[Bug c++/81486] Class template argument deduction fails with (), succeeds with {}

[vittorio.romeo at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81486

--- Comment #2 from Vittorio Romeo <vittorio.romeo at outlook dot com> ---
Here's a snippet that seems to reproduce this bug, even without an explicit
deduction guide:

    template <typename... Ts>
    struct foo
    {
        template <typename... Us>
        foo(Us...) { }
    };

    int main()
    {
        auto f = foo();
    }

On godbolt:
https://godbolt.org/g/CgqLq5