How to detect the presence of a static member function with certain signature?

Question

I found several questions & answers on SO dealing with detecting at compile time (via SFINAE) whether a given class has a member of certain name, type, or signature. How

Answer 1

Simply invoke the member function and discard the result in a SFINAE context. If it succeeds, the method exists. If it fails, the method does not.

// not needed in C++1y
templateusing enable_if_t=typename enable_if::type;

// If the other tests fail, the type T does not have a method `foo` of
// signature Sig.  The class=void parameter is an implementation detail
// that in an industrial quality implementation we would hide in a helper
// template type.
templatestruct has_foo:std::false_type{};

// For R(Args...), we attempt to invoke `T::foo` with (Args...), then check
// if we can assign the return value to a variable of type R.
template
struct has_foo()... ) ),
      R
    >::value
    && !std::is_same::value
  >
>: std::true_type {};
// for `void` return value, we only care if the function can be invoked,
// no convertible test required:
template
struct has_foo()... ) ) )
>: std::true_type {};

use:

 has_foo< bar, int(int) >::value

which checks if int r = T::foo( 7 ) is a valid expression, and not for exact signature match.