C++ std:：reference_wrapper上的类型特征<；标准：：任何>；

std:：reference_wrapper

应该是可复制构造的，而不考虑包含的类型（它只是内部的指针）。然而，当断言使用

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

---

您看到的错误在顶部清楚地指出，

std:：is\u copy\u constructible

没有您检查的专门化的

值成员。现在，您传递的类型不会违反
std:：is\u copy\u constructible
的约定，因此不存在未定义行为的风险

这意味着只有一件事，这不是编译器的问题，而是标准库实现的问题。Clang使用默认安装的标准库实现，该实现在运行的系统
godbolt
上是
libstdc++
（GNU实现）。由于某些可能的软不兼容性，这显然会导致问题

如果在使用Clang构建时指定
-stdlib=libc++
选项（
libc++
是标准库的LLVM实现），则它可以接受代码。请在问题中包含编译器错误。关于“…而不是编译失败”：静态断言的全部要点是在编译时失败，如果您只想查看值，请不要将其放入静态断言用户463035818：您完全正确。但是Clang中的编译不会失败，因为
静态断言失败。如果我删除
static\u assert
或否定参数，它也会失败。问题是，对
static\u assert
的（constexpr）参数求值时出错。是的，我只是认为
static\u assert
有点模糊，您的句子有点错误/有点误导。如果您给
std:：is_copy_constructible
的类型不是copy constructible，则编译时将为false，并且代码将在编译时失败（因为静态断言），这是有意义的。我没有想到STL。然而：Clang难道不能用标准库的GNU实现进行编译吗？@ephimetheus——通常是这样。每个标准库版本都针对一个编译器进行了更多的定制。有时候，它会引起问题，就像你看到的。
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