https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592
--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
{} as a template argument is currently only supported by GCC as an extension,
but I raised this on the core C++ list and it seems that the the conclusion is
that we want this to work, though there's no CWG for it yet.
