After doing a binary search on debian/patches/series (on amd64), it
looks like the culprit for this bug is pr33763.diff. And looking at
that Bugzilla report, it certainly makes sense that this would affect
s_isnan.i, which contains an always_inline definition of __isnan
followed by a non-inline definition.

I've also created a reduced test case, attached.
--
Daniel Schepler