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.
