Why @specialized annotation doesn't work for asInstanceOf?

Question

As far as i understand, @specialized annotation should generate some unboxed code for every primitive type i mentioned, but this doesn't work:

scala> def aaa[@specialized(Int, Double, Float, Long) T] = (5.0).doubleValue.asInstanceOf[T]
aaa: [T]=> T

scala> aaa[Int]
unrecoverable error (inside interpreter/compiler)

This compiles:

scala> def aaa[@specialized(Int, Double, Float, Long) T](a: T) = (5.0).doubleValue.asInstanceOf[T]
aaa: [T]=> T

scala> aaa[Int](0)
ClassCastException

But it still uses boxed type for asInstanceOf[T]. This obviously works:

scala> (5.0).asInstanceOf[Int]
res28: Int = 5

UPDATE: Type erasure and answers like that Writing a generic cast function Scala has nothing to do with my problem. Type erasure just preventing compiler from adding typecast byte-code operation for generics, but eventually it will be added - see ClassCastException (generated by this op) in my REPL

Answer 1

The problem was in Scala REPL - @specialized doesn't work there. Compiling def aaa[@specialized(Int) T] = (5.0).asInstanceOf[T] with scalac gives:

  public <T> T aaa();
    Code:
       0: ldc2_w        #15                 // double 5.0d
       3: invokestatic  #22                 // Method scala/runtime/BoxesRunTime
.boxToDouble:(D)Ljava/lang/Double;
       6: areturn

  public int aaa$mIc$sp();
    Code:
       0: ldc2_w        #15                 // double 5.0d
       3: d2i
       4: ireturn

d2i is exactly what i was expecting. And of course everything works fine with scalac (so I don't need pattern matching for every possible type). So it's just the issue with interpreter.

Answer 2

The method is specialized, but under separate compilation (i.e., different lines), the specialized method isn't invoked.

In the following, b.B.f works, c.B.f is broken.

$ scala -Xprint:typer,cleanup

scala> :pa -raw
// Entering paste mode (ctrl-D to finish)

package a { object A { def aaa[@specialized(Int) T] = (5.0).doubleValue.asInstanceOf[T] }}
package b { object B { def f = a.A.aaa[Int] }}

// Exiting paste mode, now interpreting.

[[syntax trees at end of                     typer]] // <pastie>
package <empty> {
  package a {
    object A extends scala.AnyRef {
      def <init>(): a.A.type = {
        A.super.<init>();
        ()
      };
      def aaa[@specialized(scala.Int) T]: T = scala.this.Predef.double2Double(5.0).doubleValue().asInstanceOf[T]
    }
  };
  package b {
    object B extends scala.AnyRef {
      def <init>(): b.B.type = {
        B.super.<init>();
        ()
      };
      def f: Int = a.A.aaa[Int]
    }
  }
}

[[syntax trees at end of                   cleanup]] // <pastie>
package <empty> {
  package a {
    object A extends Object {
      def aaa(): Object = scala.Double.box(scala.this.Predef.double2Double(5.0).doubleValue());
      <specialized> def aaa$mIc$sp(): Int = scala.this.Predef.double2Double(5.0).doubleValue().toInt();
      def <init>(): a.A.type = {
        A.super.<init>();
        ()
      }
    }
  };
  package b {
    object B extends Object {
      def f(): Int = a.A.aaa$mIc$sp();
      def <init>(): b.B.type = {
        B.super.<init>();
        ()
      }
    }
  }
}


scala> :pa -raw
// Entering paste mode (ctrl-D to finish)

package c { object B { def f = a.A.aaa[Int] }}

// Exiting paste mode, now interpreting.

[[syntax trees at end of                     typer]] // <pastie>
package c {
  object B extends scala.AnyRef {
    def <init>(): c.B.type = {
      B.super.<init>();
      ()
    };
    def f: Int = a.A.aaa[Int]
  }
}

[[syntax trees at end of                   cleanup]] // <pastie>
package c {
  object B extends Object {
    def f(): Int = scala.Int.unbox(a.A.aaa());
    def <init>(): c.B.type = {
      B.super.<init>();
      ()
    }
  }
}

Answer 3

Perhaps you just need a more recent compiler? here's what I get with 2.11.2:

Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_72).
Type in expressions to have them evaluated.
Type :help for more information.

scala> def aaa[@specialized(Int, Double, Float, Long) T] = (5.0).doubleValue.asInstanceOf[T]
aaa: [T]=> T

scala> aaa[Int]
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
  at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:105)
  ... 33 elided

scala> aaa[Float]
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Float
  at scala.runtime.BoxesRunTime.unboxToFloat(BoxesRunTime.java:113)
  ... 33 elided

scala> aaa[Double]
res2: Double = 5.0

scala> 5.0.asInstanceOf[Int]
res3: Int = 5

scala> 5.0.asInstanceOf[Integer]
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
  ... 33 elided

Note that you can directly do 5.0.asInstanceOf[Int] but not 5.0.asInstanceOf[Integer]. Also note that the ClassCastException that you get with aaa[Int] is referring to java.lang.Integer not Scala's Int class. I suspect what goes on here is that the @specialized annotation is generating a function, aaa, that has an "Int" specialization but because of type erasure and boxing is converting that to java.lang.Integer and there is no automatic conversion with asInstanceOf between Double and Integer.