如何用tuple类型列表与std::constructible_from实现构造性检查？

用tuple存储类型列表TypeList，需求是：给定类型X，检查TypeList中是否至少有一个类型可由X构造，且必须使用标准库概念std::constructible_from。

编写的概念定义如下：

template <typename From, typename To, std::size_t... Is>
concept ConstructibleFromHelper = requires(From from, To to) {
    requires (std::constructible_from<typename std::tuple_element<Is, To>::type,From> || ...);
};

template <typename From, typename To>
concept ConstructibleFrom = requires {
    requires ConstructibleFromHelper<From,To,std::make_index_sequence<std::tuple_size_v<To>>()>;
};

编译时报错：

./type_traits.h:36:46: error: template argument for non-type template parameter is treated as function type 'std::make_index_sequence<std::tuple_size_v<To>> ()' (aka '__make_integer_seq<std::integer_sequence, unsigned long, std::tuple_size_v<To>> ()')
    requires ConstructibleFromHelper<From,To,std::make_index_sequence<std::tuple_size_v<To>>()>;

核心问题是无法直接将make_index_sequence展开为std::size_t... Is参数包，且概念不支持偏特化，需要调整实现方式。

方法一：通过辅助概念推导参数包 #

错误根源是直接把std::make_index_sequence<...>()当作模板参数传递，这会被解析为函数类型而非参数包。正确的做法是传递std::index_sequence实例让编译器推导参数包：

#include <concepts>
#include <tuple>
#include <utility>

template <typename From, typename ToTuple, std::size_t... Is>
concept ConstructibleFromHelper = (std::constructible_from<std::tuple_element_t<Is, ToTuple>, From> || ...);

template <typename From, typename ToTuple>
concept AnyConstructibleFromTuple = requires {
    requires ConstructibleFromHelper<From, ToTuple, std::make_index_sequence<std::tuple_size_v<ToTuple>>::type{}>;
};

这里使用std::make_index_sequence<...>::type{}生成std::index_sequence<Is...>的实例，编译器会自动推导出参数包Is...，进而展开折叠表达式检查每个tuple元素的构造性。

方法二：紧凑实现（无需辅助概念） #

利用立即调用的lambda表达式直接处理索引序列，代码更简洁：

#include <concepts>
#include <tuple>

template <typename From, typename ToTuple>
concept AnyConstructibleFromTuple = []<std::size_t... Is>(std::index_sequence<Is...>) {
    return (std::constructible_from<std::tuple_element_t<Is, ToTuple>, From> || ...);
}(std::make_index_sequence<std::tuple_size_v<ToTuple>>{});

lambda表达式通过模板参数推导接收索引序列，内部用折叠表达式遍历tuple的所有类型，检查是否存在可由From构造的类型。

验证代码 #

struct A {};
struct B { B(A) {} };
struct C {};

using TypeList = std::tuple<A, B, C>;

static_assert(AnyConstructibleFromTuple<A, TypeList>); // 成立：A可构造自身，B可由A构造
static_assert(!AnyConstructibleFromTuple<B, TypeList>); // 不成立：A、C都无法由B构造

内容的提问来源于stack exchange，提问作者Debashish