Many existing constructors have this form:
map(std::initializer_list<value_type> il, const Compare& comp = Compare(), const allocator_type& a = allocator_type())
The issue is that a temporary allocator_type is constructed, and
passed to the base class where it is used to copy-constructed the
rebound allocator.
This temporary allocator_type here is always destroyed at the end
of the map() constructor. For stateful allocators this is not
desirable.
The solution is to adopt what libc++ is doing and have to constructors:
map(std::initializer_list<value_type> il, const Compare& comp = Compare(), const allocator_type& a)
and
map(std::initializer_list<value_type> il, const Compare& comp = Compare())
This way, unless an allocator is provided by the client, no extra temporary
creation/destruction occurs.