c++ - std::reference_wrapper 上的类型特征

Question

std::reference_wrapper无论包含的类型如何（它只是内部的一个指针），都应该是可复制构造的。但是，当断言使用

static_assert(std::is_copy_constructible<std::reference_wrapper<std::any> >::value, "!");

Clang 以 STL 内部的编译错误终止，而 GCC 没有报告任何问题。其他类型不是这种情况（我直接测试过std::reference_wrapper<int>）std::any。

我有一个示例显示在https://godbolt.org/z/7YwkWb的 GCC 和 Clang 中的编译。

据我了解，即使给定的类型std::is_copy_constructible不是可复制构造的，它也应该只给出编译时错误，而不是编译失败。这里有什么问题？这是编译器错误还是我遗漏了什么？

Clang的编译错误是：

In file included from <source>:1:
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:132:31: error: no member named 'value' in 'std::is_copy_constructible<std::reference_wrapper<std::any> >'
    : public conditional<_B1::value, _B2, _B1>::type
                         ~~~~~^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/any:170:17: note: in instantiation of template class 'std::__and_<std::is_copy_constructible<std::reference_wrapper<std::any> >, std::is_constructible<std::reference_wrapper<std::any>, const std::reference_wrapper<std::any> &> >' requested here
      enable_if<__and_<is_copy_constructible<_Tp>,
                ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/any:175:5: note: in instantiation of template type alias '__any_constructible' requested here
    using __any_constructible_t =
    ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/any:181:56: note: in instantiation of template type alias '__any_constructible_t' requested here
              __any_constructible_t<_Tp, _ValueType&&> = true,
                                                       ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/any:183:7: note: while substituting prior template arguments into non-type template parameter [with _ValueType = const std::reference_wrapper<std::any> &, _Tp = std::reference_wrapper<std::any>, _Mgr = std::any::_Manager_internal<std::reference_wrapper<std::any> >]
      any(_ValueType&& __value)
      ^~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:921:56: note: while substituting deduced template arguments into function template 'any' [with _ValueType = const std::reference_wrapper<std::any> &, _Tp = (no value), _Mgr = (no value), $3 = (no value), $4 = (no value)]
      : public __bool_constant<__is_constructible(_Tp, _Args...)>
                                                       ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:933:14: note: in instantiation of template class 'std::is_constructible<std::reference_wrapper<std::any>, const std::reference_wrapper<std::any> &>' requested here
    : public is_constructible<_Tp, const _Tp&>
             ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:939:14: note: in instantiation of template class 'std::__is_copy_constructible_impl<std::reference_wrapper<std::any>, true>' requested here
    : public __is_copy_constructible_impl<_Tp>
             ^
<source>:12:20: note: in instantiation of template class 'std::is_copy_constructible<std::reference_wrapper<std::any> >' requested here
static_assert(std::is_copy_constructible<std::reference_wrapper<std::any> >::value, "!");
                   ^
In file included from <source>:1:
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:137:31: error: no member named 'value' in 'std::is_copy_constructible<std::reference_wrapper<std::any> >'
    : public conditional<_B1::value, __and_<_B2, _B3, _Bn...>, _B1>::type
                         ~~~~~^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/any:192:27: note: in instantiation of template class 'std::__and_<std::is_copy_constructible<std::reference_wrapper<std::any> >, std::__not_<std::is_constructible<std::reference_wrapper<std::any>, const std::reference_wrapper<std::any> &> >, std::__not_<std::__is_in_place_type<std::reference_wrapper<std::any> > > >' requested here
              enable_if_t<__and_<is_copy_constructible<_Tp>,
                          ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/any:196:7: note: while substituting prior template arguments into non-type template parameter [with _ValueType = const std::reference_wrapper<std::any> &, _Tp = std::reference_wrapper<std::any>, _Mgr = std::any::_Manager_internal<std::reference_wrapper<std::any> >]
      any(_ValueType&& __value)
      ^~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:921:56: note: while substituting deduced template arguments into function template 'any' [with _ValueType = const std::reference_wrapper<std::any> &, _Tp = (no value), _Mgr = (no value), $3 = (no value)]
      : public __bool_constant<__is_constructible(_Tp, _Args...)>
                                                       ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:933:14: note: in instantiation of template class 'std::is_constructible<std::reference_wrapper<std::any>, const std::reference_wrapper<std::any> &>' requested here
    : public is_constructible<_Tp, const _Tp&>
             ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:939:14: note: in instantiation of template class 'std::__is_copy_constructible_impl<std::reference_wrapper<std::any>, true>' requested here
    : public __is_copy_constructible_impl<_Tp>
             ^
<source>:12:20: note: in instantiation of template class 'std::is_copy_constructible<std::reference_wrapper<std::any> >' requested here
static_assert(std::is_copy_constructible<std::reference_wrapper<std::any> >::value, "!");
                   ^
2 errors generated.
Compiler returned: 1

标签： c++gccclangc++17

Answer 1

您看到的错误在顶部清楚地表明std::is_copy_constructible没有value您检查的专业化成员。现在，您传递的类型不会违反 的合同std::is_copy_constructible，因此不存在未定义行为的风险。

这仅意味着一件事，它不是编译器问题，而是标准库实现问题。Clang 使用默认安装的标准库实现，在系统godbolt上运行的是libstdc++（GNU 实现）。由于某些可能的软不兼容，这显然会导致问题。

如果您在使用 Clang 构建时指定-stdlib=libc++选项（libc++是标准库的 LLVM 实现），那么它可以正常接受代码。