error: overloads have similar conversions

[rjs123]

I have these overload functions which take different parameters...and basically concatenate two strings together except for the first declaration. The error that arises is c2666 which basically says that they have similar conversions...any ideas on fixing this?

String& operator+(const String& s);

friend String operator+(const String& s, const String& t);
friend String operator+(const String& s, const char* t);
friend String operator+(const char* s, const String& t);
friend String operator+(const String& s, char t);
friend String operator+(char s, const String& t);