[PATCH] D22557: [libcxx] Diagnose invalid memory order arguments in <atomic>. Fixes PR21179.

[Eric Fiselier via cfe-commits]

EricWF created this revision.
EricWF added reviewers: mclow.lists, rsmith.
EricWF added subscribers: cfe-commits, rsmith.

This patch uses the __attribute__((enable_if)) hack suggested by @rsmith to diagnose invalid arguments when possible.

In order to diagnose an invalid argument `m` to `f(m)` we provide an additional overload of `f` that is only enabled when `m` is invalid. When that function is enabled it uses __attribute__((unavailable)) to produce a diagnostic message.

https://reviews.llvm.org/D22557

Files:
  include/atomic
  test/libcxx/atomics/diagnose_invalid_memory_order.fail.cpp

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