evidence-for-absence-of-implicit.scala

// CONTEXT:  https://groups.google.com/forum/#!topic/scala-user/8YpX1VkIkDs

import scala.annotation.implicitNotFound
import scala.language.experimental.macros
import scala.reflect.macros.whitebox

/**
  * Macro-driven type for proving that an implicit parameter does not exist
  * in scope.
  */
@implicitNotFound("Cannot prove that Not[${T}] because an implicit for ${T} exists in scope")
sealed trait Not[T]

object Not {
  /**
    * Implicit instance for [[Not]], auto-generated via a macro.
    */
  implicit def proof[T]: Not[T] = macro proofMacro[T]

  def proofMacro[T : c.WeakTypeTag](c: whitebox.Context): c.Tree = {
    import c.universe._
    if (c.inferImplicitValue(weakTypeOf[T], silent=true) == EmptyTree)
      reify(new Not[T] {}).tree
    else
      c.abort(c.macroApplication.pos, "Found an implicit, so can't find Not[T]")
  }
}

// Example

scala> trait Foo 
scala> trait Bar

scala> def something[T <: Foo](value: T)(implicit ev: Not[T <:< Bar]) = println("it works!")
//=> something: [T <: Foo](value: T)(implicit ev: experiments.NotMacro.Not[<:<[T,Bar]])Unit

scala> something(new Foo {})
//=> it works!

scala> something(new Foo with Bar {})
//=> <console>:12: error: cannot prove that Not[<:<[Foo with Bar,Bar]] because an implicit for <:<[Foo with Bar,Bar] exists in scope
//=>               something(new Foo with Bar {})
//=>                         ^