MixedVec: clarify dynamic indexing of heterogeneous elements

1 files changed, 7 insertions, 4 deletions

diff --git a/src/main/scala/chisel3/util/MixedVec.scala b/src/main/scala/chisel3/util/MixedVec.scala
index e09cdc4a..eeac76ea 100644
--- a/src/main/scala/chisel3/util/MixedVec.scala
+++ b/src/main/scala/chisel3/util/MixedVec.scala
@@ -77,18 +77,21 @@ final class MixedVec[T <: Data](private val eltsIn: Seq[T]) extends Record with
 
   /**
     * Dynamically (via a mux) retrieve the element at the given index.
+    * This is implemented via a mux with the width of the widest element in this MixedVec.
+    * For example, a MixedVec of type Seq(UInt(4.W), UInt(8.W)) will create an 8-bit mux for this operation.
+    * Note: it is up to the user to process the resultant UInt (e.g. unflatten, etc).
     *
-    * @param index Index with which to retrieve.
-    * @return Retrieved index.
+    * @param index Index to retrieve. If the index is out of range, it will return the first element.
+    * @return Retrieved index as a UInt with the width of the widest element.
     */
-  def apply(index: UInt): T = {
+  def apply(index: UInt): UInt = {
     requireIsHardware(index, "index must be hardware")
 
     if (length < 1) {
       throw new IndexOutOfBoundsException("Collection is empty")
     }
 
-    MuxLookup(index, elts.head, elts.zipWithIndex.map { case (el, el_index) => el_index.U -> el })
+    MuxLookup(index, elts.head.asUInt, elts.zipWithIndex.map { case (el, el_index) => el_index.U -> el.asUInt })
   }
 
   /** Strong bulk connect, assigning elements in this MixedVec from elements in a Seq.