Extended Description

This doesn't compile:

#include
#include
#include

template
struct Alloc : std::allocator
{
template struct rebind { using other = Alloc; };
Alloc(int id) : id(id) { }
template Alloc(const Alloc& a) : id(a.id) { }
int id;
};

using string = std::basic_string<char, std::char_traits, Alloc>;
using stringbuf = std::basic_stringbuf<char, std::char_traits, Alloc>;

int main()
{
string s(Alloc(1));
stringbuf b(s);
assert( b.str().get_allocator() == s.get_allocator() );
}

The basic_stringbuf constructor default-initializes its basic_string member, which default-initializes its allocator.