https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101485
--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> --- Gašper has confirmed that the intention is for it to be SFINAE-friendly, so declcall would work here (and for [comparisons.general], [comparisons.three.way], and [range.cmp], although P2434 might make it unnecessary for those).