I wanted to have a fold-expressioned function that concatenates string_views. But the code I tried didn’t work. What am I makging wrong ?

template<typename Allocator, typename Char, typename Traits>
auto sv_concat( std::basic_string_view<Char, Traits> svs ... )
{
    basic_string<Char, Traits, Allocator> str;
    str.reserve( (svs.length() + ...) );
    ((str += svs), ...);
    return str;
}

This are the clang++ 18 errors:

test.cpp:14:31: error: pack expansion does not contain any unexpanded parameter packs
   14 |         str.reserve( (svs.length() + ...) );
      |                       ~~~~~~~~~~~~   ^
test.cpp:15:17: error: pack expansion does not contain any unexpanded parameter packs
   15 |         ((str += svs), ...);
      |          ~~~~~~~~~~~~  ^