Why MacOS clang can't use C++17 std? - c++

Question

I want to use c++1z in MacOS10.14.4, like this g++ -std=c++1z test.cpp -o test. But the clang can't compile the code.
The error as follows.
In file included from test.cpp:3:
/Library/Developer/CommandLineTools/usr/include/c++/v1/any:599:5: error: static_assert failed due to requirement 'is_constructible<basic_string<char> &&, _RawValueType
&>::value' "ValueType is required to be an lvalue reference or a CopyConstructible type"
static_assert(is_constructible<_ValueType, _RawValueType &>::value,
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:29:19: note: in instantiation of function template specialization 'std::__1::any_cast<std::__1::basic_string<char> &&>' requested here
auto b = std::any_cast<std::string&&>(a); //< rvalue reference (no need for std::move)
^
1 error generated.
But the same code can work on Linux. The code as follows.
#include <string>
#include <iostream>
#include <any>
int main()
{
// simple example
auto a = std::any(12);
std::cout << std::any_cast<int>(a) << '\n';
try {
std::cout << std::any_cast<std::string>(a) << '\n';
}
catch(const std::bad_any_cast& e) {
std::cout << e.what() << '\n';
}
// advanced example
a = std::string("hello");
auto& ra = std::any_cast<std::string&>(a); //< reference
ra[1] = 'o';
std::cout << "a: " << std::any_cast<const std::string&>(a) << '\n'; //< const reference
auto b = std::any_cast<std::string&&>(a); //< rvalue reference (no need for std::move)
// Note, 'b' is a move-constructed std::string, 'a' is now empty
std::cout << "a: " << *std::any_cast<std::string>(&a) //< pointer
<< "b: " << b << '\n';
}
clang version: Apple LLVM version 10.0.1 (clang-1001.0.46.4)
And I use the gcc to compile this code, but didn't work either.

Answer 1

This seems to be from https://developercommunity.visualstudio.com/content/problem/200281/stdany-cast-static-assert-error-because-of-is-cons.html, which has an answer from Microsoft. The answer there was that this was a defect LWG 2768, so the fix may not have been implemented in older implementations of the standard library. For example, this compiles in clang 6.0.0 and gcc 7.4, but not clang 7.0.0 and gcc 8.1.
The reason is that you can't take an rvalue reference of an lvalue any. Fix this by either taking an lvalue reference and moving that or taking an rvalue reference of a moved any:
std::move(std::any_cast<std::string&>(a));
std::any_cast<std::string&&>(std::move(a));