[Bug c++/84782] Rejects a maybe C++ code snippet

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84782

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
They don't look like duplicates to me. PR 70431 involves a union, no SFINAE,
and a completely different error message.