Rationale:

Not a defect. [Originally classified as a defect, later reclassified.] basic_string, unlike the other standard library template containers, is severely constrained by its use of char_traits. Those types are dictated by the traits class, and are far from implementation defined.

In Morristown we changed the size_type and difference_type typedefs for all the other containers to implementation defined with a reference to [container.requirements]. This should probably also have been done for strings.