[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
"is_nothrow_invocable_v<CompletionFunction&> shall be true."

Failing to meet that requirement is undefined, so we're not required to reject
it as ill-formed.