lrwxr-xr-x 1 martin wheel 12 Jan 31 13:19 ./9.0/usr/include/g++/bits/gthr-default.h@ -> gthr-posix.h
-rwxr-xr-x 1 martin wheel 0 Jan 31 13:19 ./updated/usr/include/g++/bits/gthr-default.h*
Indeed, I have an empty (executable)
/usr/include/g++/bits/gthr-default.h regular file. Replacing it
with a symbolic link to gthr-posix.h seems to work (at least your
previous example compiles now).
Sounds like a bsdtar bug :-( which also explains why it did not happen
for your beta updates.
Then it would be nice to have it fixed before 9.0.