Rationale (04/99): In 9.8.4 [namespace.udir] paragraph 4:
If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed.The term entity applied to typedefs refers to the underlying type or class (Clause 6 [basic] , paragraph 3); therefore both declarations of size_t declare the same entity and the above example is well-formed.