'switch' when out of date

[John Arbash Meinel]

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I've started working with a lightweight checkout, pointing at various
branches. I've started getting this error from time to time:

bzr: ERROR: cannot switch threads with an out of date tree. Please run
bzr update.

Could someone clarify why we can't switch if the tree is not up-to-date?
We know the working tree's current revision, and that seems trivial to
use that rather than the branch's last revision when we do the switch.

Is it something to do with not wanting changes to go missing? Perhaps in
a heavy-checkout something could happen, but not really in a lightweight.

John
=:->

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