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http://dev.w3.org/2006/webapi/WebIDL/#defineownproperty [[ If creating is false and O does not implement an interface with a named property setter, then Reject. If creating is false or O implements an interface with a named property creator, then: ]] The algorithm doesn't say what to do (presumably Reject?) when creating is true and O implements an interface without a named property creator
When 'creating' is true, you do neither 2.2.1 nor 2.2.2. Which means you continue on to steps 3 and 4. Just a naive translation of this algorithm into (pseudo-)code gives that.
oops, indeed, sorry for the noise