Fix SInt literals to reject too small of widths (#4786)

Co-authored-by: Schuyler Eldridge <[email protected]>

Author: jackkoenig

core/src/main/scala/chisel3/internal/firrtl/IR.scala

@@ -171,7 +171,9 @@ private[chisel3] object ir {
       val unsigned = if (n < 0) (BigInt(1) << width.get) + n else n
       s"asSInt(${ULit(unsigned, width).name})"
     }
-    def minWidth: Int = (if (w.known) 0 else 1) + n.bitLength
+
+    // Special case for 0 which can be specified to zero-width (but defaults to 1 bit).
+    def minWidth: Int = if (n == 0 && w.known) 0 else 1 + n.bitLength
 
     def cloneWithWidth(newWidth: Width): this.type = {
       SLit(n, newWidth).asInstanceOf[this.type]

src/test/scala-2/chiselTests/SIntOps.scala

@@ -284,4 +284,18 @@ class SIntOpsSpec extends AnyPropSpec with Matchers with ShiftRightWidthBehavior
     blit.x.litOption should be(Some(2))
     blit.y.litOption should be(Some(9))
   }
+
+  property("SInt literals with too small of a width should be rejected") {
+    // Sanity checks.
+    0.S.getWidth should be(1)
+    0.S(0.W).getWidth should be(0)
+    -1.S.getWidth should be(1)
+    1.S.getWidth should be(2)
+    // The real check.
+    -2.S.getWidth should be(2)
+    an[IllegalArgumentException] shouldBe thrownBy(-2.S(1.W))
+    0xde.S.getWidth should be(9)
+    an[IllegalArgumentException] shouldBe thrownBy(0xde.S(8.W))
+    an[IllegalArgumentException] shouldBe thrownBy(0xde.S(4.W))
+  }
 }