[gpderetta]

   template<range R, regular X, invocable<range_value<R>, X> F>
     requires same_as<invoke_result_t<F, R, X>, X>
   auto fold(R&& range, F f, X accumulator) {
      for(auto x: range) 
         accumulator = f(x, accumulator);
      return accumulator;
   }

I can call that with a function returning a custom unit type:

   enum class void_t { Void };

   fold(my_range, [](auto&& elem, void_t) { return Void; }, Void);

But not with void:

   fold(my_range, [](auto&& elem, void) { return; }, void{});

which is very annoying and requires fold to special case 'void' via metaprogramming.