77ac36547a
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
84 lines
2.2 KiB
C++
84 lines
2.2 KiB
C++
//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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// <functional>
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// namespace placeholders {
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// // M is the implementation-defined number of placeholders
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// extern unspecified _1;
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// extern unspecified _2;
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// .
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// .
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// .
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// extern unspecified _Mp;
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// }
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// The Standard recommends implementing them as `inline constexpr` in C++17.
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//
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// Libc++ implements the placeholders as `extern const` in all standard modes
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// to avoid an ABI break in C++03: making them `inline constexpr` requires removing
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// their definition in the shared library to avoid ODR violations, which is an
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// ABI break.
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//
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// Concretely, `extern const` is almost indistinguishable from constexpr for the
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// placeholders since they are empty types.
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#include <functional>
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#include <type_traits>
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#include "test_macros.h"
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template <class T>
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TEST_CONSTEXPR_CXX17 void test(const T& t) {
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// Test default constructible.
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{
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T x; (void)x;
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}
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// Test copy constructible.
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{
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T x = t; (void)x;
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static_assert(std::is_nothrow_copy_constructible<T>::value, "");
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static_assert(std::is_nothrow_move_constructible<T>::value, "");
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}
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// It is implementation-defined whether placeholder types are CopyAssignable.
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// CopyAssignable placeholders' copy assignment operators shall not throw exceptions.
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#ifdef _LIBCPP_VERSION
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{
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T x;
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x = t;
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static_assert(std::is_nothrow_copy_assignable<T>::value, "");
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static_assert(std::is_nothrow_move_assignable<T>::value, "");
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}
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#endif
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}
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TEST_CONSTEXPR_CXX17 bool test_all() {
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test(std::placeholders::_1);
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test(std::placeholders::_2);
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test(std::placeholders::_3);
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test(std::placeholders::_4);
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test(std::placeholders::_5);
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test(std::placeholders::_6);
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test(std::placeholders::_7);
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test(std::placeholders::_8);
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test(std::placeholders::_9);
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test(std::placeholders::_10);
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return true;
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}
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int main(int, char**) {
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test_all();
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#if TEST_STD_VER >= 17
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static_assert(test_all(), "");
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#endif
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return 0;
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}
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