"Sound" minimal Gradual Typing in Scala through Implicits...

Gradual.scala

import scala.language.implicitConversions

class Unknown (value : Any) {
        def coercion[ Type ] : Type =
                value.asInstanceOf[ Type ]
}

implicit def promotion[ Type ] (value : Type) : Unknown =
        new Unknown (value)

implicit def downcast[ Type ] (value : Unknown) : Type =
        value.coercion[ Type ]

// end

shell-test.txt

λ(bash). scala -feature -i Gradual.scala
Loading Gradual.scala...
import scala.language.implicitConversions
defined class Unknown
promotion: [Type](value: Type)Unknown
downcast: [Type](value: Unknown)Type

Welcome to Scala 2.11.8 (Java HotSpot(TM) Server VM, Java 1.6.0_30).
Type in expressions for evaluation. Or try :help.

scala> val x : Unknown = 5
x: Unknown = Unknown@1a2d3be

scala> val y : Int = x // OK, Unknown ~~ Int
y: Int = 5

scala> val z : String = x // OK, Unknown ~~ String
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
  ... 32 elided

scala> val a : String = y // OK, (String ~/~ Int)
<console>:17: error: type mismatch;
 found   : Int
 required: String
       val a : String = y
                        ^

scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def cast[ Type ] (implicit value : Any) : Type = value.asInstanceOf[ Type ]
cast: [Type](implicit value: Any)Type

scala> val b : Any = 7 // OK, Int <: Any
b: Any = 7

scala> val c : Int = b // OK, implicit cast : Any => Type where Type := Int
c: Int = 7

scala> val d : String = b // OK, implicit cast : Any => Type where Type := Int
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
  ... 32 elided

scala> val e : String = c // NOT OK, implicit cast : (Int <: Any) => Type where Type := String, so Int => String
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
  ... 32 elided

scala>