ambiguous2.scala

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scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Foo[A]
object Foo {
  @annotation.implicitAmbiguous("Custom ambiguous message")
  implicit def a = new Foo[String] { }
  implicit def b = new Foo[String] { }
  implicit def c = new Foo[String] { }
}

trait Bar[A]
object Bar {
  implicit def a = new Bar[String] { }
  implicit def b = new Bar[String] { }
  @annotation.implicitAmbiguous("Custom ambiguous message")
  implicit def c = new Bar[String] { }
}

// Exiting paste mode, now interpreting.

defined trait Foo
defined object Foo
defined trait Bar
defined object Bar

scala> implicitly[Foo[String]]
<console>:14: error: Custom ambiguous message
       implicitly[Foo[String]]
                 ^

scala> implicitly[Bar[String]]
<console>:14: error: ambiguous implicit values:
 both method a in object Bar of type => Bar[String]
 and method b in object Bar of type => Bar[String]
 match expected type Bar[String]
       implicitly[Bar[String]]
                 ^