, 2013 · 10 comments
Closed

scalac sometimes ignores companion implicits #7291

scabug opened this issue Mar 23, 2013 · 10 comments
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@scabug
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@scabug scabug commented Mar 23, 2013

trait Fooable[T]
object Fooable {
  implicit def conjure[T]: Fooable[T] = {
    println("conjure")
    new Fooable[T]{}
  }

}

object Main extends App {
  implicit def traversable[T, Coll[_] <: Traversable[_]](implicit
elem: Fooable[T]): Fooable[Coll[T]] = {
    println("traversable")
    new Fooable[Coll[T]]{}
  }
  implicitly[Fooable[List[Any]]] 
}

Should print "conjure", then "traversable". Currently it prints just "conjure". Details: http://groups.google.com/group/scala-internals/browse_thread/thread/f284df8ee19c2aba

@scabug
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@scabug scabug commented Mar 23, 2013

Imported From: https://issues.scala-lang.org/browse/SI-7291?orig=1
Reporter: @xeno-by
Affected Versions: 2.10.1, 2.11.0-M2

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@scabug
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@scabug scabug commented Mar 24, 2013

@xeno-by said (edited on Mar 24, 2013 10:41:52 AM UTC):
An interesting twist of events is that this bug is fixed in scalad, the type debugger. Now the question is how exactly this happened.

11:40 ~/Projects/Master/sandbox (master)$ scalad Test.scala -Xprint:typer
Initial compilation of sources...
[[syntax trees at end of                     typer]] // Test.scala
package <empty> {
  abstract trait Fooable[T >: Nothing <: Any] extends scala.AnyRef;
  object Fooable extends scala.AnyRef {
    def <init>(): Fooable.type = {
      Fooable.super.<init>();
      ()
    };
    implicit def conjure[T >: Nothing <: Any]: Fooable[T] = {
      scala.this.Predef.println("conjure");
      {
        final class $anon extends AnyRef with Fooable[T] {
          def <init>(): anonymous class $anon = {
            $anon.super.<init>();
            ()
          };
          <empty>
        };
        new $anon()
      }
    }
  };
  object Test extends AnyRef with App {
    def <init>(): Test.type = {
      Test.super.<init>();
      ()
    };
    implicit def traversable[T >: Nothing <: Any, Coll[_ >: Nothing <: Any] >: [_]Nothing <: [_]Traversable[_]](implicit elem: Fooable[T]): Fooable[Coll[T]] = {
      scala.this.Predef.println("traversable");
      {
        final class $anon extends AnyRef with Fooable[Coll[T]] {
          def <init>(): anonymous class $anon = {
            $anon.super.<init>();
            ()
          };
          <empty>
        };
        new $anon()
      }
    };
    scala.this.Predef.implicitly[Fooable[List[Any]]](Test.this.traversable[Any, List](Fooable.conjure[Any]))
  }
}

Initial compilation finished. Starting UI...

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@scabug
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@scabug scabug commented Mar 24, 2013

@xeno-by said:
Hubert has kindly provided a fix for this one. I will submit a pull request once we're done with the paper (somewhen in the beginning of April).

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@scabug
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@scabug scabug commented Apr 21, 2013

@xeno-by said:
I'll do the backporting next week.

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@scabug
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@scabug scabug commented Apr 22, 2013

@hubertp said:
I cleaned it up already yesterday (https://github.com/hubertp/scala-dev/tree/topic/minor-typer-cleanup). PR is coming today. Sorry Eugene ;)

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@scabug
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@scabug scabug commented Apr 22, 2013

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@scabug scabug commented May 5, 2013

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@scabug scabug commented May 11, 2013

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@scabug
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@scabug scabug commented Feb 5, 2016

Michael Khan (michael.khan) said (edited on Feb 5, 2016 11:10:06 AM UTC):
This issue has re-surfaced.

I have a case class with an associated companion class defined in a domains package (Play project) as follows:

case class Registration(serviceName: String, serviceUrl: String, metadata: Option[Map[String, String]] = None)

object Registration {
implicit val format = Json.format[Registration]
}

We have a custom HTTP library we have built for our organisation which allows us to make HTTP calls - it is publicly available here (https://github.com/hmrc/http-verbs). The following call is made:

http.POST(s"$serviceUrl/registration", registration, Seq("Content-Type" -> "application/json")) 

where the second parameter is of type Registration.

When compiling the compiler complains as follows:

[error] /foo/app/connectors/ServiceLocatorConnector.scala:40: diverging implicit expansion for type uk.gov.hmrc.play.http.HttpReads[P] [error] starting with method readFromJson in trait JsonHttpReads [error] http.POST(s"$serviceUrl/registration", registration, [error] ^ [error] one error found [error] (compile:compileIncremental) Compilation failed

When I explicitly import the companion object into the same translation unit where this http.POST method is called, the compiler is able to pick up the implicit and compilation is a success. If I don't, then it complains about diverging implicit expansion error.

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@som-snytt
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@som-snytt som-snytt commented May 6, 2020

This particular example has worked in the modern era, the PPP epoch, or post-paulp.

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@som-snytt som-snytt closed this Sep 1, 2020
@SethTisue SethTisue removed this from the Backlog milestone Sep 1, 2020
@SethTisue SethTisue added this to the 2.11.0 milestone Sep 1, 2020
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