Cannot initialize std::variant with various lambda expressions

I'm playing with std::variant, lambdas and std::future, and got super weird results when I tried to compose them together. Here are examples:

using variant_t = std::variant<
    std::function<std::future<void>(int)>,
    std::function<void(int)>
>;
auto f1 = [](int) { return std::async([] { return 1; }); };
auto f2 = [](int) { return std::async([] {  }); };

variant_t v1(std::move(f1)); // !!! why DOES this one compile when it SHOULDN'T?
auto idx1 = v1.index(); //equals 1. WHY?

variant_t v2(std::move(f2)); // !!! why DOESN'T this one compile when it SHOULD?

Here is the compilation error:

Error C2665 'std::variant<std::function<std::future<void> (int)>,std::function<void (int)>>::variant': none of the 2 overloads could convert all the argument types

OK, lets change variant's items signatures from returning void to int:

using variant_t = std::variant<
    std::function<std::future<int>(int)>,
    std::function<int(int)>
>;

variant_t v1(std::move(f1)); // COMPILES (like it should)
auto idx1 = v1.index(); // equals 0

variant_t v2(std::move(f2)); // DOESN'T compile (like it should)

What the hell is going on here? Why is std::future<void> so special?