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]
                ^

3 files changed, 7 insertions, 4 deletions

diff --git a/src/reflect/scala/reflect/internal/Definitions.scala b/src/reflect/scala/reflect/internal/Definitions.scala
index 02fa3c882b..231b6a8a66 100644
--- a/src/reflect/scala/reflect/internal/Definitions.scala
+++ b/src/reflect/scala/reflect/internal/Definitions.scala
@@ -1103,6 +1103,7 @@ trait Definitions extends api.StandardDefinitions {
     lazy val BridgeClass                = requiredClass[scala.annotation.bridge]
     lazy val ElidableMethodClass        = requiredClass[scala.annotation.elidable]
     lazy val ImplicitNotFoundClass      = requiredClass[scala.annotation.implicitNotFound]
+    lazy val ImplicitAmbiguousClass     = getClassIfDefined("scala.annotation.implicitAmbiguous")
     lazy val MigrationAnnotationClass   = requiredClass[scala.annotation.migration]
     lazy val ScalaStrictFPAttr          = requiredClass[scala.annotation.strictfp]
     lazy val SwitchClass                = requiredClass[scala.annotation.switch]
diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index 8a52f0b9d8..1113da2eff 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -884,10 +884,11 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     // string.  So this needs attention.  For now the fact that migration is
     // private[scala] ought to provide enough protection.
     def hasMigrationAnnotation = hasAnnotation(MigrationAnnotationClass)
-    def migrationMessage    = getAnnotation(MigrationAnnotationClass) flatMap { _.stringArg(0) }
-    def migrationVersion    = getAnnotation(MigrationAnnotationClass) flatMap { _.stringArg(1) }
-    def elisionLevel        = getAnnotation(ElidableMethodClass) flatMap { _.intArg(0) }
-    def implicitNotFoundMsg = getAnnotation(ImplicitNotFoundClass) flatMap { _.stringArg(0) }
+    def migrationMessage     = getAnnotation(MigrationAnnotationClass) flatMap { _.stringArg(0) }
+    def migrationVersion     = getAnnotation(MigrationAnnotationClass) flatMap { _.stringArg(1) }
+    def elisionLevel         = getAnnotation(ElidableMethodClass) flatMap { _.intArg(0) }
+    def implicitNotFoundMsg  = getAnnotation(ImplicitNotFoundClass) flatMap { _.stringArg(0) }
+    def implicitAmbiguousMsg = getAnnotation(ImplicitAmbiguousClass) flatMap { _.stringArg(0) }
 
     def isCompileTimeOnly       = hasAnnotation(CompileTimeOnlyAttr)
     def compileTimeOnlyMessage  = getAnnotation(CompileTimeOnlyAttr) flatMap (_ stringArg 0)
diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
index ea213cadd9..a2232d1963 100644
--- a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
+++ b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -370,6 +370,7 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
     definitions.BridgeClass
     definitions.ElidableMethodClass
     definitions.ImplicitNotFoundClass
+    definitions.ImplicitAmbiguousClass
     definitions.MigrationAnnotationClass
     definitions.ScalaStrictFPAttr
     definitions.SwitchClass