Can't use Mux with DontCare while equivalent when() statement is fine

[edwardcwang]

Type of issue: bug report

Development Phase: request

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

Consider the following two functions below, muxLess and whenLess. As a user, they appear to implement the same logic and I would expect them to both function. However, the whenLess case elaborates and works as expected while muxLess flames out with an error:

java.lang.AssertionError: can't create Mux with heterogeneous types class chisel3.core.UInt and class chisel3.core.DontCare$
  chisel3.core.cloneSupertype$$anonfun$1.apply(Data.scala:132)
  chisel3.core.cloneSupertype$$anonfun$1.apply(Data.scala:112)
[...]
  scala.collection.AbstractTraversable.reduce(Traversable.scala:104)
  chisel3.core.cloneSupertype$.apply(Data.scala:112)
  chisel3.core.Mux$.do_apply(Mux.scala:32)
[...]

// doesn't work
def muxLess(in: UInt, outWire: UInt): Unit = {
    outWire := Mux(in < 48.U, in, DontCare)
}

// works fine
def whenLess(in: UInt, outWire: UInt): Unit = {
    when(in < 48.U) {
        outWire := in
    } .otherwise {
        outWire := DontCare
    }
}

class MyMyModule extends Module {
  val io = IO(new Bundle {
    val in = Input(UInt(12.W))
    val out = Output(UInt(12.W))
  })
  
  whenLess(io.in, io.out) // works
  muxLess(io.in, io.out) // doesn't work
}

Please tell us about your environment: Chisel 3.1

What is the use case for changing the behavior? Seems like an arbitrary restriction to a user that we can't use DontCare in a Mux() but can use it in a when() statement to accomplish the exact same thing.