Add a test for issue 4159 (#4161)

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

src/test/scala/chiselTests/ChiselEnum.scala

@@ -401,6 +401,30 @@ class ChiselEnumSpec extends ChiselFlatSpec with Utils {
     assertTesterPasses(new CastToUIntTester)
   }
 
+  // This is a bug, but fixing it may break user code.
+  // See: https://github.com/chipsalliance/chisel/issues/4159
+  it should "preserve legacy width behavior" in {
+    val verilog = ChiselStage.emitSystemVerilog(new RawModule {
+      val out1, out2, out3 = IO(Output(UInt(8.W)))
+      val e = EnumExample.e1
+      val x = e.asUInt
+      val y = e.asTypeOf(UInt())
+      val z = e.asTypeOf(UInt(e.getWidth.W))
+      out1 := Cat(1.U, x)
+      out2 := Cat(1.U, y)
+      out3 := Cat(1.U, z)
+      // The bug is that the width of x is 7 but the value of out1 is 3
+      x.getWidth should be(7)
+      x.getWidth should be(EnumExample.getWidth)
+      y.widthOption should be(None)
+      z.getWidth should be(7)
+    })
+    // The bug is that all of these should be the same as out3, or the widths above are wrong
+    verilog should include("assign out1 = 8'h3;")
+    verilog should include("assign out2 = 8'h3;")
+    verilog should include("assign out3 = 8'h81;")
+  }
+
   it should "cast literal UInts to enums correctly" in {
     assertTesterPasses(new CastFromLitTester)
   }