decltype - C++11 - emplacing a variable to any std container (vector, list, set, unordered_set)

Trying to write a generic code to emplace a default constructed object into an std container and return an iterator to it.

with insert the solution is

template
typename C::iterator insert(C& container)
{
    return container.insert(container.end(), typename C::value_type());
}

But emplace has no such common interface so I had to specialize it for 2 container types.
My best attempt (that did not compile) was:

template
auto emplace(C& container)
    -> decltype(container.emplace_back())
{
    return container.emplace_back();
}


template
auto emplace(C& container)
    -> decltype(container.emplace().first)
{
    return container.emplace().first;
}

This causes: error: void value not ignored as it ought to be

and I don't understand why my decltype was deduced as void.

Used gcc 4.8.5 with -std=c++11

Edit:

reproduced by

int main()

{
std::vector v;
std::vector::iterator iv = emplace(v);

    std::set s;
    std::set::iterator is = emplace(s);
    return 0;

}

Edit 2:

second attempt

template
auto emplace(C& container)
    -> decltype(container.emplace().first)
{
    return container.emplace().first;

}

template
auto emplace(C& container)
    -> decltype(container.emplace(container.end()))
{
    return container.emplace(container.end());
}

works for verctor, but set is ambiguous.

Answer

Finally solved it thanks to @Quentin:

// For vector, list and anything push_backable
template
auto emplace(C& container)
    -> decltype(container.emplace_back(),container.rbegin())
{
    container.emplace_back();

    return container.rbegin();
}

// For set,unordered_set, and anything that emplace returns a pair.
template
auto emplace(C& container)
    -> decltype(container.emplace().first)
{
    return container.emplace().first;
}