effective way to select last parameter of variadic template

by

With C++17, the cleanest way is

template<typename T>
struct tag
{
    using type = T;
};

template<typename... Ts>
struct select_last
{
    // Use a fold-expression to fold the comma operator over the parameter pack.
    using type = typename decltype((tag<Ts>{}, ...))::type;
};

with O(1) instantiation depth.

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