[temp.variadic] Rearrange description of pack expansion (#4543)

Author: jensmaurer

source/templates.tex

@@ -2800,24 +2800,25 @@
 \end{example}
 
 \pnum
-The instantiation of a pack expansion
-that is neither a \tcode{sizeof...} expression
-nor a \grammarterm{fold-expression}
-produces a
-list of elements
-$\mathtt{E}_1,$ $\mathtt{E}_2,$ $\cdots,$ $\mathtt{E}_N$,
+The instantiation of a pack expansion considers
+items $\tcode{E}_1, \tcode{E}_2, \dotsc, \tcode{E}_N$,
 where
-$N$ is the number of elements in the pack expansion parameters. Each
-$\mathtt{E}_i$ is generated by instantiating the pattern and
+$N$ is the number of elements in the pack expansion parameters.
+Each $\tcode{E}_i$ is generated by instantiating the pattern and
 replacing each pack expansion parameter with its $i^\text{th}$ element.
 Such an element, in the context of the instantiation, is interpreted as
 follows:
 \begin{itemize}
 \item
-if the pack is a template parameter pack, the element is a template
-parameter\iref{temp.param} of the corresponding kind (type or
-non-type) designating the $i^\text{th}$
-corresponding type or value template argument;
+if the pack is a template parameter pack, the element is
+an \grammarterm{id-expression}
+(for a non-type template parameter pack),
+a \grammarterm{typedef-name}
+(for a type template parameter pack declared without \tcode{template}), or
+a \grammarterm{template-name}
+(for a type template parameter pack declared with \tcode{template})
+if the pack was introduced by a pack expansion,
+designating the $i^\text{th}$ corresponding type or value template argument;
 
 \item
 if the pack is a function parameter pack, the element is an
@@ -2835,37 +2836,17 @@
 that resulted from instantiation of
 the \grammarterm{init-capture} pack.
 \end{itemize}
-
-All of the $\mathtt{E}_i$ become items in the enclosing list.
-\begin{note}
-The variety of list varies with the context:
-\grammarterm{expression-list},
-\grammarterm{base-specifier-list},
-\grammarterm{template-argument-list}, etc.
-\end{note}
-When $N$ is zero, the instantiation of the expansion produces an empty list.
-Such an instantiation does not alter the syntactic interpretation of the
-enclosing construct, even in cases where omitting the list entirely would
+When $N$ is zero, the instantiation of a pack expansion
+does not alter the syntactic interpretation of the enclosing construct,
+even in cases where omitting the pack expansion entirely would
 otherwise be ill-formed or would result in an ambiguity in the grammar.
-\begin{example}
-\begin{codeblock}
-template<class... T> struct X : T... { };
-template<class... T> void f(T... values) {
-  X<T...> x(values...);
-}
-
-template void f<>();    // OK: \tcode{X<>} has no base classes
-                        // \tcode{x} is a variable of type \tcode{X<>} that is value-initialized
-\end{codeblock}
-\end{example}
 
 \pnum
 The instantiation of a \tcode{sizeof...} expression\iref{expr.sizeof} produces
-an integral constant containing the number of elements in the pack
-it expands.
+an integral constant with value $N$.
 
 \pnum
-The instantiation of a \grammarterm{fold-expression} produces:
+The instantiation of a \grammarterm{fold-expression}\iref{expr.prim.fold} produces:
 \begin{itemize}
 \item
 \tcode{((}$\mathtt{E}_1$
@@ -2896,11 +2877,8 @@
 \end{itemize}
 
 In each case,
-\placeholder{op} is the \grammarterm{fold-operator},
-$N$ is the number of elements in the pack expansion parameters,
-and each $\mathtt{E}_i$ is generated by instantiating the pattern
-and replacing each pack expansion parameter with its $i^\text{th}$ element.
-For a binary fold-expression,
+\placeholder{op} is the \grammarterm{fold-operator}.
+For a binary fold,
 $\mathtt{E}$ is generated
 by instantiating the \grammarterm{cast-expression}
 that did not contain an unexpanded pack.
@@ -2916,7 +2894,7 @@
 \tcode{((true \&\& true) \&\& true) \&\& false},
 which evaluates to \tcode{false}.
 \end{example}
-If $N$ is zero for a unary fold-expression,
+If $N$ is zero for a unary fold,
 the value of the expression is shown in \tref{temp.fold.empty};
 if the operator is not listed in \tref{temp.fold.empty},
 the instantiation is ill-formed.
@@ -2931,6 +2909,28 @@
 \tcode{,}       & \tcode{void()} \\
 \end{floattable}
 
+\pnum
+The instantiation of any other pack expansion
+produces a list of elements $\tcode{E}_1, \tcode{E}_2, \dotsc, \tcode{E}_N$.
+\begin{note}
+The variety of list varies with the context:
+\grammarterm{expression-list},
+\grammarterm{base-specifier-list},
+\grammarterm{template-argument-list}, etc.
+\end{note}
+When $N$ is zero, the instantiation of the expansion produces an empty list.
+\begin{example}
+\begin{codeblock}
+template<class... T> struct X : T... { };
+template<class... T> void f(T... values) {
+  X<T...> x(values...);
+}
+
+template void f<>();    // OK: \tcode{X<>} has no base classes
+                        // \tcode{x} is a variable of type \tcode{X<>} that is value-initialized
+\end{codeblock}
+\end{example}
+
 \rSec2[temp.friend]{Friends}
 
 \pnum