This issue was originally reported by Sandy Gao.

There are a number of areas where the recommendations state certain
rules, but there are no corresponding constraints for them, hence
there are no proper error codes.

In addition, there should be an explicit constraint that all schemas
must conform to the schema for schemas, thus providing an error code
to refer to.

See
http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2003Jun/0088.html

See also
http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2002Nov/0237.html

This item was classified in the meeting of 2003-10-24
(http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2003Oct/0085.html)

This item was discussed in the meeting of 2003-11-04
(http://www.w3.org/XML/Group/2003/11/xml-schema-ftf-minutes.html)

This item was discussed, and phase-1 agreement was reached, in the
meeting of 2004-03-18
(http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2004Mar/0060.html).

The underlying issue applies to both Structures and Datatypes.  This
Bugzilla entry is for Structures.  According to the requirements
document, phase-1 agreement has been reached.