https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64697
--- Comment #19 from Václav Haisman <vhaisman at gmail dot com> --- There appears to be some sort of interaction with the `inline` attribute of the `get_ptd()` function. If the `get_ptd()` function is just declared `extern` in `def.hxx` and defined in `def.cxx`, the link error goes away.