c++ - Warning: direct base class inaccessible in derived due to ambiguity; is this serious? -

i don't understand ambiguity here. did identify line causes ambiguity , marked it.

#include <string> #include <unordered_map>  class spell {     protected:         struct exemplar {};         spell() = default;         spell (exemplar, const std::string&); };  class spellfromscroll : virtual public spell {     private:         static std::unordered_map<std::string, spellfromscroll*> prototypesmap;     public:         static void insertinprototypesmap (const std::string& tag, spellfromscroll* spell) {             prototypesmap.emplace (tag, spell);         }         template <typename t> static spellfromscroll* createfromspell (t*); }; std::unordered_map<std::string, spellfromscroll*> spellfromscroll::prototypesmap;  class spellwithtargets : virtual public spell {};  // *** note: virtual  class sleep : public spellwithtargets {     private:         static const sleep prototype;     public:         static std::string spellname() {return "sleep";}     private:         sleep (exemplar e) : spell (e, spellname()) {} }; const sleep sleep::prototype (exemplar{});  template <typename t> class scrollspell : /*virtual*/ public t, public spellfromscroll {};  spell::spell (exemplar, const std::string& spellname) {     // ambiguity warning!     spellfromscroll::insertinprototypesmap (spellname, spellfromscroll::createfromspell(this)); }  template <typename t> spellfromscroll* spellfromscroll::createfromspell (t*) {     return new scrollspell<t>; }  int main() {}  /* c:\adandd>g++ -std=c++14 ambiguity.cpp -o a.exe -wall -wextra -pedantic-errors ambiguity.cpp: in instantiation of 'class scrollspell<spell>': ambiguity.cpp:32:13:   required 'static spellfromscroll* spellfromscroll::createfromspell(t*) [with t = spell]' ambiguity.cpp:27:90:   required here ambiguity.cpp:23:7: warning: direct base 'spell' inaccessible in 'scrollspell<spell>' due ambiguity  class scrollspell : public t, public spellfromscroll {};        ^ ambiguity.cpp:23:7: warning: virtual base 'spell' inaccessible in 'scrollspell<spell>' due ambiguity [-wextra]  c:\adandd> */ 

how serious it, , can go wrong later on program evolves?

update: solution found letting t virtual base of scrollspell<t>. in program t derived class of spell, , spell virtual base of t. see diagram below.

                  spell                   /   \               v  /     \ v                 /       \                /         \      spellfromscroll      spellwithtargets           \                \            \                \             \               sleep              \               /               \             / v                \           /               scrollspell<sleep> 

in above diagram, why sleep being virtual base of scrollspell<sleep> solve problem?

template <typename t> class scrollspell : public t, public spellfromscroll {}; 

here, t = spell, scrollspell class has spell class direct, non-virtual base class, , virtual base class through spellfromscroll. ambiguity. declaring base class t virtual might solve problem.

also don't understand point behind design, might introduce new issues.