[Bug libstdc++/81381] New: std::basic_stringbuf only works with DefaultConstructible allocators

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81381

            Bug ID: 81381
           Summary: std::basic_stringbuf only works with
                    DefaultConstructible allocators
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

#include <memory>
#include <sstream>
#include <cassert>

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

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

struct SB : std::basic_stringbuf<char, std::char_traits<char>, Alloc<char>>
{
  SB(const string& s) : basic_stringbuf(s) { }
  using basic_stringbuf::overflow;
};

int main()
{
  string s(Alloc<char>(1));
  SB b(s);
  b.overflow('a');
  assert( b.str().get_allocator() == s.get_allocator() );
}