Calling .asUInt on a Bool returns the Bool itself (rather than a true UInt)

[jackkoenig]

Type of issue: Bug Report

Please provide the steps to reproduce the problem:

Consider the following example:

//> using scala "2.13.12"
//> using dep "org.chipsalliance::chisel:6.4.0"
//> using plugin "org.chipsalliance:::chisel-plugin:6.4.0"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
// _root_ disambiguates from package chisel3.util.circt if user imports chisel3.util._
import _root_.circt.stage.ChiselStage

class Foo extends Module {
  val in = IO(Input(Bool()))
  val out = IO(Output(UInt(8.W)))

  out := in.asUInt.pad(8)
}

object Main extends App {
  println(
    ChiselStage.emitSystemVerilog(
      gen = new Foo,
      firtoolOpts = Array("-disable-all-randomization", "-strip-debug-info")
    )
  )
}

What is the current behavior?

The same error as in https://github.com/chipsalliance/chisel/issues/4162, i.e.

Exception in thread "main" java.lang.IllegalArgumentException: requirement failed
	at ... ()
	at Foo.$anonfun$new$1(chisel-example.scala:14)
	at chisel3.Data.$anonfun$$colon$eq$1(Data.scala:754)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
	at chisel3.experimental.prefix$.apply(prefix.scala:33)
	at chisel3.Data.$colon$eq(Data.scala:754)
	at Foo.<init>(chisel-example.scala:14)
	at Main$.$anonfun$new$2(chisel-example.scala:20)
	at chisel3.Module$.evaluate(Module.scala:88)
	at chisel3.Module$.do_apply(Module.scala:35)
	at chisel3.stage.phases.Elaborate.$anonfun$transform$2(Elaborate.scala:52)
	at chisel3.internal.Builder$.$anonfun$buildImpl$1(Builder.scala:1018)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
	at chisel3.internal.Builder$.buildImpl(Builder.scala:1012)
	at chisel3.internal.Builder$.$anonfun$build$1(Builder.scala:1003)
	at logger.Logger$.$anonfun$makeScope$4(Logger.scala:148)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
	at logger.Logger$.makeScope(Logger.scala:146)
	at logger.Logger$.makeScope(Logger.scala:133)
	at ... ()
	at ... (Stack trace trimmed to user code only. Rerun with --full-stacktrace to see the full stack trace)

What is the expected behavior?

I would expect this to work, the verilog should be:

assign out = {7'h0, in};

Bool inheriting from UInt and then not properly fulfilling the inheritance contract is bad enough, but not even being able to work around it by casting the Bool to a UInt is even worse. Calling .asUInt on a Bool should definitely return UInt(1.W).

Please tell us about your environment:

Other Information

What is the use case for changing the behavior?