xieite::fold<>

template<auto fn, typename T, typename... Ts>
using fold = /* ??? */;

template<typename... Ts>
struct type_list {
	template<typename T>
	static constexpr bool has = (... || std::same_as<Ts, T>);

	template<typename T>
	using append = type_list<Ts..., T>;

	template<template<typename...> typename M>
	using to = M<Ts...>;
};

template<typename... Ts>
using unique_variant = xieite::fold<
	[]<typename List, typename T> static {
		if constexpr (!List::template has<T>) {
			return typename List::template append<T>();
		} else {
			return List();
		}
	},
	type_list<>,
	Ts...
>::template to<std::variant>;

static_assert(std::same_as<
	unique_variant<int, int, char, int, float, char>,
	std::variant<int, char, float>
>);