Detecting constexpr with SFINAE

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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 <type_traits>

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

template<class T>
struct has_constexpr_f : decltype(detail::check<T>(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.

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