https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623
--- Comment #7 from jim x <xmh970252187 at gmail dot com> ---
In a simple way, the rule just requires that, for a function template, the
template parameter that is declared after a template parameter pack should
either appear in the parameter-declaration-clause before the template
pack(deducible) or just have a default argument.
````
auto f(auto..., auto a, auto...) { return a; }
````
IIUC, this is just disallowed since all arguments would only match the first
function parameter pack.
