Fix literal handling for views of empty Aggregates (backport #4071) (#…

…4074)

* Fix literal handling for views of empty Aggregates (#4071)

Previously, a view of an empty Record or Vec would always have a
litValue of 0.

(cherry picked from commit 11c3f51)

# Conflicts:
#	src/test/scala/chiselTests/experimental/DataView.scala

* Fixup backport conflicts

---------

Co-authored-by: Jack Koenig <koenig@sifive.com>

core/src/main/scala/chisel3/Aggregate.scala

@@ -42,6 +42,15 @@ sealed abstract class Aggregate extends Data {
     }
 
     topBindingOpt match {
+      // Don't accidentally invent a literal value for a view that is empty
+      case Some(_: AggregateViewBinding) if this.getElements.isEmpty =>
+        reifySingleData(this) match {
+          case Some(target: Aggregate) => target.checkingLitOption(checkForDontCares)
+          case _ =>
+            val msg =
+              s"It should not be possible to have an empty Aggregate view that doesn't reify to a single target, but got $this"
+            Builder.exception(msg)(UnlocatableSourceInfo)
+        }
       case Some(_: BundleLitBinding | _: VecLitBinding | _: AggregateViewBinding) =>
         // Records store elements in reverse order and higher indices are more significant in Vecs
         this.getElements.foldRight(Option(BigInt(0)))(shiftAdd)

src/test/scala/chiselTests/experimental/DataView.scala

@@ -6,6 +6,8 @@ import chisel3._
 import chisel3.experimental.conversions._
 import chisel3.experimental.dataview._
 import chisel3.experimental.{Analog, HWTuple2}
+import chisel3.experimental.BundleLiterals._
+import chisel3.experimental.VecLiterals._
 import chisel3.reflect.DataMirror.internal.chiselTypeClone
 import chisel3.util.{Decoupled, DecoupledIO, Valid, ValidIO}
 import chiselTests.ChiselFlatSpec
@@ -999,6 +1001,38 @@ class DataViewSpec extends ChiselFlatSpec {
     ChiselStage.emitCHIRRTL(new MyModule)
   }
 
+  it should "NOT invent literal values for views of empty Aggregates" in {
+    class MyModule extends Module {
+      val emptyBundle = IO(Output(new Bundle {}))
+      val emptyVec = IO(Output(Vec(0, UInt(8.W))))
+
+      val bundleView = emptyBundle.viewAs[Bundle]
+      val vecView = emptyVec.viewAs[Vec[UInt]]
+
+      emptyBundle.litOption should be(None)
+      bundleView.litOption should be(None)
+      emptyVec.litOption should be(None)
+      vecView.litOption should be(None)
+    }
+    ChiselStage.emitCHIRRTL(new MyModule)
+  }
+
+  it should "support literal values for views of empty Aggregates" in {
+    class MyModule extends Module {
+      val emptyBundle = (new Bundle {}).Lit()
+      val emptyVec = Vec(0, UInt(8.W)).Lit()
+
+      val bundleView = emptyBundle.viewAs[Bundle]
+      val vecView = emptyVec.viewAs[Vec[UInt]]
+
+      emptyBundle.litOption should be(Some(0))
+      bundleView.litOption should be(Some(0))
+      emptyVec.litOption should be(Some(0))
+      vecView.litOption should be(Some(0))
+    }
+    ChiselStage.emitCHIRRTL(new MyModule)
+  }
+
   behavior.of("PartialDataView")
 
   it should "still error if the mapping is non-total in the view" in {