A meets the proto-allocator
CopyConstructible (C++Std [copyconstructible]),
Destructible (C++Std [destructible]),
the allocator requirements (C++Std [allocator.requirements]), where
U is an object type. [Note:
the proto-allocator requirements but not the allocator requirements. —end
note] No constructor, comparison operator, copy operation, move
operation, or swap operation on these types shall exit via an exception.