How does <:< work?

Question

scala> implicitly[Int <:< AnyVal]
res0: <:<[Int,AnyVal] = <function1>

scala> class Foo
defined class Foo

scala> class Bar extends Foo
defined class Bar

scala> implicitly[Foo <:< Bar]
<console>:8: error: could not find implicit value for parameter e: <:<[Foo,Bar]
       implicitly[Foo <:< Bar]
                 ^

scala> implicitly[Bar <:< Foo]
res2: <:<[Bar,Foo] = <function1>

How does <:< constraint work? Or more precisely, where is the implicit definition that supplies the instances of <:<?

Answer 1

You can find it in Predef. Implicit method conforms[A] provides these evidences:

implicit def conforms[A]: A <:< A = new (A <:< A) { def apply(x: A) = x }

You can actually try to implement it yourself in order to make it more clear:

abstract class subclassOf[-From, +To] extends (From => To)
implicit def subclassOfCheck[A]: A subclassOf A = new (A subclassOf A) { def apply(x: A) = x }

implicitly[Int subclassOf AnyVal]

class Foo
class Bar extends Foo

implicitly[Bar subclassOf Foo]