std :: visit for variant无法在clang 5下编译

问题描述

std :: visit的以下用法在gcc 7.2下可正确编译，但在clang 5.0下无法编译。有人知道问题出在哪里吗？

The following uses of std::visit compiles properly under gcc 7.2 but fails to compile under clang 5.0. Does anyone know what the problem is?

#include <variant>

struct S1 {int foo() { return 0; }};
struct S2 {int foo() { return 1; }};

using V = std::variant<S1, S2>;

int  bar() {
  V v;
  return std::visit([](auto& s) { return s.foo(); }, v);
}

第一个错误是：

include/c++/7.2.0/variant:238:46: error: cannot cast 'std::variant<S1, S2>' to its private base class
  'std::__detail::__variant::_Variant_storage<true, S1, S2>'
  return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);

以下是godbolt的链接显示此错误：

Here is a link to godbolt shows this error: https://godbolt.org/g/5iaKUm

推荐答案

这是已知的，似乎只影响libstdc ++的 std :: variant （以及其他使用相同组合的构造）。问题与模板的朋友功能有关-请参见线程。

This is known bug 33222 that seems to only affect libstdc++'s std::variant (and other constructs using the same combination). The problem is related to friend function to templates - see the thread for more detail.

libc ++的变体似乎没有使用libstdc ++所使用的朋友技术，因此，您可能希望暂时将其更改为libc ++。

The variant from libc++ doesn't seem to use the technique of friends that libstdc++ used, so you might want to temporarily change to libc++ in the meantime.

小更新：此问题已在最新的主干中修复。

Small update: This has been fixed in the latest trunk.

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