Fix the implementation of max_align_t in lang/Align.h.

[brianpl]

Folly's Align.h header overwrites max_align_t, which is usually taken from stdlib. This is to correct an error in legacy implementations of max_align_t, by logically reconstructing the maximum alignment value in a constexpr.

max_align_t depends on the function folly::details::max_align_, which is supposed to implement a recursive max value function using templates. However, the expression

!(a < max_align_(e, es...)) ? a : max_align_(e, es...)

does not recursively compute the maximum value over e and es...

This implementation actually selects the first value of e and es... that is followed by a lower value.

The correct implementation is provided in this pull request.

This error prevents folly from compiling on newer versions of clang, targeting x86, because CachelinePadded.h statically checks max_align_t in an assertion, and then fails, because it thinks that AtomicStruct<LifoSemHead, atomic> is over-aligned.

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[yfeldblum]

Yikes! Thanks for the PR.

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@yfeldblum has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[yfeldblum]

Thanks!

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@yfeldblum merged this pull request in 72c08f7.