[temp.variadic] Use consistent utterances associated with pack expansion

[temp.variadic] p5 and [temp.variadic] p8 states

A pack expansion consists of a pattern and an ellipsis, the instantiation of which produces zero or more instantiations of the pattern in a list (described below).

The instantiation of a pack expansion considers items...

Such two rules explicitly state that productions are produced from the instantiation of a pack expansion. However, in [temp.variadic] p8.2, we use a different utterance. [temp.variadic] p8.2 says:

if the pack is a function parameter pack, the element is an id-expression designating the i_th function parameter that resulted from instantiation of the function parameter pack declaration;

[dcl.fct] p25 explicit defines that

When it is part of a parameter-declaration-clause, the parameter-declaration declares a function parameter pack ([temp.variadic]).

A function parameter pack is a pack expansion ([temp.variadic]).

From the above, we can get the following conclusion that

• the particular parameter-declaration declares a function parameter pack
• a function parameter pack is a pack expansion

Hence, we shouldn't say the instantiation of the function parameter pack declaration(i.e., the parameter-declaration is not a pack expansion). We should consistently say

if the pack is a function parameter pack, the element is an id-expression designating the i_th function parameter that resulted from instantiation of the function parameter pack declaration;

https://github.com/cplusplus/draft/issues/5165 has a similar issue.

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[temp.variadic] p5.1

Pack expansions can occur in the following contexts:

• In a function parameter pack ([dcl.fct]); the pattern is the parameter-declaration without the ellipsis.

This bullet is a bit different from other bullets after it. The other context all refers to the grammar component that contains the ellipsis. In the initial of p5, we have emphasized that a pack expansion consists of a pattern and an ellipsis. That is, the grammar component that contains the ellipsis conforms to the definition of a pack expansion. Not only from this point but also from [temp.param] p17, they all expose that the definition in [dcl.fct] p25 is contradictory.

From [temp.param] p17

If a template-parameter is a type-parameter with an ellipsis prior to its optional identifier or is a parameter-declaration that declares a pack ([dcl.fct]), then the template-parameter is a template parameter pack.

we can get a conclusion that

• if a template-parameter that is a parameter-declaration that declares a pack, then the template-parameter is a template parameter pack,
• since template-parameter is the parameter-declaration, the parameter-declaration is a template parameter pack.

Conversely, [dcl.fct] p25 says

Otherwise, the parameter-declaration is part of a template-parameter-list and declares a template parameter pack; see [temp.param].

This rule uses "declares" not "is". we may read the contradiction from such two formal rules. Moreover, [temp.variadic] p5.1 is uncoordinated. I think we may change [dcl.fct] p25

A declarator-id or abstract-declarator containing an ellipsis shall only be used in a parameter-declaration. When it is part of a parameter-declaration-clause, the parameter-declaration declares a function parameter pack ([temp.variadic]). Otherwise, the parameter-declaration is part of a template-parameter-list and declares a template parameter pack; see [temp.param]. Such parameter-declaration is a pack expansion ([temp.variadic]).

change [temp.param] p17 to

If a template-parameter is a type-parameter with an ellipsis prior to its optional identifier or is a parameter-declaration that declares a pack ([dcl.fct]), then the template-parameter is declares a template parameter pack. A parameter-declaration declaring a template parameter pack whose type contains one or more unexpanded packs is a pack expansion. Similarly, a type-parameter, with a template-parameter-list containing one or more unexpanded packs, that declares a template parameter pack is a pack expansion. A type-parameter with a type-constraint that contains an unexpanded parameter pack is a pack expansion. A template-parameter that is a pack expansion shall not expand a template parameter pack declared in the same template-parameter-list.

change [temp.variadic] p5.1 and p5.3 to

Pack expansions can occur in the following contexts:

• In a parameter-declaration that declares a function parameter pack; the pattern is the parameter-declaration without the ellipsis.
• [...]
• In a template-parameter that is a pack expansion ([temp.param]):

• if the template-parameter is a parameter-declaration; the pattern is the parameter-declaration without the ellipsis;
• if the template-parameter is a type-parameter; the pattern is the corresponding type-parameter without the ellipsis.

Start by comparing [temp.variadic] p5, p5.1, p5.3, and p8 with [dcl.fct] p25 and [temp.param] p17, including the related issue #5165. Determine whether the proposed terminology and pack-expansion rules are consistent across these clauses; done means an agreed wording change that resolves the identified contradictions.