def runtimeClass

The opening of SI-4696 reminded me I have this lying around. What do we think about it.

https://github.com/paulp/scala-dev/tree/runtime-class

Summarized via scaladoc:

/** This class exists to install the runtimeClass() method on any target.
* It improves on Object.getClass (as seen from scala) in the following ways:
*
* - it correctly returns classOf[Int], not classOf[Integer], if called on an Int.
* - it can be used on Any and AnyVal, whereas getClass only exists on AnyRef.
* - the return type is a more precise Class[_ <: T], where T is the static type of the target.
*
* The return type should be consistent with getClass as seen from java. According
* to the javadoc, "The actual result type [of getClass] is Class<? extends |X|> where
* |X| is the erasure of the static type of the expression on which getClass is called."
{{{
object RuntimeClassVsGetClass {
class A
class B extends A

// scala expressions, with the expression's type commented:
(new B).getClass().newInstance() // Any
(new B).runtimeClass().newInstance() // B
(new B: A).runtimeClass().newInstance() // A
5.getClass // doesn't compile
5.asInstanceOf[AnyRef].getClass // classOf[_ <: java.lang.Integer]
5.runtimeClass() // classOf[_ <: Int] (i.e. java.lang.Integer.TYPE or classOf[Int])
(5: AnyVal).runtimeClass() // classOf[_ <: java.lang.Integer] (AnyVal implies boxed)
(5: Any).runtimeClass() // classOf[_ <: java.lang.Integer] (as does Any)
}
}}}