fusion::pair not compatible with std::is_default_constructible

[crmoore@…]

When the Second type of pair is not default constructible, std::is_default_constructible either returns true or fails to compile for fusion::pair.

This is on OSX with apple and macports compilers.

Adding enable_if< is_default_constructible<Second> > to pair's default constructor makes this work as expected.

struct NotDefaultConstructible {
  NotDefaultConstructible(int){};
};

// prints 0
std::cout << std::is_default_constructible< NotDefaultConstructible >::value << std::endl;

using fusion_pair1 = boost::fusion::pair<int, NotDefaultConstructible>;

// should print 0
// fails to compile on clang (xcode 7, 3.4, 3.6, 3.7), gcc 4.9
// prints 1 on gcc 5, clang 3.4
std::cout << std::is_default_constructible< fusion_pair1 >::value << std::endl;

Test and patch attached. Patch is unlikely to meat portability requirements to be applied as-is.

[crmoore@…]

Test app