[all-commits] [llvm/llvm-project] 77ac36: [libc++] Fix ODR violation with placeholders
Louis Dionne via All-commits
all-commits at lists.llvm.org
Thu Apr 27 07:57:35 PDT 2023
Branch: refs/heads/main
Home: https://github.com/llvm/llvm-project
Commit: 77ac36547a2d15dc465d723179ce7047a59583ce
https://github.com/llvm/llvm-project/commit/77ac36547a2d15dc465d723179ce7047a59583ce
Author: Louis Dionne <ldionne.2 at gmail.com>
Date: 2023-04-27 (Thu, 27 Apr 2023)
Changed paths:
M libcxx/include/__functional/bind.h
M libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.place/placeholders.pass.cpp
Log Message:
-----------
[libc++] Fix ODR violation with placeholders
In 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.
Differential Revision: https://reviews.llvm.org/D149292
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