Ignore empty aggregates elements when binding aggregate direction (#946)

Previously, including an empty aggregate in a Bundle would cause
a MixedDirectionAggregateException because it has no elements and thus
doesn't have a direction

* Add SampleElementBinding for Vec sample elements
* Add ActualDirection.Empty for bound empty aggregates

1 files changed, 1 insertions, 5 deletions

diff --git a/chiselFrontend/src/main/scala/chisel3/core/Bits.scala b/chiselFrontend/src/main/scala/chisel3/core/Bits.scala
index 7270d92b..43a851ed 100644
--- a/chiselFrontend/src/main/scala/chisel3/core/Bits.scala
+++ b/chiselFrontend/src/main/scala/chisel3/core/Bits.scala
@@ -27,11 +27,7 @@ abstract class Element extends Data {
   private[chisel3] override def bind(target: Binding, parentDirection: SpecifiedDirection) {
     binding = target
     val resolvedDirection = SpecifiedDirection.fromParent(parentDirection, specifiedDirection)
-    direction = resolvedDirection match {
-      case SpecifiedDirection.Unspecified | SpecifiedDirection.Flip => ActualDirection.Unspecified
-      case SpecifiedDirection.Output => ActualDirection.Output
-      case SpecifiedDirection.Input => ActualDirection.Input
-    }
+    direction = ActualDirection.fromSpecified(resolvedDirection)
   }
 
   private[core] override def topBindingOpt: Option[TopBinding] = super.topBindingOpt match {