I haven't had the chance to see what is happening inside union-build yet, but
did confirm that propagating expat/fixed instead of expat does not have the
same problem.
I tried with
(define fontconfig-fixed
(package
(inherit fontconfig)
(propagated-inputs
(modify-inputs (package-propagated-inputs fontconfig)
(replace "expat" (@@ (gnu packages xml) expat/fixed))))))
and then (fhs-union (list fontconfig-fixed)) did not have any broken links
trying to go to the wrong version. Using this also works around my original
problem of a multi-arch union build having collisions (so that seems to stem
from whatever is happening here).
