This fails to compile if you cannot instanciate the default
constructor T.

The problem showed up with a std::tuple<T> where T is
not default constructible. This is a really nasty issue:
std::tuple<T> implements a default constructor but
instanciating this fails in this case.

As a consequence std::is_constructible<std::tuple<T>> is
true, but trying to instanciate it gives a compile error.
Similar behaviour can be observed with operator<.
At least for the default constructor of std::tuple
this seems to be fixed in C++17 by removing it from the
overload set if you can't instanciate one for T.