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[Bug 12185] Conditional Type Assignment and substitutability

From: <bugzilla@jessica.w3.org>
Date: Fri, 22 Apr 2011 00:51:26 +0000
To: www-xml-schema-comments@w3.org
Message-Id: <E1QD4ak-0000Zd-1Z@jessica.w3.org>
http://www.w3.org/Bugs/Public/show_bug.cgi?id=12185

--- Comment #5 from C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com> 2011-04-22 00:51:25 UTC ---
A second comment, on a side point.  The bug description says 

    it's very unsatisfactory that problems with the schema should be
    detected during instance validation.

It may be unsatisfactory to find schema errors at instance
validation time (it doesn't bother me much, but I agree that it does
bother some intelligent observers), but violations of Conditional
Type Substitutable are not defined as problems with the schema; they
lead to the determination that the instance is invalid, not to the
determination that the schema is non-conforming.

This is pointed out by the note at the end of 3.4.6.3.

The bug description may be taking the view that the 'real' problem
is in the schema, not the instance, and that by making the
constraint affect instance validity instead of schema conformance
the WG was simply lying to itself.  But that presupposes some clear
accepted rule for deciding what problems are schema problems and
what problems are instance problems; if the WG has ever found
consensus on such a rule, I don't remember it.  Many things in the
spec might be clearer and cleaner if we had.  But in the absence of
such a rule, I think this is an appeal to a Platonic reality of
schemas that is not accessible to most of the WG, let alone to
readers of our spec.

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