None of the rules you quoted say anything about when a template parameter does or does not participate in template argument deduction, so I'm not seeing your point. (I believe the T pack does participate in template argument deduction here.) Note [temp.deduct.type] p4

In certain contexts, however, the value does not participate in type deduction, but instead uses the values of template arguments that were either deduced elsewhere or explicitly specified.

(Nothing is explicitly specified and this is not a non-deducible context.)

None of the rules you quoted say anything about when a template parameter does or does not participate in template argument deduction, so I'm not seeing your point. (I believe the T pack does participate in template argument deduction here.) Note [temp.deduct.type] p4

In certain contexts, however, the value does not participate in type deduction, but instead uses the values of template arguments that were either deduced elsewhere or explicitly specified.

(Nothing is explicitly specified and this is not a non-deducible context.)

I think the quote you cited here might apply to such a case:

template<class T>
T fun(T);

In a function call, T in the return type does not participate in template argument deduction, however the second T in function parameter does. Hence it's the case of deduced elsewhere.

In the first snippet, pack T does not appear in function parameters which will be used to determine the value of template arguments in a context of deduction for function call. I think if there's no certain rule rules that "template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments", how to deduced the pack to a empty of template arguments?

So, the point you're making is that the T pack doesn't appear in the function parameters, so template argument deduction can't possibly deduce it. Which means we need wording elsewhere to say what T is in the example above.

The text

A trailing template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments.

was made a note in P1787R6.

Asked CWG http://lists.isocpp.org/core/2021/03/10688.php

This seems to be an issue in the wording, but it feels far from editorial.

So, the point you're making is that the T pack doesn't appear in the function parameters, so template argument deduction can't possibly deduce it. Which means we need wording elsewhere to say what T is in the example above.

The text

A trailing template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments.

was made a note in P1787R6.

Asked CWG http://lists.isocpp.org/core/2021/03/10688.php

Yes, That's exactly what I'm concerning here. Moreover, it seems such a case can only apply to those packs that do not parcipate in deduction.

template<class...T>
struct Tuple{
  Tuple(int);
};
template<class...T>
void fun(Tuple<T...>);
int main(){
   fun(0);  //invalid
}

Since it violates [temp.deduct.call#4]