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Regression: cannot pattern match on type T[_] when T's type parameter has bounds #7716

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scabug opened this issue Aug 4, 2013 · 5 comments
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@scabug
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@scabug scabug commented Aug 4, 2013

Upon entering this code into the REPL:

trait TA[X <: CharSequence]
val ta: TA[_] = new TA[String] {}

ta match {
case _ => "hi"
}

I get this error:

error: type arguments [_$1] do not conform to trait TA's type parameter bounds [X <: CharSequence]
              ta match {
              ^

The code compiles on Scala 2.9.2, but fails on at least 2.10.0 and 2.10.2. Thus it appears to be a regression.

@scabug
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@scabug scabug commented Aug 4, 2013

Imported From: https://issues.scala-lang.org/browse/SI-7716?orig=1
Reporter: Alex Hvostov (argv)
Affected Versions: 2.10.0, 2.10.2
See #7694

@scabug
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@scabug scabug commented Aug 4, 2013

@paulp said:
Some workarounds:

  • compile with -Xoldpatmat
  • val ta: TA[_ <: CharSequence] = new TA[String] {}
  • val ta = new TA[String] {}

In some similar scenarios this already works again in 2.11, e.g.

def f(ta: TA[_]) = ta match { case _ => "hi" }

But the val example isn't compiling even now.

@scabug
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@scabug scabug commented Aug 5, 2013

Alex Hvostov (argv) said:
Sorry about the overstated priority, by the way. JIRA wasn't letting me adjust it from "blocker" for some reason.

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@scabug scabug commented Aug 16, 2013

@retronym said:
Another example from the mailing list:

scala> val e: java.lang.Enum[_] = java.util.concurrent.TimeUnit.SECONDS

scala> e match { case _ => 0 }
<console>:9: error: type arguments [_$1] do not conform to class Enum's type parameter bounds [E <: Enum[E]]
              e match { case _ => 0 }
              ^
[[syntax trees at end of                 refchecks]] // <console>
package $line4 {
  object $read extends scala.AnyRef {
    def <init>(): $line4.$read.type = {
      $read.super.<init>();
      ()
    };
    object $iw extends scala.AnyRef {
      def <init>(): type = {
        $iw.super.<init>();
        ()
      };
      object $iw extends scala.AnyRef {
        def <init>(): type = {
          $iw.super.<init>();
          ()
        };
        private[this] val res0: Int = {
          case <synthetic> val x1: Enum[_$1] = $line3.$read.$iw.$iw.e;
          case4(){
            matchEnd3(0)
          };
          matchEnd3(x: Int){
            x
          }
        };
        <stable> <accessor> def res0: Int = $iw.this.res0
      }
    }
  }
}

This is somewhat related to #7694; the pattern matcher translation now happens before refchecks, and the TypeTree in the tpt of the synthetic temporary x1 is subject to bounds checking.

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@scabug scabug commented Aug 16, 2013

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