Consider

struct S {

template <typename T>
friend class Foo<T>::Nested; // #1

template <typename ...Ts>
friend class Foo<Ts>::Nested...; //#2

};

Here #1 declares a template friend declaration such that Foo<T>::Nested is a friend of S for any T, per
https://eel.is/c++draft/temp.friend#4 and https://eel.is/c++draft/temp.friend#5

However #2 doesn't really mean anything sensible. Yet there does not seem to be any wording specifying exactly what
variadic friend declarations are or are not allowed.

P2893 incorrectly claimed that #1 was ill-formed and consequently did not add wording to make #2 ill-formed.