Add test for behavior of bundles with cloneType.

chiselFrontend/src/main/scala/chisel3/internal/Builder.scala

@@ -72,6 +72,8 @@ trait InstanceId {
 
 }
 
+class GetReferenceException(message: String) extends Exception(message)
+
 private[chisel3] trait HasId extends InstanceId {
   private[chisel3] def _onModuleClose: Unit = {} // scalastyle:ignore method.name
   private[chisel3] val _parent: Option[BaseModule] = Builder.currentModule
@@ -111,7 +113,7 @@ private[chisel3] trait HasId extends InstanceId {
   private[chisel3] def setRef(parent: HasId, name: String): Unit = setRef(Slot(Node(parent), name))
   private[chisel3] def setRef(parent: HasId, index: Int): Unit = setRef(Index(Node(parent), ILit(index)))
   private[chisel3] def setRef(parent: HasId, index: UInt): Unit = setRef(Index(Node(parent), index.ref))
-  private[chisel3] def getRef: Arg = _ref.get
+  private[chisel3] def getRef: Arg = _ref.getOrElse(throw new GetReferenceException(s"bad .getRef on None"))
   private[chisel3] def getOptionRef: Option[Arg] = _ref
 
   // Implementation of public methods.

src/main/scala/chisel3/internal/firrtl/Emitter.scala

@@ -2,8 +2,9 @@
 
 package chisel3.internal.firrtl
 import chisel3._
-import chisel3.core.{SpecifiedDirection, EnumType}
+import chisel3.core.{EnumType, SpecifiedDirection}
 import chisel3.experimental._
+import chisel3.internal.GetReferenceException
 import chisel3.internal.sourceinfo.{NoSourceInfo, SourceLine}
 
 private[chisel3] object Emitter {
@@ -26,6 +27,7 @@ private class Emitter(circuit: Circuit) {
     s"$dirString ${e.id.getRef.name} : ${emitType(e.id, clearDir)}"
   }
 
+  //scalastyle:off cyclomatic.complexity
   private def emitType(d: Data, clearDir: Boolean = false): String = d match {
     case d: Clock => "Clock"
     case d: chisel3.core.EnumType => s"UInt${d.width}"
@@ -41,7 +43,14 @@ private class Emitter(circuit: Circuit) {
         case (true, _) =>
           s"${elt.getRef.name} : ${emitType(elt, true)}"
         case (false, SpecifiedDirection.Unspecified | SpecifiedDirection.Output) =>
-          s"${elt.getRef.name} : ${emitType(elt, false)}"
+          try {
+            s"${elt.getRef.name} : ${emitType(elt, false)}"
+          }
+          catch {
+            case getRef: GetReferenceException =>
+              throw new GetReferenceException(s"Cannot emit ports for ${d.className}" +
+                      s" with fields ${d.elements.keys.mkString(",")}, perhaps you forgot a cloneType")
+          }
         case (false, SpecifiedDirection.Flip | SpecifiedDirection.Input) =>
           s"flip ${elt.getRef.name} : ${emitType(elt, false)}"
       }

src/test/scala/chiselTests/BundleSpec.scala

@@ -4,6 +4,7 @@ package chiselTests
 
 import chisel3._
 import chisel3.core.IgnoreSeqInBundle
+import chisel3.internal.GetReferenceException
 import chisel3.testers.BasicTester
 
 trait BundleSpecUtils {
@@ -116,3 +117,29 @@ class BundleSpec extends ChiselFlatSpec with BundleSpecUtils {
     }
   }
 }
+
+class BundleWithoutCloneTypeProblem extends ChiselPropSpec {
+  property("Bundles should not freak out if you forget cloneType") {
+    intercept[GetReferenceException] {
+      val a = new Bundle {
+        val b = UInt(4.W)
+      }
+
+      runTester(
+        new BasicTester {
+          val m = Module(new Module {
+            val d = Wire(new Bundle {
+              val e = a // forgot cloneType
+            })
+            val io = IO(new Bundle {
+              val c = Output(a.cloneType)
+            })
+            io.c := d.e
+            d.e.b := 0.U
+            stop()
+          })
+        }
+      )
+    }.getMessage should include("Cannot emit ports for Bundle with fields e, perhaps you forgot a cloneType")
+  }
+}