I think from the classic STL's point of view that'd be simply

    auto it = std::find_if(v.begin(), v.end(), pred);

    if (it != v.end()) {

        auto index = std::distance(v.begin(), it);

        // do something with index

    }

The thing is, in terms of the classic STL, you never actually want to know indices because they're simply not as useful as iterators. Anything you might want to do with "that element" requires an iterator. Anything you might want to do "for each element up to that one" requires an iterator.  All you can do with an integer index is use it to increment an existing iterator, which might be slow if the iterator type isn't random-access.

However, the STL does provide std::count and std::count_if, which do something similar to what you want and return integers. So I guess maybe the classic-STL name for what you want is `count_until`. The problem is that it's not clear what value `count_until` could return to indicate failure. The obvious answer is "length of the entire range."

    auto trim(string_view sv) {

        auto leading_spaces = nonstd::count_until(sv.begin(), sv.end(), std::not_fn(isSpace));

        sv.remove_prefix(leading_spaces);

        auto trailing_spaces = nonstd::count_until(sv.rbegin(), sv.rend(), std::not_fn(isSpace));

        sv.remove_suffix(trailing_spaces);

        return sv;

    }

Compare to one way you might write that today (if I haven't made an off-by-one error):

    auto trim(string_view sv) {

        auto first_nonspace = std::find_if(sv.begin(), sv.end(), std::not_fn(isSpace));

        sv.remove_prefix(first_nonspace - sv.begin());

        auto last_nonspace = std::find_if(sv.rbegin(), sv.rend(), std::not_fn(isSpace)).base();

        sv.remove_suffix(sv.end() - last_nonspace);

        return sv;

    }

I think you should use std::iter_difference_t<InputIterator> instead of std::size_t.

OTOH, it's a little weird to talk about the difference_type of an input iterator.

OTOOH, it seems that all standard input iterators have `ptrdiff_t` as their difference_type, so that's good.

my $.02,

Arthur