[Bug 26453] [xp3.1] Lowest common supertype

https://www.w3.org/Bugs/Public/show_bug.cgi?id=26453

--- Comment #1 from Michael Kay <mike@saxonica.com> ---
I propose we make "lowest common supertype" a defined term and define it as
follows:

Given two atomic types A and B, the lowest common supertype of A and B is the
atomic type determined as follows:

* If A and B are derived from the same primitive type, then the lowest common
ancestor of A and B in the tree formed by the itemType-subType relation over
atomic types.

* If A and B are derived from different primitive types P and Q, and P is
promotable to Q, then Q (or vice-versa, if Q is promotable to P, then P). The
only type pairs that are promotable in this way are decimal to float, float to
double, decimal to double, and anyURI to string.

* If A and B are derived from different primitive types P and Q, and P is not
promotable to Q and Q is not promotable to P, then xs:anyAtomicType.

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Received on Tuesday, 16 September 2014 19:26:28 UTC