Variadic constructor of indirect is too greedy

[Ukilele]

In PR #128 we removed the parameter std::in_place_t from the variadic constructor of indirect:

template <class... Ts>
- explicit constexpr indirect(std::in_place_t, Ts&&... ts);
+ explicit constexpr indirect(Ts&&... ts);

The following snippet shows a simplified version of indirect and a (potentially contrived) scenario, where the variadic constructor might be too greedy:

#include <concepts>

template<class T>
struct indirect {
    indirect() = default;

    template<class ...Ts>
    explicit indirect(Ts&&... ts)
        requires std::constructible_from<T, Ts...> {}
};

struct Evil {
    Evil(indirect<Evil>&) {}
};

int main() {
    indirect<Evil> i1;
    indirect<Evil> i2(i1);
}

https://compiler-explorer.com/z/KWvzcnjY9

In the line indirect<Evil> i2(i1); we don't call the copy constructor of indirect (as I would expect as a user of indirect), but we call the variadic constructor as it is a better match. Note that it is only a better match as i1 is non-const. For a const indirect<Evil> the copy-constructor would still be called.
The first question is whether you all agree with me that indirect<Evil> i2(i1); should indeed call the copy-constructor or if you think that it is intentional design that it calls the variadic constructor. And if you agree, I think a fix would be to constraint the variadic constructor further. Something like:

template <class... Ts>
explicit constexpr indirect(Ts&&... ts)
- requires std::constructible_from<T, Ts&&...>;
+ requires (std::constructible_from<T, Ts...> &&
+     ( sizeof...(Ts) != 1 || !(std::same_as<std::remove_reference_t<Ts>, indirect> || ...) )
+ );

In prose: If the pack Ts consists of only one type U, make sure that U is not indirect&.

[jbcoe]

I agree that this is an issue and that your fix is good. Are you happy to raise a PR?