Trick to allow incomplete types in templates?

Question

C++ STL containers don't allow instantiation with incomplete types; it is undefined behavior.

Is this a valid "trick" to get around that restriction? Or does this trick still have undefined behavior?

#include <vector>

template<template<class, class> class Vector = std::vector>
struct my_incomplete_vector
{
    struct Element;

    // Element is incomplete here, but does it matter anymore?
    typedef Vector<Element, std::allocator<Element> > type;

    struct Element { typename type::iterator value; };
};

int main()
{
    my_incomplete_vector<>::type v;
    v.resize(1);

    // this isn't normally possible without incomplete types
    v[0].value = v.begin();
    return 0;
}

Answer 1

It's undefined behavior. The standard requires a type to be complete if it is used as the argument of a template, at the point where the template is instantiated. And my_incomplete_vector::Element is not complete when you use it inside Element. No problems will occur until you actually instantiate your template, of course, but g++ fails to compile your code with the usual debugging options (-D_GLIBCXX_CONCEPT_CHECKS -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC).