UML22 — String is primitive but has structure.

String is primitive but has structure. Section 17.4 (PrimitiveTypes) has String, even though the definition of primitive type in Section 7.3.43 excludes any structure: "A primitive type defines a predefined data type, without any relevant substructure (i.e. it has no parts). A primitive datatype may have an algebra and operations defined outside of UML, for example, mathematically."

see above