Chisel 3.5.1 incorrectly warns on some Bundle traits

[jackkoenig]

Type of issue: bug report

Impact: no functional change

Development Phase: request | proposal

Other information

If the current behavior is a bug, please provide the steps to reproduce the problem:

It seems to happen on Bundle traits that only have virtual methods, eg.

trait Foo extends Bundle {
  val foo: UInt
}

class Bar extends Foo {
  val foo = UInt(8.W)
  val bar = UInt(8.W)
}

class Example extends Module {
  val in = IO(Input(new Bar))
  val out = IO(Output(new Bar))
  out := in
}

This seems to be due to the changes that make it possible to generate _elementsImpl since we know process non-concrete Bundles. That being said, this only seems to affect abstract Bundles that only have virtual methods so I don't think it affects the correctness of capturing the elements since those virtual methods will need to be implemented later.

What is the current behavior?

Chisel 3.5.1 warns on trait Foo: https://scastie.scala-lang.org/0J6M9n2URSCvzfAEnkZPcw

What is the expected behavior?

Chisel 3.5.0 does not: https://scastie.scala-lang.org/l1P8OyIMQ9aX3Ge22WGSEA

Please tell us about your environment:

Chisel v3.5.1

What is the use case for changing the behavior?