Fix #163 - invalid generated SystemVerilog for bit slicing on expressions

lib/src/modules/bus.dart

@@ -39,7 +39,10 @@ class BusSubset extends Module with InlineSystemVerilog {
           'Index out of bounds, indices $startIndex and $endIndex must be less than width-1');
     }
 
-    _original = Module.unpreferredName('original_' + bus.name);
+    // original name can't be unpreferred because you cannot do a bit slice on expressions
+    // in SystemVerilog, and other expressions could have been in-lined
+    _original = 'original_' + bus.name;
+
     _subset =
         Module.unpreferredName('subset_${endIndex}_${startIndex}_' + bus.name);
 

test/bus_test.dart

@@ -42,6 +42,7 @@ class BusTestModule extends Module {
     var aBJoined = addOutput('a_b_joined', width: a.width + b.width);
     var aPlusB = addOutput('a_plus_b', width: a.width);
     var a1 = addOutput('a1');
+    var expressionBitSelect = addOutput('expression_bit_select', width: 4);
 
     aBar <= ~a;
     aAndB <= a & b;
@@ -52,6 +53,8 @@ class BusTestModule extends Module {
     aBJoined <= [b, a].swizzle();
     a1 <= a[1];
     aPlusB <= a + b;
+    expressionBitSelect <=
+        [aBJoined, aShrunk, aRange, aRSliced, aPlusB].swizzle().slice(3, 0);
   }
 }
 
@@ -148,7 +151,8 @@ void main() {
       'a_reversed': 8,
       'a_range': 3,
       'a_b_joined': 16,
-      'a_plus_b': 8
+      'a_plus_b': 8,
+      'expression_bit_select': 4,
     };
     test('NotGate bus', () async {
       var gtm = BusTestModule(Logic(width: 8), Logic(width: 8));
@@ -291,5 +295,18 @@ void main() {
           signalToWidthMap: signalToWidthMap);
       expect(simResult, equals(true));
     });
+
+    test('expression bit select', () async {
+      var gtm = BusTestModule(Logic(width: 8), Logic(width: 8));
+      await gtm.build();
+      var vectors = [
+        Vector({'a': 1, 'b': 1}, {'expression_bit_select': 2}),
+      ];
+      await SimCompare.checkFunctionalVector(gtm, vectors);
+      var simResult = SimCompare.iverilogVector(
+          gtm.generateSynth(), gtm.runtimeType.toString(), vectors,
+          signalToWidthMap: signalToWidthMap, dontDeleteTmpFiles: true);
+      expect(simResult, equals(true));
+    });
   });
 }