Improve P1789 description

[Tsche]

The previous description did not capture the intended use for this feature properly. There is not much point in introducing an index sequence to operate on a single pack/tuple-like - it's more interesting for operating on more than one of those things at the same time.

I've changed the example to a simple tuple comparison - this should showcase how this change can improve pre-C++26 code.

Additionally a few little nitpicks in the "What it does" section:

• we don't specialize get, we provide ADL-discoverable overloads
• the point of introducing packs via structured binding is to stay within the same function scope (this matters because of P3096, see paper)

[Tsche]

To be fair, there are uses for this with a single pack/tuple-like. For example, consider a HOF that reverses the argument list:

#include <functional>
#include <utility>

template<class F, class... Args>
decltype(auto) reverse_parameter_pack(F&& fnc, Args&&... args) {
  static constexpr auto [...Idx] = std::index_sequence_for<Args...>();
  return std::invoke(std::forward<F>(fnc), 
      std::forward<Args...[sizeof...(args)-Idx-1]>(args...[sizeof...(args)-Idx-1])...);
}

[cdervis]

Let's use ! instead of not, just to be consistent with other example codes across our data files.

[Tsche]

I feel like that makes it a lot less readable, but sure :P

[cdervis]

Hi @Tsche , thanks for your contribution. I agree with your points, and besides some nitpicks we can definitely integrate this. Could you please make the adjustments I commented?

[cdervis]

To be fair, there are uses for this with a single pack/tuple-like. For example, consider a HOF that reverses the argument list:

#include <functional>
#include <utility>

template<class F, class... Args>
decltype(auto) reverse_parameter_pack(F&& fnc, Args&&... args) {
  static constexpr auto [...Idx] = std::index_sequence_for<Args...>();
  return std::invoke(std::forward<F>(fnc), 
      std::forward<Args...[sizeof...(args)-Idx-1]>(args...[sizeof...(args)-Idx-1])...);
}

I think your current changes are fine as-is. They get the point across well.