Rationale:

Recommend NAD. operator+() with string already requires the desired semantics of copying the allocator from one of the strings (lhs when there is a choice).

1. The essence of the problem.

User-defined allocators without default constructor are not explicitly supported by the standard but they can be supported just like std::vector supports elements without default constructor.

As a result, there exist implementations that work well with such allocators and implementations that don't.

2. The cause of the problem.

1) The standard doesn't explicitly state this intent but it should. In particular, 20.1.5p5 explicitly state the intent w.r.t. the allocator instances that compare non-equal. So it can similarly state the intent w.r.t. the user-defined allocators without default constructor.

2) Some container operations are obviously underspecified. In particular, 21.3.7.1p2 tells:

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator> operator+(
    const charT* lhs,
    const basic_string<charT,traits,Allocator>& rhs
  );

Returns: basic_string<charT,traits,Allocator>(lhs) + rhs.

That leads to the basic_string<charT,traits,Allocator>(lhs, Allocator()) call. Obviously, the right requirement is:

Returns: basic_string<charT,traits,Allocator>(lhs, rhs.get_allocator()) + rhs.

It seems like a lot of DRs can be submitted on this "Absent call to get_allocator()" topic.

3. Proposed actions.

1) Explicitly state the intent to allow for user-defined allocators without default constructor in 20.1.5 Allocator requirements.

2) Correct all the places, where a correct allocator object is available through the get_allocator() call but default Allocator() gets passed instead.

4. Code sample.

Let's suppose that the following memory pool is available:

class mem_pool {
      // ...
      void* allocate(size_t size);
      void deallocate(void* ptr, size_t size);
};

So the following allocator can be implemented via this pool:

class stl_allocator {
      mem_pool& pool;

 public:
      explicit stl_allocator(mem_pool& mp) : pool(mp) {}
      stl_allocator(const stl_allocator& sa) : pool(sa.pool) {}
      template <class U>
      stl_allocator(const stl_allocator<U>& sa)  : pool(sa.get_pool()) {}
      ~stl_allocator() {}

      pointer allocate(size_type n, std::allocator<void>::const_pointer = 0)
      {
       return (n!=0) ? static_cast<pointer>(pool.allocate(n*sizeof(T))) : 0;
      }

      void deallocate(pointer p, size_type n)
      {
       if (n!=0) pool.deallocate(p, n*sizeof(T));
      }

      // ...
};

Then the following code works well on some implementations and doesn't work on another:

typedef basic_string<char, char_traits<char>, stl_allocator<char> > 
  tl_string;
mem_pool mp;
tl_string s1("abc", stl_allocator<int>(mp));
printf("(%s)\n", ("def"+s1).c_str());

In particular, on some implementations the code can't be compiled without default stl_allocator() constructor.

The obvious way to solve the compile-time problems is to intentionally define a NULL pointer dereferencing default constructor

stl_allocator() : pool(*static_cast<mem_pool*>(0)) {}

in a hope that it will not be called. The problem is that it really gets called by operator+(const char*, const string&) under the current 21.3.7.1p2 wording.