back.pysim: fix RHS codegen for Cat() and Repl(..., 0).

Fixes #325.

nmigen/back/pysim.py

@@ -492,7 +492,9 @@ class _RHSValueCompiler(_ValueCompiler):
             part_mask = (1 << len(part)) - 1
             gen_parts.append(f"(({self(part)} & {part_mask}) << {offset})")
             offset += len(part)
+        if gen_parts:
             return f"({' | '.join(gen_parts)})"
+        return f"0"
 
     def on_Repl(self, value):
         part_mask = (1 << len(value.value)) - 1
@@ -502,7 +504,9 @@ class _RHSValueCompiler(_ValueCompiler):
         for _ in range(value.count):
             gen_parts.append(f"({gen_part} << {offset})")
             offset += len(value.value)
+        if gen_parts:
             return f"({' | '.join(gen_parts)})"
+        return f"0"
 
     def on_ArrayProxy(self, value):
         index_mask = (1 << len(value.index)) - 1

nmigen/test/test_sim.py

@@ -712,3 +712,15 @@ class SimulatorIntegrationTestCase(FHDLTestCase):
             with sim.write_vcd(open(os.path.devnull, "wt")):
                 with sim.write_vcd(open(os.path.devnull, "wt")):
                     pass
+
+
+class SimulatorRegressionTestCase(FHDLTestCase):
+    def test_bug_325(self):
+        dut = Module()
+        dut.d.comb += Signal().eq(Cat())
+        Simulator(dut).run()
+
+    def test_bug_325_bis(self):
+        dut = Module()
+        dut.d.comb += Signal().eq(Repl(Const(1), 0))
+        Simulator(dut).run()