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SethTisue opened this issue Jan 29, 2018 · 4 comments · Fixed by scala/scala#6420
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2.13 regression in @tailrec + singleton types #467

SethTisue opened this issue Jan 29, 2018 · 4 comments · Fixed by scala/scala#6420
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@milessabin
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2.13.0-M4

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@SethTisue
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at e.g. https://scala-ci.typesafe.com/view/scala-2.13.x/job/scala-2.13.x-integrate-community-build/919/consoleFull we see:

[akka-actor] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/akka-actor-75f78e641cd66395240926f2b01f241f7dee1dce/akka-actor/src/main/scala/akka/util/ByteIterator.scala:277:33: could not optimize @tailrec annotated method dropWhile: it has a singleton result type which cannot be erased correctly
[akka-actor] [error]     @tailrec final override def dropWhile(p: Byte ⇒ Boolean): this.type =
[akka-actor] [error]                                 ^
[akka-actor] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/akka-actor-75f78e641cd66395240926f2b01f241f7dee1dce/akka-actor/src/main/scala/akka/util/ByteIterator.scala:312:34: could not optimize @tailrec annotated method getToArray: it has a singleton result type which cannot be erased correctly
[akka-actor] [error]     @tailrec protected final def getToArray[A](xs: Array[A], offset: Int, n: Int, elemSize: Int)(getSingle: ⇒ A)(getMult: (Array[A], Int, Int) ⇒ Unit): this.type =
[akka-actor] [error]                                  ^
[akka-actor] [warn] 109 warnings found
[akka-actor] [error] two errors found

@milessabin since it involves singleton types, I'm tentatively assigning this to you for investigation.
@SethTisue SethTisue mentioned this issue Jan 29, 2018
Closed
SethTisue added a commit to scalacommunitybuild/akka that referenced this issue Jan 29, 2018
@SethTisue
remove some @tailrec annotations for 2.13 community build
938698b 
see scala/scala-dev#467
@2m 2m mentioned this issue Feb 19, 2018
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@jrudolph
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jrudolph commented Mar 5, 2018

The change here is not just a nuisance but a regression since, as the error message says, tail call optimization isn't done any more.

@milessabin
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Agreed. I'm on it.

@jrudolph
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jrudolph commented Mar 5, 2018

Cool, thanks Miles.

@SethTisue SethTisue added this to the 2.13.0-M4 milestone Mar 5, 2018
@2m 2m mentioned this issue Mar 7, 2018
Closed
2m added a commit to akka/akka that referenced this issue Mar 7, 2018
@2m
#24507 Remove tailrec annotation
494426a 
Until scala/scala-dev#467 is fixed and
released
jrudolph added a commit to jrudolph/akka-http that referenced this issue Mar 8, 2018
@jrudolph
Remove @tailrec annotations where 2.13.0-M3 doesn't allow it
5450370 
This is due to scala/scala-dev#467.

This might lead to stack overflows, so don't merge but wait for a
fix in the next version.
jrudolph added a commit to jrudolph/akka-http that referenced this issue Mar 8, 2018
@jrudolph
Remove @tailrec annotations where 2.13.0-M3 doesn't allow it
2afaa59 
This is due to scala/scala-dev#467.

This might lead to stack overflows, so don't merge but wait for a
fix in the next version.
milessabin added a commit to milessabin/scala that referenced this issue Mar 13, 2018
@milessabin
Singleton typed args/results don't interfere with tailcalls
08bcf83 
At one point the literal types PR modified the semantics of asInstanceOf
to check for equality of the target value with the value embedded in the
singleton type. This interacted badly with mkAttributedCastHack in
Tailcalls,

  dotta@c2534bf

because the inserted casts could end up referring to a non-existent
Symbol which would be needed to implement the equality check during
Erasure.

The fix at the time was to make the use of a singleton type as a method
argument or result type disqualify the method from having the tailcall
optimization applied. A later decision by the SIP committee decided for
unrelated reasons that an asInstanceOf test should not perform an
equality check, so this disqualification became unnecessary. It was not
removed, however.

This came to light because singleton types are used as method argument
and result types in methods which are expected to have the tail call
optimizatino applied, resulting in,

  scala/scala-dev#467

We fix that here by doing what should have been done earlier: removing
the disqualification.
@milessabin milessabin mentioned this issue Mar 13, 2018
Merged
@milessabin
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Fixed in scala/scala#6420.

@marcospereira marcospereira mentioned this issue Mar 14, 2018
Merged
adriaanm pushed a commit to scala/scala that referenced this issue Apr 11, 2018
@milessabin @adriaanm
Singleton typed args/results don't interfere with tailcalls (#6420)
77db1d2 
Singleton typed args/results don't interfere with tailcalls

At one point the literal types PR modified the semantics of asInstanceOf
to check for equality of the target value with the value embedded in the
singleton type. This interacted badly with mkAttributedCastHack in
Tailcalls,

  dotta@c2534bf

because the inserted casts could end up referring to a non-existent
Symbol which would be needed to implement the equality check during
Erasure.

The fix at the time was to make the use of a singleton type as a method
argument or result type disqualify the method from having the tailcall
optimization applied. A later decision by the SIP committee decided for
unrelated reasons that an asInstanceOf test should not perform an
equality check, so this disqualification became unnecessary. It was not
removed, however.

This came to light because singleton types are used as method argument
and result types in methods which are expected to have the tail call
optimizatino applied, resulting in,

  scala/scala-dev#467

We fix that here by doing what should have been done earlier: removing
the disqualification.
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Singleton typed args/results don't interfere with tailcalls milessabin/scala
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@jrudolph @milessabin @SethTisue