Cleaner elaboration of void functions.

This fixed githun issue # 281.

Statement.h

@@ -232,6 +232,8 @@ class PCallTask  : public Statement {
       NetProc*elaborate_method_(Design*des, NetScope*scope,
                                 bool add_this_flag = false) const;
       NetProc*elaborate_function_(Design*des, NetScope*scope) const;
+      NetProc*elaborate_void_function_(Design*des, NetScope*scope,
+				       NetFuncDef*def) const;
 
       NetProc*elaborate_build_call_(Design*des, NetScope*scope,
 				    NetScope*task, NetExpr*use_this) const;

elab_lval.cc

@@ -204,7 +204,7 @@ NetAssign_* PEIdent::elaborate_lval(Design*des,
 	   print a useful error message. */
       if (reg == 0) {
 	    if (use_scope->type()==NetScope::FUNC
-		&& use_scope->func_def()->return_sig()==0
+		&& use_scope->func_def()->is_void()
 		&& use_scope->basename()==peek_tail_name(path_)) {
 		  cerr << get_fileline() << ": error: "
 		       << "Cannot assign to " << path_

elaborate.cc

@@ -3672,6 +3672,9 @@ NetProc* PCallTask::elaborate_function_(Design*des, NetScope*scope) const
 	// call with a missing return assignment.
       if (! func) return 0;
 
+      if (gn_system_verilog() && func->is_void())
+	    return elaborate_void_function_(des, scope, func);
+
 	// Generate a function call version of this task call.
       PExpr*rval = new PECallFunction(package_, path_, parms_);
       rval->set_file(get_file());
@@ -3686,6 +3689,21 @@ NetProc* PCallTask::elaborate_function_(Design*des, NetScope*scope) const
       return tmp->elaborate(des, scope);
 }
 
+NetProc* PCallTask::elaborate_void_function_(Design*des, NetScope*scope,
+					     NetFuncDef*def) const
+{
+      NetScope*dscope = def->scope();
+
+      if (debug_elaborate) {
+	    cerr << get_fileline() << ": PCallTask::elaborate_void_function_: "
+		 << "function void " << scope_path(dscope)
+		 << endl;
+      }
+
+      ivl_assert(*this, dscope->elab_stage() >= 3);
+      return elaborate_build_call_(des, scope, dscope, 0);
+}
+
 NetProc* PCallTask::elaborate_build_call_(Design*des, NetScope*scope,
 					  NetScope*task, NetExpr*use_this) const
 {

netlist.h

@@ -3650,6 +3650,13 @@ class NetFuncDef : public NetBaseDef {
 		 const std::vector<NetExpr*>&pd);
       ~NetFuncDef();
 
+	// Return true if the function returns "void". We still treat
+	// it as a function since we need to check that the contents
+	// meet the requirements of a function, but we need to know
+	// that it is void because it can be evaluated differently.
+      inline bool is_void() const { return result_sig_ == 0; }
+
+	// Non-void functions have a return value as a signal.
       const NetNet*return_sig() const;
 
 	// When we want to evaluate the function during compile time,

t-dll.cc

@@ -596,14 +596,14 @@ static void fill_in_scope_function(ivl_scope_t scope, const NetScope*net)
       const NetFuncDef*def = net->func_def();
       assert(def);
 
-      const NetNet*return_sig = def->return_sig();
-      if (return_sig == 0) {
+      if (def->is_void()) {
 	      // Special case: If there is no return signal, this is
 	      // apparently a VOID function.
 	    scope->func_type = IVL_VT_VOID;
 	    scope->func_signed = 0;
 	    scope->func_width = 0;
       } else {
+	    const NetNet*return_sig = def->return_sig();
 	    scope->func_type = return_sig->data_type();
 	    scope->func_signed = return_sig->get_signed();
 	    scope->func_width = return_sig->vector_width();