Canonicalize construction of Decoupled with no payload (#785)

* Canonicalize construction of empty Decoupled

* Change signature after dev meeting discussion
* Make EmptyBundle private and final

* Add test case for Decoupled with no payload

* Apply suggestions from code review

Co-authored-by: Richard Lin <richard.lin@berkeley.edu>

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: Adam Izraelevitz <azidar@gmail.com>
Co-authored-by: Richard Lin <richard.lin@berkeley.edu>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/test/scala/chiselTests/DecoupledSpec.scala b/src/test/scala/chiselTests/DecoupledSpec.scala
new file mode 100644
index 00000000..c251df82
--- /dev/null
+++ b/src/test/scala/chiselTests/DecoupledSpec.scala
@@ -0,0 +1,19 @@
+// See LICENSE for license details.
+
+package chiselTests
+
+import chisel3._
+import chisel3.util.Decoupled
+
+class DecoupledSpec extends ChiselFlatSpec {
+  "Decoupled() and Decoupled.empty" should "give DecoupledIO with empty payloads" in {
+    elaborate(new Module {
+      val io = IO(new Bundle {
+        val in = Flipped(Decoupled())
+        val out = Decoupled.empty
+      })
+      io.out <> io.in
+      assert(io.asUInt.widthOption.get === 4)
+    })
+  }
+}