[over.match.funcs.general] p2 says

a member function that does not have an explicit object parameter is considered to have an extra first parameter, called the implicit object parameter, which represents the object for which the member function has been called.

[over.match.funcs.general] p4 says

where X is the class of which the function is a member and cv is the cv-qualification on the member function declaration.

Consider this example:

struct A{
   void show(){}
};
struct B:A{
};
B bobj;
bobj.show();

In this case, the object for which the member function show is called is bobj that is of type B. [class.derived.general] p2 says

Members of a base class are also members of the derived class.

That is, show is also a member of class B. So, whether the type of the implicit object parameter is considered to be A& or B&? The intent should be the former. [over.match.funcs.general] p4 might be changed to

where X is the class of which the function is a direct member

For conversion functions that are implicit object member functions, the function is considered to be a direct member of the class of the implied object argument for the purpose of defining the type of the implicit object parameter.

For non-conversion functions that are implicit object member functions nominated by a using-declaration in a derived class, the function is considered to be a direct member of the derived class for the purpose of defining the type of the implicit object parameter.