[PATCH] D151952: [clang] adds `__type_pack_index` so we can get a type's parameter pack index

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

Taking a step back for a moment: what is the intended use case for this? My concern is that most of the time you're going to want to make O(N) such queries into a pack of N elements, resulting in O(N^2) compile time, much like we regrettably get from uses of `__type_pack_element`. If we can avoid the regret in this case by providing a different intrinsic, perhaps we should do so. (For example, we could take two packs of types, and produce a sequence of integers giving the indexes of each of the first types in the second list, in O(N) time. And similarly we could add a `__type_pack_elements` that takes a pack of types and a pack of indexes and performs N indexing operations at once, in O(N) time, rather than having the program do it in O(N^2) time.)


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https://reviews.llvm.org/D151952