如果包为空，是否对可变参数包类型执行替换?

考虑以下程序:

#include <type_traits>

enum class dummy {};
template <typename T>
using EnableIf = typename std::enable_if<T::value, dummy>::type;
template <typename T>
using DisableIf = typename std::enable_if<!T::value, dummy>::type;

template <typename T>
struct dependent_true_type : std::true_type {};

template <typename T,
          EnableIf<dependent_true_type<T>>...>
std::true_type f();
template <typename T,
          DisableIf<dependent_true_type<T>>...>
std::false_type f();

static_assert(decltype(f<int>())::value, "");

int main() {}

GCC 4.7 高兴接受这个程序.我最近的 clang 3.1 版本声称对 f 的调用不明确.

test.c++:22:24: fatal error: call to 'f' is ambiguous
static_assert(decltype(f<int>())::value, "");
                       ^~~~~~
test.c++:17:16: note: candidate function [with T = int, $1 = <>]
std::true_type f();
               ^
test.c++:20:17: note: candidate function [with T = int, $1 = <>]
std::false_type f();
                ^
1 error generated.