type_traits must assert type completeness for some traits

[Antony Polukhin]

Without that check hard detectable errors are possible:

#include <boost/type_traits.hpp>

struct foo;

void kill_sanity() {
    (void)boost::is_constructible<foo, foo>::value;
}

struct foo{};

int main() {
    static_assert(boost::is_constructible<foo, foo>::value, "foo must be constructible from foo"); // Ooops
}

All the is_*constructible, is_base_of, is_convertible, is_destructible, is_*swappable, has_virtual_destructor traits must assert completeness.

Solution could be to create an asserting structure (pseudocode):

template <class T>
struct assert_complete {
    template <class U> yes_type test(mpl::int_<sizeof(U)>* = 0);
    template <class U> no_type test(...);

    static_assert(sizeof(test<T>(0)) == sizeof(yes_type), "T must be a complete type!");
};

[John Maddock]

Nod, there were a few traits which gave incorrect results when instantiated with an incomplete type, fixed in ​https://github.com/boostorg/type_traits/commit/ac351390b215b73d8a9e16d2ca37ba2d9d88a14f (plus a few subsequent commits for older compiler workarounds).