Why compile error with enable_if

Question

Why this does not compile with gcc48 and clang32?

#include 

template  
struct S {

    template 
    typename         


        

Answer 1

To get std::enable_if to work like this, you are relying on SFINAE. Unfortunately, at the point where you declare

S<1> s1;

it will instantiate all of S<1>'s member declarations. SFINAE will only come into play at this point if S<1> were an ill-formed construct. It is not. Unfortunately, it contains a function which is invalid, thus the instantiation of S<> is invalid.

For things like this, I might defer to a seperate template struct:

template 
struct f_functor {
    template 
    static int f(T t) { return 1; }
};

template <>
struct f_functor {
    template 
    static int f(T t) { return 2; }
};

template  
struct S {

    template 
    typename int f(T t) { return f_functor::f(t); }
};