Allow adapt engaged=1, num_warmup=0 for fixed_param sampling

Author: WardBrian

src/cmdstan/command.hpp

@@ -560,17 +560,20 @@ int command(int argc, const char *argv[]) {
     bool save_warmup
         = get_arg_val<bool_argument>(parser, "method", "sample", "save_warmup");
 
+    list_argument *algo = dynamic_cast<list_argument *>(
+        parser.arg("method")->arg("sample")->arg("algorithm"));
+    std::string algo_name = algo->value();
+    bool use_fixed_param = model.num_params_r() == 0 || algo_name == "fixed_param";
+
     bool adapt_engaged = get_arg_val<bool_argument>(parser, "method", "sample",
                                                     "adapt", "engaged");
-    if (adapt_engaged == true && num_warmup == 0) {
+    if (!use_fixed_param && adapt_engaged == true && num_warmup == 0) {
       msg << "The number of warmup samples (num_warmup) must be greater than "
           << "zero if adaptation is enabled." << std::endl;
       throw std::invalid_argument(msg.str());
     }
-    list_argument *algo = dynamic_cast<list_argument *>(
-        parser.arg("method")->arg("sample")->arg("algorithm"));
-    std::string algo_name = algo->value();
-    if (model.num_params_r() == 0 || algo_name == "fixed_param") {
+
+    if (use_fixed_param) {
       if (algo_name != "fixed_param") {
         info(
             "Model contains no parameters, running fixed_param sampler, "