c++ - Why SFINAE trick doesn't work for non-class type when tried for class member pointer? -

with curiosity, trying alternate implementation of is_class construct using sizeof() trick. following code:

template<typename t> struct is_class {   typedef char (&yes)[7];   typedef char (&no)[3];    static yes check (int t::*);   static no check (...);    enum { value = (sizeof(check(0)) == sizeof(yes)) }; }; 

problem when instantiate is_class<int>, gives compile error:

error: creating pointer member of non-class type ‘int’ 

now, question is, if int t::* not applicable int (or void* etc.) why doesn't substitution fail yes check. shouldn't compiler select no check ?

yes , no not templates, sfinae cannot possibly apply them. need this:

template<typename t> struct is_class {   typedef char (&yes)[7];   typedef char (&no)[3];    template <typename u>   static yes check (int u::*);    template <typename>   static no check (...);    enum { value = (sizeof(check<t>(0)) == sizeof(yes)) }; }; 

now sfinae can kick in.