Updated code such that it will compile

modules/sign_extender.sv

@@ -8,9 +8,9 @@ module sign_extender #(in_width=4, out_width=5)(
 );
     always_comb begin
         if (op) begin
-            assign ext_val = { {(2**(out_width-in_width)){val[2**in_width-1]}}, val};
+            ext_val = { {(2**(out_width-in_width)){val[2**in_width-1]}}, val};
         end else begin
-            assign ext_val = ({{(2**(out_width-in_width)){1'b0}}, val});
+            ext_val = ({{(2**(out_width-in_width)){1'b0}}, val});
         end
     end
 endmodule