Scala: Implicit evidence for class with type parameter

Question

Here is a simple setup with two traits, a class with a covariant type parameter bounded by the previous traits, and a second class with a type parameter bounded by the other cla

Answer 1

This is not an answer to the question, but to show that the 'type constraint' is really just an implicit conversion:

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_33).
Type in expressions to have them evaluated.
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scala> trait A { def test() {} }
defined trait A

scala> class WhatHappens[T] { def test(t: T)(implicit ev: T <:< A) = t.test() }
defined class WhatHappens

scala> :javap -v WhatHappens
...
public void test(java.lang.Object, scala.Predef$$less$colon$less);
  Code:
   Stack=2, Locals=3, Args_size=3
   0:   aload_2
   1:   aload_1
   2:   invokeinterface #12,  2; //InterfaceMethod scala/Function1.apply:(Ljava/lang/Object;)Ljava/lang/Object;
   7:   checkcast   #14; //class A
   10:  invokeinterface #17,  1; //InterfaceMethod A.test:()V
   15:  return
...
  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      16      0    this       LWhatHappens;
   0      16      1    t       Ljava/lang/Object;
   0      16      2    ev       Lscala/Predef$$less$colon$less;
...