std::reference_wrapper应该是可复制构造的，而不管所包含的类型如何（它只是内部的指针）。但是，当断言使用
static_assert(std::is_copy_constructible<std::reference_wrapper<std::any> >::value, "!");
Clang在STL内部终止并出现编译错误，而GCC报告没有问题。其他类型则不是这种情况（我直接测试了std::reference_wrapper<int>和std::any）。
我有一个示例，显示了https://godbolt.org/z/7YwkWb中使用GCC和Clang进行编译的情况。
据我了解，即使给std::is_copy_constructible的类型不是可复制构造的，它也应该给出一个编译时错误，而不是编译失败。
这里有什么问题？那是编译器错误还是我缺少什么？
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
答案 0 :(得分：3)
您看到的错误在顶部清楚地表明std::is_copy_constructible没有要检查的专业化的value成员。现在，您传递的类型不会违反std::is_copy_constructible的约定，因此不存在未定义行为的风险。
这仅意味着一件事，这不是编译器问题，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）。这显然是由于某些可能的软件不兼容造成的。
如果在使用Clang进行编译时指定了-stdlib=libc++选项（libc++是标准库的LLVM实现），那么它将接受代码。