[PATCH] D144503: [WIP][STLExtras] Allow `llvm::enumerate` to enumerate over multiple ranges

[Jakub Kuderski via Phabricator via llvm-commits]

kuhar added a comment.

In D144503#4142656 <https://reviews.llvm.org/D144503#4142656>, @dblaikie wrote:

>> C++17 does not allow for recursive expansion of structured bindings.
>
> Could we address this issue more directly, perhaps - like having a "flatten" (maybe it only goes one level deep - a range over pair or tuple and any of the elements of that tuple that are themselves tuples would be flattened into one big tuple (but only that one layer deep - could use the wrapper multiple times if you wanted to flatten even deeper))?

Can you give an example? I don't see how this would work. My starting point is that I would like to enable this: `for (auto [Idx, X, Y, Z] : ...(Xs, Ys, Zs)...)`.

If we tried to compose `enumerate` and `zip_equal`, we would end up with `flatten` used like this: `for (auto [Idx, X, Y, Z] : flatten(enumerate(zip_equal(Xs, Ys, Zs))))`. Here flatten would have to a `map_range`-style range adaptor, and I think we would lose `it.index()`?


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