implicitNotFound

To customize the error message that's emitted when an implicit of type C[T1,..., TN] cannot be found, annotate the class C with @implicitNotFound. Assuming C has type parameters X1, ..., XN, the error message will be the result of replacing all occurrences of ${Xi} in the string msg with the string representation of the corresponding type argument Ti. The annotation is effectively inherited by subtypes if they are not annotated.

The annotation can also be attached to implicit parameters. In this case, ${Xi} can refer to type parameters in the current scope. The @implicitNotFound message on the parameter takes precedence over the one on the parameter's type.

import scala.annotation.implicitNotFound

@implicitNotFound("Could not find an implicit C[${T}, ${U}]")
class C[T, U]

class K[A] {
  def m[B](implicit c: C[List[A], B]) = 0
  def n[B](implicit @implicitNotFound("Specific message for C of list of ${A} and ${B}") c: C[List[A], B]) = 1
}

object Test {
  val k = new K[Int]
  k.m[String]
  k.n[String]
}

The compiler issues the following error messages:

Test.scala:13: error: Could not find an implicit C[List[Int], String]
 k.m[String]
    ^
Test.scala:14: error: Specific message for C of list of Int and String
 k.n[String]
    ^