make JsValueFormat low priority

src/main/scala/spray/json/AdditionalFormats.scala

@@ -20,12 +20,7 @@ package spray.json
 /**
   * Provides additional JsonFormats and helpers
  */
-trait AdditionalFormats {
-
-  implicit object JsValueFormat extends JsonFormat[JsValue] {
-    def write(value: JsValue) = value
-    def read(value: JsValue) = value
-  }
+trait AdditionalFormats extends AdditionalFormats.LowPriorityImplicits {
 
   implicit object RootJsObjectFormat extends RootJsonFormat[JsObject] {
     def write(value: JsObject) = value
@@ -114,4 +109,17 @@ trait AdditionalFormats {
     }
   }
 
-}
+}
+
+object AdditionalFormats {
+
+  trait LowPriorityImplicits {
+
+    implicit object JsValueFormat extends JsonFormat[JsValue] {
+      def write(value: JsValue) = value
+      def read(value: JsValue) = value
+    }
+
+  }
+
+}

src/test/scala/spray/json/AdditionalFormatsSpec.scala

@@ -70,4 +70,21 @@ class AdditionalFormatsSpec extends Specification {
         """{"id":1,"name":"a","foos":[{"id":2,"name":"b","foos":[{"id":3,"name":"c"}]},{"id":4,"name":"d"}]}""".parseJson
     }
   }
+
+  "JsValueFormat" should {
+    import DefaultJsonProtocol._
+
+    class Box[-T]
+    implicit def box[T : JsonFormat]: Box[T] = new Box
+
+    "not cause ambiguity when summoning Box[JsObject]" in {
+      implicitly[Box[JsObject]]
+      true mustEqual true // just checking that it compiles
+    }
+
+    "not cause ambiguity when summoning Box[JsArray]" in {
+      implicitly[Box[JsArray]]
+      true mustEqual true // just checking that it compiles
+    }
+  }
 }