Support === on empty Aggregate (#3747)

(cherry picked from commit da6ae7d)

core/src/main/scala/chisel3/Data.scala

@@ -915,7 +915,8 @@ object Data {
             thiz.elementsIterator
               .zip(that.elementsIterator)
               .map { case (thisData, thatData) => thisData === thatData }
-              .reduce(_ && _)
+              .reduceOption(_ && _) // forall but that isn't defined for Bool on Seq
+              .getOrElse(true.B)
           }
         case (thiz: Record, that: Record) =>
           if (thiz._elements.size != that._elements.size) {
@@ -939,7 +940,8 @@ object Data {
                     )
                 }
             }
-              .reduce(_ && _)
+              .reduceOption(_ && _) // forall but that isn't defined for Bool on Seq
+              .getOrElse(true.B)
           }
         // This should be matching to (DontCare, DontCare) but the compiler wasn't happy with that
         case (_: DontCare.type, _: DontCare.type) => true.B

src/test/scala/chiselTests/DataEqualitySpec.scala

@@ -70,6 +70,9 @@ class DataEqualitySpec extends ChiselFlatSpec with Utils {
     val a = UInt(8.W)
     val b: Bundle = gen
   }
+  class MaybeEmptyBundle(x: Boolean) extends Bundle {
+    val a = Option.when(x)(UInt(8.W))
+  }
 
   behavior.of("UInt === UInt")
   it should "pass with equal values" in {
@@ -140,6 +143,14 @@ class DataEqualitySpec extends ChiselFlatSpec with Utils {
       )
     }
   }
+  it should "support empty Vecs" in {
+    assertTesterPasses {
+      new EqualityTester(
+        Wire(Vec(0, UInt(8.W))),
+        Wire(Vec(0, UInt(8.W)))
+      )
+    }
+  }
   it should "fail with equal sizes, differing values" in {
     assertTesterFails {
       new EqualityTester(
@@ -168,6 +179,14 @@ class DataEqualitySpec extends ChiselFlatSpec with Utils {
       )
     }
   }
+  it should "support empty Bundles" in {
+    assertTesterPasses {
+      new EqualityTester(
+        (new MaybeEmptyBundle(false)).Lit(),
+        (new MaybeEmptyBundle(false)).Lit()
+      )
+    }
+  }
   it should "fail with equal type, differing values" in {
     assertTesterFails {
       new EqualityTester(