Imported From: https://issues.scala-lang.org/browse/SI-5285?orig=1
Reporter: Marc Millstone (splittingfield)
Affected Versions: 2.9.1
See #5267

Wouldn't think it is necessary to specialize for Boolean. Boolean only has two values true and false. When it is "boxed", it is actually referencing two pre-defined java.lang.Boolean objects, which does not impact performance at all. Can definitely close this one.

agree

  val q: Boolean => Boolean = b => !b
  def h(x: Boolean) = q(x)

Am I supposed to expect boxing? The same with Int param does not box.

Here, h boxes before calling the lambda, which unboxes before calling my static !_.

You can mix in a specialized trait:

  trait P { def apply(b: Boolean): Boolean }
  val p: (Boolean => Boolean) with P = new Function1[Boolean, Boolean] with P {
    override def apply(b: Boolean) = !b
  }

Boxed apply unboxes and calls apply(Z) as you would hope. Of course h would use unboxed P#apply.

I recently

  def f(xs: List[Int]): Option[Int] =
    xs.collectFirst { case i: Int if i > 42 => i > 50 }
      .map {
        if (_) 1
        else  -1
      }

I see that this form is actually worse, as you don't get to handle the boxed value; it always unboxes, so it reboxes for the eq!

  def ffs(xs: List[Int]): Option[Int] =
    xs.collectFirst { case i: Int if i > 42 => i > 50 }
      .map(b => if (b eq java.lang.Boolean.TRUE) 1 else -1)

Presumably, only by taking Any (and casting to AnyRef) can you wind up doing the obvious:

def testit[A >: Boolean <: Any](v: A): Int  = if (v.asInstanceOf[AnyRef] eq java.lang.Boolean.TRUE) 1 else -1