static_cast，带有显式右值转换运算符

Question

I am writing a simple wrapper class, and I want to provide explicit conversion operators to the wrapped type. The following code compiles fine with gcc

class wrap
{
    double value;
public:
    explicit wrap(double x) : value(x) {}
    explicit operator double&&() && { return std::move(value); }
};

int main() {
    wrap w(5);
    double && x (std::move(w) ); //ok
    double && y = static_cast<double&&>(std::move(w)); //clang reports an error here
}

But clang reports an error: cannot cast from lvalue of type 'typename std::remove_reference<wrap &>::type' (aka 'wrap') to rvalue reference type 'double &&'; types are not compatible error: cannot cast from lvalue of type 'typename std::remove_reference<wrap &>::type' (aka 'wrap') to rvalue reference type 'double &&'; types are not compatible .

As far as I know (see the latest draft, 5.2.9 §4 ) static_cast<T>(e) has the same semantic has T t(e) , but clang does not refuse the latter.

Which compiler is right?

Answer 1

This is clang bug 19917 . From the section of the standard you mentioned, §5.2.9/4:

An expression e can be explicitly converted to a type T using a static_cast of the form static_cast<T>(e) if the declaration T t(e); is well-formed, for some invented temporary variable t .

In this case, T t(e); is well-formed and compiles on both compilers, so static_cast<T>(e) should too. GCC correctly accepts it.