Provide require message for negative widths (#4008)

(cherry picked from commit 751a4cf)

# Conflicts:
#	core/src/main/scala/chisel3/Width.scala

core/src/main/scala/chisel3/Width.scala

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chisel3
+
+object Width {
+  def apply(x: Int): Width = KnownWidth(x)
+  def apply(): Width = UnknownWidth()
+}
+
+sealed abstract class Width {
+  type W = Int
+  def min(that: Width): Width = this.op(that, _ min _)
+  def max(that: Width): Width = this.op(that, _ max _)
+  def +(that:   Width): Width = this.op(that, _ + _)
+  def +(that:   Int):   Width = this.op(this, (a, b) => a + that)
+  @deprecated(
+    "The width of shift-right now differs by type, use unsignedShiftRight and signedShiftRight",
+    "Chisel 7.0.0"
+  )
+  def shiftRight(that:         Int): Width = this.op(this, (a, b) => 0.max(a - that))
+  def unsignedShiftRight(that: Int): Width = this.op(this, (a, b) => 0.max(a - that))
+  def signedShiftRight(that:   Int): Width = this.op(this, (a, b) => 1.max(a - that))
+  def dynamicShiftLeft(that:   Width): Width =
+    this.op(that, (a, b) => a + (1 << b) - 1)
+
+  def known: Boolean
+  def get:   W
+  protected def op(that: Width, f: (W, W) => W): Width
+}
+
+sealed case class UnknownWidth() extends Width {
+  def known: Boolean = false
+  def get:   Int = None.get
+  def op(that: Width, f: (W, W) => W): Width = this
+  override def toString: String = ""
+}
+
+sealed case class KnownWidth(value: Int) extends Width {
+  require(value >= 0, s"Widths must be non-negative, got $value")
+  def known: Boolean = true
+  def get:   Int = value
+  def op(that: Width, f: (W, W) => W): Width = that match {
+    case KnownWidth(x) => KnownWidth(f(value, x))
+    case _             => that
+  }
+  override def toString: String = s"<${value.toString}>"
+}

src/test/scala/chiselTests/UIntOps.scala

@@ -479,4 +479,9 @@ class UIntOpsSpec extends ChiselPropSpec with Matchers with Utils {
     chirrtl should include("y <= a")
     chirrtl should include("z <= b")
   }
+
+  property("UInts with negative widths should have a decent error message") {
+    val e = the[IllegalArgumentException] thrownBy (UInt(-8.W))
+    e.getMessage should include("Widths must be non-negative, got -8")
+  }
 }