If one alternative is larger than the others and it has an enum tag or pointer inside of it, such that you can squeeze the other alternatives before/after that

If one alternative is larger than the others and it has an enum tag or pointer inside of it, such that you can squeeze the other alternatives before/after that tag word, you're good to go. If you had multiple alternatives of maximum size, they'd need some word in them, at the same offset/size, such that they don't have conflicting representations. An enum tag could overlap with a non-null pointer, and two enum tags with user-declared tag values (such that they do not overlap) could work too. Or if you're crazy you could discard having each type's representation be computed solely as a function of what the type is made of, choosing their representations based on how well they pack into other types. (Or worse yet, if you've got a borrow checker and your types are memcpyable, you could pack things and rejigger bytes however you wanted and then unfurl them into a temporary whenever something uses the interior value, unless I'm overlooking something. So if you had Either<Either<u32, u32>, Either<u32, u32>>, you could represent that with a single tag whose value is 0, 1, 2, or 3, and when if the tag's 2 or 3 and you make a reference to the interior value, either fix the tag in-place and fix it back when you're done (if you're the sole owner of the Either<Either...>), or copy the value out and let the borrower borrow that (and copy it back in when it's done, if it was a mutable borrow).)