如何检测元组类型重复场景下的重载决议失败并触发static_assert？

以下代码实现了元函数tuple_index，用于获取std::tuple中目标类型对应的索引：

#include <tuple>
#include <type_traits>
#include <utility>
#include <cstddef>

template <typename ...>
struct always_false : std::false_type {};

template <>
struct always_false<decltype([] {})> : std::true_type {};

template <typename ...Ts>
inline constexpr bool always_false_v = always_false<Ts...>::value;

template <typename>
struct type_wrapper {};

template <std::size_t I, typename T>
struct tuple_index_impl_base
{
    auto foo(type_wrapper<T>) -> std::integral_constant<std::size_t, I>;
};

template <typename ...Ts>
struct tuple_index_impl_base_fallback
{
    auto foo(...)
    {
        static_assert(always_false_v<Ts...>, "Type not exist.");
    }
};

template <typename, typename ...>
struct tuple_index_impl;

template <std::size_t ...Is, typename ...Ts>
struct tuple_index_impl<std::index_sequence<Is...>, Ts...> : tuple_index_impl_base<Is, Ts>..., tuple_index_impl_base_fallback<Ts...>
{
    using tuple_index_impl_base<Is, Ts>::foo...;
    using tuple_index_impl_base_fallback<Ts...>::foo;
};

template <typename, typename>
struct tuple_index;

template <typename T, typename ...Ts>
struct tuple_index<T, std::tuple<Ts...>> : decltype(std::declval<tuple_index_impl<std::make_index_sequence<std::tuple_size_v<std::tuple<Ts...>>>, Ts...>>().foo(std::declval<type_wrapper<T>>())) {};

template <typename T, typename U>
inline constexpr std::size_t tuple_index_v = tuple_index<T, U>::value;

不同场景的表现 #

• 类型唯一存在时：代码可正常返回索引

  constexpr std::size_t i = tuple_index_v<char&, std::tuple<int, float, char, char&, const char*, char>>;    // i == 3
  

• 类型不存在时：触发static_assert输出友好错误

  constexpr std::size_t i = tuple_index_v<long, std::tuple<int, float, char, char&, const char*, char>>;    // 错误 C2338: static_assert failed: 'Type not exist.'
  

• 类型重复存在时：编译器抛出重载歧义错误，而非预设的友好提示

  constexpr std::size_t i = tuple_index_v<char, std::tuple<int, float, char, char&, const char*, char>>;    // 错误 C2668: 'tuple_index_impl_base_fallback<int,float,char,char &,const char *,char>::foo': 对重载函数的调用不明确
  

当重载集合中存在多个匹配的foo函数时，编译器不会选择回退的foo(...)重载。如何在这种场景下检测重载决议失败，并通过static_assert输出友好的错误信息？

内容的提问来源于stack exchange，提问作者El Mismo Sol