[Bug c++/57063] Valid static_cast from data member to rvalue reference fails to compile

[tsoae at mail dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57063

--- Comment #4 from Nikolka <tsoae at mail dot ru> 2013-04-25 13:51:21 UTC ---
It looks like the root of the issue is that static_cast produces an expression
with wrong value category sometimes.

    #include <iostream>
    #include <type_traits>

    #define PRINT_VALUE(...) \
        std::cout << #__VA_ARGS__ " = " << __VA_ARGS__ << std::endl

    template <class T>
        struct X
    {
        void f()
        {
            PRINT_VALUE(std::is_lvalue_reference<
                decltype(static_cast<T &&>(value))>{});

            PRINT_VALUE(std::is_rvalue_reference<
                decltype(static_cast<T &&>(value))>{});

            std::cout << std::endl;
        }
        T &&value;
    };

    struct A {};

    int main()
    {
        X<int>{0}.f();
        X<A>{A()}.f();
    }

When T = int, static_cast<T &&>(value) is wrongfully treated as an lvalue. In
the first example the compiler thinks that we want to return lvalue of type int
by rvalue reference int &&, which would be invalid.