Why I can’t use types which can only be constructed by default and move in std::variant?

#include <variant>
struct M1 { M1(M1&&){} M1(){} };
struct M2 { M2(M2&&){} M2(){} };

int main()
{
    std::variant<M1,M2> var=M1{};
    var=M1{};
}

I want to use it with classes containing std::unique_ptr…