print for debug: the ostream need explicit string not char[].

src/musredit_qt5/mupp/var/include/PAst.hpp

@@ -155,7 +155,7 @@ namespace mupp { namespace ast
     typedef std::list<statement> statement_list;
 
     // print functions for debugging
-    inline std::ostream& operator<<(std::ostream& out, nil) { out << "nil"; return out; }
+    inline std::ostream& operator<<(std::ostream& out, nil) { out << std::string("nil"); return out; }
     inline std::ostream& operator<<(std::ostream& out, variable const& var) { out << var.name; return out; }
 }}
 

src/musredit_qt6/mupp/var/include/PAst.hpp

@@ -155,7 +155,7 @@ namespace mupp { namespace ast
     typedef std::list<statement> statement_list;
 
     // print functions for debugging
-    inline std::ostream& operator<<(std::ostream& out, nil) { out << "nil"; return out; }
+    inline std::ostream& operator<<(std::ostream& out, nil) { out << std::string("nil"); return out; }
     inline std::ostream& operator<<(std::ostream& out, variable const& var) { out << var.name; return out; }
 }}