An implementation may derive any class in the C++ standard library from a class with a name reserved to the implementation.
Certain classes defined in the C++ standard library are required to be derived from other classes in the C++ standard library. An implementation may derive such a class directly from the required base or indirectly through a hierarchy of base classes with names reserved to the implementation.
In any case:
Every base class described as non-virtual shall not be virtual;
Unless explicitly stated otherwise, types with distinct names shall be distinct types.183
All types specified in the C++ standard library shall be non-final types unless otherwise specified.
There is an implicit exception to this rule for types that are described as synonyms for basic integral types, such as size_t ([support.types]) and streamoff ([stream.types]).