DontCare Influences Type Inference

[jackkoenig]

Consider the following:

class DontCareTest extends Module {
  class Bun extends Bundle {
    val foo = UInt(8.W)
  }
  val io = IO(new Bundle {
    val uint = Output(UInt())
    val bun = Output(Valid(new Bun))
  })
  val uint = WireInit(UInt(8.W), DontCare)
  io.uint := uint

  val bun = WireInit(Valid(new Bun), DontCare)
  bun.valid := false.B
  io.bun := bun
}

This gives the following error:

[info] Compiling 1 Scala source to /Users/jack.koenig3/ucb-bar/chisel3-raw/target/scala-2.12/classes...
[error] /Users/jack.koenig3/ucb-bar/chisel3-raw/src/main/scala/DontCare.scala:16: value valid is not a member of chisel3.core.Data
[error]   bun.valid := false.B
[error]       ^
[error] one error found

The definition of WireInit.apply being:

def apply[T <: Data](t: T, init: T)(implicit sourceInfo: SourceInfo, compileOptions: CompileOptions): T

Now changing the type of init to Data should fix this particular instance but this problem will pop up all over the place, including in libraries. Is there a way to tell the type system to ignore DontCare for type inference? @ducky64 @sdtwigg

[ducky64]

What other instances of this problem are there? := is already untyped, and changing init to type Data makes the WireInit equivalent to a Wire declaration followed by an untyped := connect.

I'd imagine it doesn't make any sense to initialize a register with DontCare, since Chisel doesn't complain about uninitialized Regs?

[jackkoenig]

In library code if you accept both a t and an init it will do the same type inference and you'll get the same issue. That being said I don't think there's a fundamental fix other than "don't write bad library code".