Fix cat of zero-width SInt (#2116)

Previously, concatenating two SInts where one is of zero-width would
return the non-zero-width SInt. This is incorrect because the output of
Cat should be of type UInt. Now the ZeroWidth transform will introduce a
cast when removing a Cat when the argument type is non-UInt.

1 files changed, 17 insertions, 0 deletions

diff --git a/src/test/scala/firrtlTests/ZeroWidthTests.scala b/src/test/scala/firrtlTests/ZeroWidthTests.scala
index 99ebbdd3..df630065 100644
--- a/src/test/scala/firrtlTests/ZeroWidthTests.scala
+++ b/src/test/scala/firrtlTests/ZeroWidthTests.scala
@@ -220,6 +220,23 @@ class ZeroWidthTests extends FirrtlFlatSpec {
         |    x <= UInt<1>(1)""".stripMargin
     (parse(exec(input))) should be(parse(check))
   }
+
+  "Cat of SInt with zero-width" should "keep type correctly" in {
+    val input =
+      """circuit Top :
+        |  module Top :
+        |    input x : SInt<0>
+        |    input y : SInt<1>
+        |    output z : UInt<1>
+        |    z <= cat(y, x)""".stripMargin
+    val check =
+      """circuit Top :
+        |  module Top :
+        |    input y : SInt<1>
+        |    output z : UInt<1>
+        |    z <= asUInt(y)""".stripMargin
+    (parse(exec(input))) should be(parse(check))
+  }
 }
 
 class ZeroWidthVerilog extends FirrtlFlatSpec {