Forward declaration for scoped allocator customization traits.

[Daniel James]

A container currently needs to include all of scoped_allocator.hpp just to customize the appropriate traits which seems a bit much. Could you declare (not define) constructible_with_allocator_suffix and constructible_with_allocator_prefix in a separate header so that they can be specialised without including all the other things?

It might be useful to include a definition of allocator_arg_t in case it needs to write a constructor using it.

[Ion Gaztañaga]

Fixed in trunk at revision 79463