Extended Description

Container requirements are all stated in terms of CopyInsertable into X, and Erasable from X, which involves indirection through the container's allocator.

The following test shows a class which can only be constructed/destroyed by its allocator, revealing that std::vector tries to construct temporaries directly without going through the allocator.

/usr/local/libcxx-head/include/c++/v1/vector:1815:24: error: call to deleted constructor of 'value_type' (aka 'X')

#include

struct Tag { };

template
class TaggingAllocator
{
public:
using value_type = T;

TaggingAllocator() = default;

template<typename U>
  TaggingAllocator(const TaggingAllocator<U>&) { }

T*
allocate(std::size_t n) { return std::allocator<T>{}.allocate(n); }

void
deallocate(T* p, std::size_t n) { std::allocator<T>{}.deallocate(p, n); }

template<typename U, typename... Args>
  void
  construct(U* p, Args&&... args)
  { ::new((void*)p) U(Tag{}, std::forward<Args>(args)...); }

template<typename U, typename... Args>
  void
  destroy(U* p)
  { p->~U(); }

};

template<typename T, typename U>
bool
operator==(const TaggingAllocator&, const TaggingAllocator&)
{ return true; }

template<typename T, typename U>
bool
operator!=(const TaggingAllocator&, const TaggingAllocator&)
{ return false; }

struct X
{
// All constructors must be passed the Tag type.

// DefaultInsertable into vector<X, TaggingAllocator>,
X(Tag) { }
// CopyInsertable into vector<X, TaggingAllocator>,
X(Tag, const X&) { }
// MoveInsertable into vector<X, TaggingAllocator>, and
X(Tag, X&&) { }

// EmplaceConstructible into vector<X, TaggingAllocator> from args.
template<typename... Args>
X(Tag, Args&&...) { }

// not DefaultConstructible, CopyConstructible or MoveConstructible.
X() = delete;
X(const X&) = delete;
X(X&&) = delete;

// CopyAssignable.
X& operator=(const X&) { return *this; }

// MoveAssignable.
X& operator=(X&&) { return *this; }

private:
// Not Destructible.
~X() { }

// Erasable from vector<X, TaggingAllocator>.
friend class TaggingAllocator;
};

int main()
{
std::vector<X, TaggingAllocator> v;
v.reserve(3);
v.emplace_back();
v.emplace(v.begin());
v.emplace(v.begin(), 1, 2, 3);
}

// template class std::vector<X, TaggingAllocator>;

Uncommenting the last line to explicitly instantiate the vector fails differently, presumably there are some SFINAE conditions which make invalid assumptions about the type.