P1061R10 Structured Bindings can introduce a Pack

Editorial notes:
 * Replace digits with numeral words in comment in example.

Author: jensmaurer

source/declarations.tex

@@ -68,19 +68,19 @@
 \end{bnf}
 
 \begin{bnf}
-\nontermdef{attributed-identifier}\br
-    identifier \opt{attribute-specifier-seq}
+\nontermdef{sb-identifier}\br
+    \opt{\terminal{...}} identifier \opt{attribute-specifier-seq}
 \end{bnf}
 
 \begin{bnf}
-\nontermdef{attributed-identifier-list}\br
-    attributed-identifier\br
-    attributed-identifier-list \terminal{,} attributed-identifier
+\nontermdef{sb-identifier-list}\br
+    sb-identifier\br
+    sb-identifier-list \terminal{,} sb-identifier
 \end{bnf}
 
 \begin{bnf}
 \nontermdef{structured-binding-declaration}\br
-    \opt{attribute-specifier-seq} decl-specifier-seq \opt{ref-qualifier} \terminal{[} attributed-identifier-list \terminal{]}
+    \opt{attribute-specifier-seq} decl-specifier-seq \opt{ref-qualifier} \terminal{[} sb-identifier-list \terminal{]}
 \end{bnf}
 
 \begin{bnf}
@@ -218,6 +218,10 @@
 \tcode{thread_local},
 \tcode{auto}\iref{dcl.spec.auto}, or
 a \grammarterm{cv-qualifier}.
+The declaration shall contain at most one \grammarterm{sb-identifier}
+whose \grammarterm{identifier} is preceded by an ellipsis.
+If the declaration contains any such \grammarterm{sb-identifier},
+it shall declare a templated entity\iref{temp.pre}.
 \begin{example}
 \begin{codeblock}
 template<class T> concept C = true;
@@ -7040,13 +7044,17 @@
 
 \pnum
 A structured binding declaration introduces the \grammarterm{identifier}{s}
-$\tcode{v}_0$, $\tcode{v}_1$, $\tcode{v}_2, \dotsc$
+$\tcode{v}_0$, $\tcode{v}_1$, $\tcode{v}_2, \dotsc, \tcode{v}_{N-1}$
 of the
-\grammarterm{attributed-identifier-list} as names
-of \defn{structured binding}{s}.
+\grammarterm{sb-identifier-list} as names.
+An \grammarterm{sb-identifier} that contains an ellipsis
+introduces a structured binding pack\iref{temp.variadic}.
+A \defn{structured binding} is either
+an \grammarterm{sb-identifier} that does not contain an ellipsis or
+an element of a structured binding pack.
 The optional \grammarterm{attribute-specifier-seq} of
-an \grammarterm{attributed-identifier}
-appertains to the structured binding so introduced.
+an \grammarterm{sb-identifier}
+appertains to the associated structured bindings.
 Let \cv{} denote the \grammarterm{cv-qualifier}{s} in
 the \grammarterm{decl-specifier-seq} and
 \placeholder{S} consist of
@@ -7080,6 +7088,42 @@
 \tcode{E} is never a reference type\iref{expr.prop}.
 \end{note}
 
+\pnum
+The \defn{structured binding size} of \tcode{E}, as defined below,
+is the number of structured bindings
+that need to be introduced by the structured binding declaration.
+If there is no structured binding pack,
+then the number of elements in the \grammarterm{sb-identifier-list}
+shall be equal to the structured binding size of \tcode{E}.
+Otherwise, the number of non-pack elements shall be no more than
+the structured binding size of \tcode{E};
+the number of elements of the structured binding pack is
+the structured binding size of \tcode{E} less
+the number of non-pack elements in the\grammarterm{sb-identifier-list}.
+
+\pnum
+Let $\textrm{SB}_i$ denote
+the $i^\textrm{th}$ structured binding in the structured binding declaration
+after expanding the structured binding pack, if any.
+\begin{note}
+If there is no structured binding pack,
+then $\textrm{SB}_i$ denotes $\tcode{v}_i$.
+\end{note}
+\begin{example}
+\begin{codeblock}
+struct C { int x, y, z; };
+
+template<class T>
+void now_i_know_my() {
+  auto [a, b, c] = C();     // OK, $\textrm{SB}_0$ is \tcode{a}, $\textrm{SB}_1$ is \tcode{b}, and $\textrm{SB}_2$ is \tcode{c}
+  auto [d, ...e] = C();     // OK, $\textrm{SB}_0$ is \tcode{d}, the pack \tcode{e} $(\tcode{v}_1)$ contains two structured bindings: $\textrm{SB}_1$ and $\textrm{SB}_2$
+  auto [...f, g] = C();     // OK, the pack \tcode{f} $(\tcode{v}_0)$ contains two structured bindings: $\textrm{SB}_0$ and $\textrm{SB}_1$, and $\textrm{SB}_2$ is \tcode{g}
+  auto [h, i, j, ...k] = C();           // OK, the pack \tcode{k} is empty
+  auto [l, m, n, o, ...p] = C();        // error: structured binding size is too small
+}
+\end{codeblock}
+\end{example}
+
 \pnum
 If a structured binding declaration appears as a \grammarterm{condition},
 the decision variable\iref{stmt.pre} of the condition is \exposid{e}.
@@ -7090,9 +7134,10 @@
 the program is ill-formed.
 
 \pnum
-If \tcode{E} is an array type with element type \tcode{T}, the number
-of elements in the \grammarterm{attributed-identifier-list} shall be equal to the
-number of elements of \tcode{E}. Each $\tcode{v}_i$ is the name of an
+If \tcode{E} is an array type with element type \tcode{T},
+the structured binding size of \tcode{E} is equal to the
+number of elements of \tcode{E}.
+Each $\textrm{SB}_i$ is the name of an
 lvalue that refers to the element $i$ of the array and whose type
 is \tcode{T}; the referenced type is \tcode{T}.
 \begin{note}
@@ -7103,6 +7148,19 @@
 auto f() -> int(&)[2];
 auto [ x, y ] = f();            // \tcode{x} and \tcode{y} refer to elements in a copy of the array return value
 auto& [ xr, yr ] = f();         // \tcode{xr} and \tcode{yr} refer to elements in the array referred to by \tcode{f}'s return value
+
+auto g() -> int(&)[4];
+
+template<size_t N>
+void h(int (&arr)[N]) {
+  auto [a, ...b, c] = arr;  // \tcode{a} names the first element of the array, \tcode{b} is a pack referring to the second and
+                            // third elements, and \tcode{c} names the fourth element
+  auto& [...e] = arr;       // \tcode{e} is a pack referring to the four elements of the array
+}
+
+void call_h() {
+  h(g());
+}
 \end{codeblock}
 \end{example}
 
@@ -7113,8 +7171,7 @@
 the expression \tcode{std::tuple_size<E>::value}
 shall be a well-formed integral constant expression
 and
-the number of elements in
-the \grammarterm{attributed-identifier-list} shall be equal to the value of that
+the structured binding size of \tcode{E} is equal to the value of that
 expression.
 Let \tcode{i} be an index prvalue of type \tcode{std::size_t}
 corresponding to $\tcode{v}_i$.
@@ -7144,7 +7201,7 @@
 \placeholder{S} \terminal{U$_i$ r$_i$ =} initializer \terminal{;}
 \end{ncbnf}
 
-Each $\tcode{v}_i$ is the name of an lvalue of type $\tcode{T}_i$
+Each $\textrm{SB}_i$ is the name of an lvalue of type $\tcode{T}_i$
 that refers to the object bound to $\tcode{r}_i$;
 the referenced type is $\tcode{T}_i$.
 The initialization of \exposid{e} and
@@ -7162,12 +7219,12 @@
 well-formed when named as \tcode{\exposidnc{e}.\placeholder{name}}
 in the context of the structured binding,
 \tcode{E} shall not have an anonymous union member, and
-the number of elements in the \grammarterm{attributed-identifier-list} shall be
+the structured binding size of \tcode{E} is
 equal to the number of non-static data members of \tcode{E}.
 Designating the non-static data members of \tcode{E} as
 $\tcode{m}_0$, $\tcode{m}_1$, $\tcode{m}_2, \dotsc$
 (in declaration order),
-each $\tcode{v}_i$ is the
+each $\textrm{SB}_i$ is the
 name of an lvalue that refers to the member \tcode{m}$_i$ of \exposid{e} and
 whose type is
 that of \tcode{\exposidnc{e}.$\tcode{m}_i$}\iref{expr.ref};

source/expressions.tex

@@ -2864,6 +2864,9 @@
 }
 \end{codeblock}
 \end{example}
+
+\pnum
+A fold expression is a pack expansion.
 \indextext{expression!fold|)}%
 
 \rSec2[expr.prim.req]{Requires expressions}

source/preprocessor.tex

@@ -1905,7 +1905,7 @@
 \defnxname{cpp_sized_deallocation}                & \tcode{201309L} \\ \rowsep
 \defnxname{cpp_static_assert}                     & \tcode{202306L} \\ \rowsep
 \defnxname{cpp_static_call_operator}              & \tcode{202207L} \\ \rowsep
-\defnxname{cpp_structured_bindings}               & \tcode{202403L} \\ \rowsep
+\defnxname{cpp_structured_bindings}               & \tcode{202411L} \\ \rowsep
 \defnxname{cpp_template_template_args}            & \tcode{201611L} \\ \rowsep
 \defnxname{cpp_threadsafe_static_init}            & \tcode{200806L} \\ \rowsep
 \defnxname{cpp_unicode_characters}                & \tcode{200704L} \\ \rowsep

source/templates.tex

@@ -2840,11 +2840,27 @@
 \end{codeblock}
 \end{example}
 
+\pnum
+A \defn{structured binding pack} is an \grammarterm{sb-identifier}
+that introduces zero or more structured bindings\iref{dcl.struct.bind}.
+\begin{example}
+\begin{codeblock}
+auto foo() -> int(&)[2];
+
+template <class T>
+void g() {
+  auto [...a] = foo();          // \tcode{a} is a structured binding pack containing two elements
+  auto [b, c, ...d] = foo();    // \tcode{d} is a structured binding pack containing zero elements
+}
+\end{codeblock}
+\end{example}
+
 \pnum
 A \defn{pack} is
 a template parameter pack,
 a function parameter pack,
-or an \grammarterm{init-capture} pack.
+an \grammarterm{init-capture} pack, or
+a structured binding pack.
 The number of elements of a template parameter pack
 or a function parameter pack
 is the number of arguments provided for the parameter pack.
@@ -2999,15 +3015,21 @@
 designating the $i^\text{th}$ function parameter
 that resulted from instantiation of
 the function parameter pack declaration;
-otherwise
 
 \item
 if the pack is an \grammarterm{init-capture} pack,
 the element is an \grammarterm{id-expression}
 designating the variable introduced by
 the $i^\text{th}$ \grammarterm{init-capture}
 that resulted from instantiation of
-the \grammarterm{init-capture} pack declaration.
+the \grammarterm{init-capture} pack declaration;
+otherwise
+
+\item
+if the pack is a structured binding pack,
+the element is an \grammarterm{id-expression}
+designating the $i^\textrm{th}$ structured binding in the pack
+that resulted from the structured binding declaration.
 \end{itemize}
 When $N$ is zero, the instantiation of a pack expansion
 does not alter the syntactic interpretation of the enclosing construct,
@@ -5338,6 +5360,28 @@
 a structured binding declaration\iref{dcl.struct.bind} whose
 \grammarterm{brace-or-equal-initializer} is type-dependent,
 \item
+associated by name lookup with a pack,
+\begin{example}
+\begin{codeblock}
+struct C { };
+
+void g(...);            // \#1
+
+template <typename T>
+void f() {
+  C arr[1];
+  auto [...e] = arr;
+  g(e...);              // calls \#2
+}
+
+void g(C);              // \#2
+
+int main() {
+  f<int>();
+}
+\end{codeblock}
+\end{example}
+\item
 associated by name lookup with
 an entity captured by copy\iref{expr.prim.lambda.capture}
 in a \grammarterm{lambda-expression}
@@ -5522,6 +5566,8 @@
 \keyword{sizeof} \terminal{...} \terminal{(} identifier \terminal{)}\br
 fold-expression
 \end{ncsimplebnf}
+unless the \grammarterm{identifier} is a structured binding pack
+whose initializer is not dependent.
 
 \pnum
 A \grammarterm{noexcept-expression}\iref{expr.unary.noexcept}