On Wed, Jun 10, 2020 at 2:06 AM Dvir Yitzchaki via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
you can get a similar behavior with operator||

template <std::size_t... is>
T test_impl(std::size_t j, std::index_sequence<is...>)
    T res;
    (void)( (j == is ? (res = f<is>(), true) : false) || ... || (throw std::range_error("Out of range"), false) );
    return res;

Unfortunately that requires `T` to be default-constructible and support move-assignment, neither of which is necessarily true in modern code. (Someone might even want to make `test_impl` return a reference.)  Worse, it requires all the `f<is>()` to have the same return type.
However, I agree, this should be mentioned in the paper, in a "Tony Table": "here's how you can kinda do it in C++20, versus how I want to be able to do it."
It would be useful to link to Jonathan Müller's "Nifty Fold Expression Tricks" https://foonathan.net/2020/05/fold-tricks/#content as well. He gives this specific trick under the heading "Find the first element that matches the predicate."