Enable Cat of Zero Element Vec (#1623)

* Return 0.U for asUInt of a zero-element Seq

Add a condition to SeqUtils.asUInt to have it return an unspecified
width 0.U when applied to an empty sequence. This enables the ability
to do a Cat of a zero-element sequence.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

* Test elaboration of Cat on zero-element Seq

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/main/scala/chisel3/util/Cat.scala b/src/main/scala/chisel3/util/Cat.scala
index 793a5463..3369eb18 100644
--- a/src/main/scala/chisel3/util/Cat.scala
+++ b/src/main/scala/chisel3/util/Cat.scala
@@ -25,6 +25,7 @@ object Cat {
     * in the sequence forms the least significant bits.
     *
     * Equivalent to r(0) ## r(1) ## ... ## r(n-1).
+    * @note This returns a `0.U` if applied to a zero-element `Vec`.
     */
   def apply[T <: Bits](r: Seq[T]): UInt = SeqUtils.asUInt(r.reverse)
 }