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[Bug 19504] [QT3TS] instanceof139

From: <bugzilla@jessica.w3.org>
Date: Fri, 12 Oct 2012 16:12:21 +0000
To: public-qt-comments@w3.org
Message-ID: <bug-19504-523-aU3qirVX4b@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=19504

--- Comment #3 from Tim Mills <tim@cbcl.co.uk> ---
I've just looked back at the formal semantics.

The judgment

statEnv |- Type1 <: Type2

holds if the first type is a subtype of the second.

Semantics

This judgment is true if and only if, for every value Value, if Value matches
Type1 holds, then Value matches Type2 also holds.

You say

"it's obvious to you and me that the set of instances of union(A,B) is a subset
of the instances of union(A, B, C, D)"

and thus this subtype relationship has to hold.  Hence I think we have a
problem.

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