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Pattern match fails on custom generic type #9648

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scabug opened this issue Feb 5, 2016 · 3 comments
Closed

Pattern match fails on custom generic type #9648

scabug opened this issue Feb 5, 2016 · 3 comments
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patmat reflection

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@scabug
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scabug commented Feb 5, 2016 •
edited by SethTisue

Following up this question on Stack Overflow:

Pattern matching a type using typeOf[T] with @unchecked gives incorrect result.

The code below

import scala.reflect.runtime.{universe => ru} 

class Foo[T](val data: T)

def matchFoo[A: ru.TypeTag](foo: Foo[A]) = {
  foo match {
    case fooStr: Foo[String @unchecked] if ru.typeOf[A] =:= ru.typeOf[String] => println("Found String")
    case _ => println("Found other")
  }
}

def main(args: Array[String]): Unit = {
  matchFoo(new Foo[Int](123))
}

expected result:

Found other

actual result:

Found String
@scabug
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scabug commented Feb 5, 2016

Imported From: https://issues.scala-lang.org/browse/SI-9648?orig=1
Reporter: Maciej Szopinski (maciej.sz)
Affected Versions: 2.11.5

@scabug scabug added this to the Backlog milestone Apr 7, 2017
@adriaanm adriaanm removed their assignment Sep 28, 2018
@SethTisue
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SethTisue commented Dec 23, 2023 •
edited

It's not clear to me that this is a bug. Matching against Foo[String @unchecked] tells the compiler "I don't need you to check that A is String — you may assume it is." And then the compiler proceeds on that assumption.

@som-snytt
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Apparently because invariant. class Foo[+A] is the expected behavior. (or case _: Foo[_])

@som-snytt som-snytt closed this as not planned Won't fix, can't repro, duplicate, stale Dec 23, 2023
@SethTisue SethTisue removed this from the Backlog milestone Dec 24, 2023
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@adriaanm @SethTisue @som-snytt @scabug