[libcxx-commits] [PATCH] D149292: [libc++] Fix ODR violation with placeholders

[Louis Dionne via Phabricator via libcxx-commits]

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In D145589 <https://reviews.llvm.org/D145589>, we made the std::bind placeholders inline constexpr to
satisfy C++17. It turns out that this causes ODR violations since the
shared library provides strong definitions for those placeholders, and
the linker on Windows actually complains about this.

Fortunately, C++17 only encourages implementations to use `inline constexpr`,
it doesn't force them. So instead, we unconditionally define the placeholders
as `extern const`, which avoids the ODR violation and is indistinguishable
from `inline constexpr` for most purposes, since the placeholders are
empty types anyway.

Note that we could also go back to the pre-D145589 state of defining them
as non-inline constexpr variables in C++17, however that is definitely
non-conforming since that means the placeholders have different addresses
in different TUs. This is all a bit pedantic, but all in all I feel that
`extern const` provides the best bang for our buck, and I can't really
find any downsides to that solution.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149292

Files:
  libcxx/include/__functional/bind.h
  libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.place/placeholders.pass.cpp

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