ambiguous1.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 val a = new Foo[String] { }
  implicit val b = new Foo[String] { }
  implicit val c = new Foo[String] { }
}

// Exiting paste mode, now interpreting.

defined trait Foo
defined object Foo

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