Description Davin McCall 2018-09-14 09:34:06 EDT

The following (self-contained) code flags an error on the last line, but compiles with no error using GCC/Clang. I've tried to reduce this testcase as much as possible:

--- begin ---
template <class A>
struct hhh
{
    using handle_t = int;
    using handle_t_r = int &;
};

template <template <class A> class tt>
struct iii
{
    using handle_t = typename tt<int>::handle_t;
    using handle_t_r = typename tt<int>::handle_t_r;
};

template<typename, typename>
struct is_same
{
    static const bool value = false;
};

template<typename Tp>
struct is_same<Tp, Tp>
{
    static const bool value = true;
};

template<bool _Cond, typename Iftrue, typename Iffalse>
    struct conditional
    { typedef Iftrue type; };

template<typename Iftrue, typename Iffalse>
    struct conditional<false, Iftrue, Iffalse>
    { typedef Iffalse type; };

template <class A> using hhh_d = hhh<A>;

constexpr bool use_empty_node = is_same<typename iii<hhh_d>::handle_t_r,
        typename iii<hhh_d>::handle_t &>::value;

using node_type = conditional<use_empty_node, int, float>::type;
--- end ---