SI-6806 Add an @implicitAmbiguous annotation

Example usage:

    trait =!=[C, D]

    implicit def neq[E, F] : E =!= F = null

    @annotation.implicitAmbiguous("Could not prove ${J} =!= ${J}")
    implicit def neqAmbig1[G, H, J] : J =!= J = null
    implicit def neqAmbig2[I] : I =!= I = null

    implicitly[Int =!= Int]

Which gives the following error:

    implicit-ambiguous.scala:9: error: Could not prove Int =!= Int
      implicitly[Int =!= Int]
                ^

Better than what was previously given:

    implicit-ambiguous.scala:9: error: ambiguous implicit values:
     both method neqAmbig1 in object Test of type [G, H, J]=> Main.$anon.Test.=!=[J,J]
     and method neqAmbig2 in object Test of type [I]=> Main.$anon.Test.=!=[I,I]
     match expected type Main.$anon.Test.=!=[Int,Int]
      implicitly[Int =!= Int]
                ^

1 files changed, 34 insertions, 0 deletions

diff --git a/src/library/scala/annotation/implicitAmbiguous.scala b/src/library/scala/annotation/implicitAmbiguous.scala
new file mode 100644
index 0000000000..46eab9ae8f
--- /dev/null
+++ b/src/library/scala/annotation/implicitAmbiguous.scala
@@ -0,0 +1,34 @@
+package scala.annotation
+
+import scala.annotation.meta._
+
+/**
+  * To customize the error message that's emitted when an implicit of type
+  * C[T1,..., TN] is found more than once, annotate the class C
+  * with @implicitAmbiguous. 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. *
+  *
+  * If more than one @implicitAmbiguous annotation is collected, the compiler is
+  * free to pick any of them to display.
+  *
+  * Nice errors can direct users to fix imports or even tell them why code
+  * intentionally doesn't compile.
+  *
+  * {{{
+  * trait =!=[C, D]
+  *
+  * implicit def neq[E, F] : E =!= F = null
+  *
+  * @annotation.implicitAmbiguous("Could not prove ${J} =!= ${J}")
+  * implicit def neqAmbig1[G, H, J] : J =!= J = null
+  * implicit def neqAmbig2[I] : I =!= I = null
+  *
+  * implicitly[Int =!= Int]
+  * }}}
+  *
+  * @author Brian McKenna
+  * @since 2.12.0
+  */
+final class implicitAmbiguous(msg: String) extends scala.annotation.StaticAnnotation {}