allocate_shared using fast_pool_allocator results in member vector iterator memory corruption on MSVC

[esas]

Reproducer:

#include <vector>
#include <boost/pool/pool_alloc.hpp>

struct TestStruct {
    std::vector<int> vec;
};

int main() {
    //std::allocator<TestStruct> allocator; // works
    boost::fast_pool_allocator<TestStruct> allocator;
    auto test = std::allocate_shared<TestStruct>(allocator);
    test->vec.push_back(1);
    auto iter = test->vec.begin();
    auto val = *iter;
}

When dereferencing iter it will assert "vector iterator not dereferencable" on MSVC (using 2015 Community Edition) everytime on 64-bit and sporadically on 32-bit.

If you put a break point (or break after the assert) and check

"iter" -> "[Raw View]" -> "std::_Vector_const_iterator ..." -> "std::_Iterator012 ..." -> "std::_Iterator_base12" -> "_Myproxy" -> "_Mycont" -> "_Myproxy"

you can see that the _Myproxy of _Mycont is "0xcccccccccccccccc" (uninitialized) when it should point to the _Myproxy of std::_Iterator_base12, forming a loop (which is the case when using std::allocator for the allocation). Note that the times it works when you compile on 32-bit the memory still seems to be corrupted (it's just not set to "0xcccccccccccccccc").