[Bug 24569] Least common types and lattices

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24569

--- Comment #2 from C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com> ---
A simple case for which two XSD simple types do not have a least common
supertype expressible in XSD:  given simple types A and B, the union of A and B
is a supertype (as defined by the XDM / XPath substitution rules, and also by
XSD substitutability), and so is the union of B and A (which is distinct in
XSD, since unions are ordered).

For a few minutes today I thought that it might be the case that function items
might also lack a least common supertype, but I have come to believe that for
any functions involving simple types that do have least common supertypes and
greatest common subtypes, there will be a least common supertype.  However,
when functions are defined on unions, the problem mentioned above will prevent
a unique solution.

The only caveat here is that the example of union(A,B) and union(B,A) applies
to types as they are defined by XSD.  Whether it also applies to XSD types as
they occur filtered through the XDM spec is currently beyond my ken.  

See also bug 24568 against XDM.

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Received on Friday, 7 February 2014 02:50:25 UTC