https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66146
--- Comment #58 from Iain Sandoe <iains at gcc dot gnu.org> --- As far as I can tell, (at least on targets with TLS support) since the variables __once_callable and __once_call have a single instance per thread, the current implementation does not support nested once_call()s in the same thread either.
