[Bug c++/81975] Unpacking two packs via alias erroneously complains about mismatched argument packs

[griwes at griwes dot info]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81975

--- Comment #1 from Michał Dominiak <griwes at griwes dot info> ---
I have made a mistake in the original testcase in the original email, here's a
correction email sent to the reflector:
http://lists.isocpp.org/core/2017/08/2828.php.

Barry's minimization of the failing code already eliminates the mistake; I'm
just providing the reply to make sure there's no confusion about the
differences between the original testcase and Barry's version.

Now, an interesting artifact of this is that both T and V fail, but U somehow
succeeds, which makes it seem like the compiler is sometimes *assuming* that
the size of a pack is 1; this'd be consistent with the Clang error message,
where it always says that one of the packs has just one element.