Warn that events can not be user task or function arguments.

This patch makes events passed as arguments to user tasks or functions
a compile time error with an appropriate error message.

elab_expr.cc

@@ -1488,6 +1488,12 @@ NetExpr* PECallFunction::elaborate_expr(Design*des, NetScope*scope,
 						   def->port(idx)->data_type(),
 						   def->port(idx)->vector_width(),
 						   tmp);
+		  if (NetEEvent*evt = dynamic_cast<NetEEvent*> (parms[idx])) {
+			cerr << evt->get_fileline() << ": error: An event '"
+			     << evt->event()->name() << "' can not be a user "
+			        "function argument." << endl;
+			des->errors += 1;
+		  }
 		  if (debug_elaborate)
 			cerr << get_fileline() << ": debug:"
 			     << " function " << path_

elaborate.cc

@@ -2644,6 +2644,13 @@ NetProc* PCallTask::elaborate_usr(Design*des, NetScope*scope) const
 	    ivl_variable_type_t lv_type = lv->expr_type();
 
 	    NetExpr*rv = elaborate_rval_expr(des, scope, lv_type, wid, parms_[idx]);
+	    if (NetEEvent*evt = dynamic_cast<NetEEvent*> (rv)) {
+		  cerr << evt->get_fileline() << ": error: An event '"
+		       << evt->event()->name() << "' can not be a user "
+		          "task argument." << endl;
+		  des->errors += 1;
+		  continue;
+	    }
 	    if (wid > rv->expr_width()) {
 		  rv->set_width(wid);
 		  rv = pad_to_width(rv, wid, *this);

expr_synth.cc

@@ -1369,6 +1369,10 @@ NetNet* NetEUFunc::synthesize(Design*des, NetScope*scope, NetExpr*root)
         /* Synthesize the arguments. */
       bool errors = false;
       for (unsigned idx = 0; idx < eparms.count(); idx += 1) {
+	    if (dynamic_cast<NetEEvent*> (parms_[idx])) {
+		  errors = true;
+		  continue;
+	    }
 	    NetNet*tmp = parms_[idx]->synthesize(des, scope, root);
 	    if (tmp == 0) {
 		  cerr << get_fileline() << ": error: Unable to synthesize "