[temp.variadic] p6 Rearrange the rule and make the meaning clearer

[temp.variadic] p6 says

For the purpose of determining whether a pack satisfies a rule regarding entities other than packs, the pack is considered to be the entity that would result from an instantiation of the pattern in which it appears.

Actually, we also want to check the requirement for syntax where the pack appears but not just the rules for entities. For example:

template<class...T>
struct S{
   std::tuple<unsigned T...> tp;  // #1
};

At #1, we want to consider the pack T as a typedef-name, and unsigned typedef-name is not a valid combination of simple-type-specifiers summarized in [dcl.type.simple] p3, hence the combination is invalid.

Rearrange [temp.variadic] p6 to the place after [temp.variadic] p8, and change it to

For the purpose of determining whether a pack satisfies the requirements defined in this document, the pack is considered to be the element that would be used for replacing in the context of the instantiation.

template<class ...T>
struct A:T...{  //#1 ok
   std::tuple<typename T::type....> tp;  // #2  ok 
};

template<class ...U>
struct B{
   std::tuple<U&...> tp; // #3 ok 
};
B<void> b; // error

The pack T interpreted as a typedef-name can form a base-specifier at #1 and a nested-name-specifier at #2. The pack U interpreted as a typedef-name can form a type-id at #3 however the typedef-name that designates the entity void in the instantiation context forms a reference to void, which is ill-formed.

Start with [temp.variadic] paragraphs 6 and 8 and compare the surrounding wording with the examples in the issue. Move paragraph 6 after paragraph 8 and replace it with the proposed rule about requirements and replacement context. Check that the resulting text covers both syntactic requirements and entity requirements illustrated by the examples.