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type conformance (or inference) difference between 2.9.2 and 2.10 #5777

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scabug opened this issue May 9, 2012 · 7 comments
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type conformance (or inference) difference between 2.9.2 and 2.10 #5777

scabug opened this issue May 9, 2012 · 7 comments
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@scabug
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scabug commented May 9, 2012

I happened across this while poking at #5340.

trait Ring {
  trait E
}

class Poly[C <: Ring](val ring: C) extends Ring
// This definition of Poly triggers the same failure on *both* versions
// class Poly(val ring: Ring) extends Ring

object BigInt extends Ring

object MyApp {
  val r = new Poly(BigInt)

  // fail on 2.10, works on 2.9.2
  (null.asInstanceOf[BigInt.E] : r.ring.E)

  // works on both versions
  val r1 = new Poly[BigInt.type](BigInt)
  (null.asInstanceOf[BigInt.E] : r1.ring.E)

  // Oddly, -Xprint:typer reports that r and r1 have the same inferred type.
  //
  // private[this] val r: Poly[BigInt.type] = new Poly[BigInt.type](BigInt);
  // <stable> <accessor> def r: Poly[BigInt.type] = MyApp.this.r;
  // (null.asInstanceOf[BigInt.E]: MyApp.r.ring.E);
  // private[this] val r1: Poly[BigInt.type] = new Poly[BigInt.type](BigInt);
  // <stable> <accessor> def r1: Poly[BigInt.type] = MyApp.this.r1;
  // (null.asInstanceOf[BigInt.E]: MyApp.r1.ring.E)

  // diff typer-2.9.2.txt typer-2.10.txt
  // ...
  // ---
  // >   object MyApp extends scala.AnyRef {
  // >     def <init>(): MyApp.type = {
  // >       MyApp.super.<init>();
  // 30c30
  // <     scala.this.Predef.implicitly[<:<[BigInt.E,MyApp.r.ring.E]](scala.this.Predef.conforms[BigInt.E]);
  // ---
  // >     scala.this.Predef.implicitly[<:<[BigInt.E,MyApp.r.ring.E]]();
}
@scabug
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scabug commented May 9, 2012

Imported From: https://issues.scala-lang.org/browse/SI-5777?orig=1
Reporter: @retronym

@scabug
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scabug commented May 9, 2012

@paulp said:
Changed between M2 and M3. I strongly suspect virtpatmat.

@scabug
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scabug commented May 9, 2012

@paulp said:
Oh, this is only five weeks ago? Oops, I strongly suspect this then: a7f68ce32c

@scabug
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scabug commented May 9, 2012

@paulp said:
Right, I removed the code which special cased inference for module classes so that "BigInt.type" would be inferred, because in theory it was no longer necessary as subtyping itself now captured the essence. But it seems there was a gap.

@scabug
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scabug commented May 9, 2012

@paulp said:
I got it, thanks for finding.

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scabug commented May 9, 2012

@paulp said:
e8e88f785d

@scabug scabug closed this as completed May 9, 2012
@scabug
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scabug commented May 9, 2012

@retronym said:
Just glad that my proclivity for stumbling over things can be put to good use.

@scabug scabug added this to the 2.10.0 milestone Apr 7, 2017
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