Detecting constexpr with SFINAE

Question

I\'m working on upgrading some C++ code to take advantage of the new functionality in C++11. I have a trait class with a few functions returning fundamental types which woul

Answer 1

NOTE: I opened a question here about whether OPs code is actually valid. My rewritten example below will work in any case.

---

but I would like to know if the code is legal C++11

It is, although the default template argument may be considered a bit unusual. I personally like the following style better, which is similar to how you (read: I) write a trait to check for a function's existence, just using a non-type template parameter and leaving out the decltype:

#include 

namespace detail{
template struct sfinae_true : std::true_type{};
template
sfinae_true<(T::f(), 0)> check(int);
template
std::false_type check(...);
} // detail::

template
struct has_constexpr_f : decltype(detail::check(0)){};

Live example.

---

Explanation time~

Your original code works^† because a default template argument's point of instantiation is the point of instantiation of its function template, meaning, in your case, in main, so it can't be substituted earlier than that.

§14.6.4.1 [temp.point] p2

If a function template [...] is called in a way which uses the definition of a default argument of that function template [...], the point of instantiation of the default argument is the point of instantiation of the function template [...].

After that, it's just usual SFINAE rules.

---

† Atleast I think so, it's not entirely clear in the standard.