使用clang和libstdc ++时，不能使用experimental ::的常量引用

Question

我正在不断引用一个实验性的::: optional <>变量，但是当我在其上使用operator->（）时，会出现编译错误，但仅当使用clang ++ 和 libstdc ++时才如此。

#include <experimental/optional>
#include <iostream>
#include <vector>

int main(void)
{
    std::experimental::optional<std::vector<int>> opt;
    const auto &rf = opt;

    opt.emplace();
    opt->push_back(1);

    std::cout << "opt->size() = " << opt->size()
              << " rf->size() = " << rf->size() << "\n";

    return 0;
}

运行该程序：

$ clang++ -W -Wall -std=c++14 -stdlib=libc++ test.cc && ./a.out
opt->size() = 1 rf->size() = 1  # OK

$ g++ -W -Wall -std=c++14 test.cc && ./a.out
opt->size() = 1 rf->size() = 1  # OK

$ clang++ -W -Wall -std=c++14 test.cc
In file included from test.cc:1:
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/experimental/optional:576:16: error: call to '__constexpr_addressof' is ambiguous
      { return __constexpr_addressof(this->_M_get()); }
               ^~~~~~~~~~~~~~~~~~~~~
test.cc:14:40: note: in instantiation of member function 'std::experimental::fundamentals_v1::optional<std::vector<int, std::allocator<int> > >::operator->' requested here
              << " rf->size() = " << rf->size() << "\n";
                                       ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/experimental/optional:173:20: note: candidate function [with _Tp = const std::vector<int, std::allocator<int> >, $1 = <>]
    constexpr _Tp* __constexpr_addressof(_Tp& __t)
                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/experimental/optional:180:17: note: candidate function [with _Tp = const std::vector<int, std::allocator<int> >, $1 = <>]
    inline _Tp* __constexpr_addressof(_Tp& __t)
                ^
1 error generated.

每当我使用常量引用时，都会发生此错误：显然，非常量引用是可以的。

我将Linux Mint 18与以下版本的g ++ / libstdc ++ 5.4.0和clang ++ 3.8.0结合使用：

$ g++ --version
g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ dpkg-query -s libstdc++-5-dev
...
Version: 5.4.0-6ubuntu1~16.04.4
...

$ clang++ --version
clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

这是我的程序，libstdc ++或clang ++中的错误吗？

更新 ：clang 3.9.0解决了问题。 （如下面由Richard Smith和Ville Voutilainen解释的那样，似乎libstdc ++期望编译器以某种与clang 3.8.0不匹配的方式运行。）

Answer 1

在我看来，这肯定像是个Clang错误，因为它诊断为模棱两可的重载具有互斥的enable_if约束，因此它们不可能模棱两可。