P2662R3 Pack Indexing

Editorial note: the proposed solitary change to strike the word "or"
from [expr.prim.id.qual] has not been applied, since it seemed
incorrect and inappropriate.

Author: jensmaurer

source/basic.tex

@@ -2182,7 +2182,7 @@
 followed by a \tcode{::} scope resolution operator
 considers only
 namespaces, types, and templates whose specializations are types.
-If a name, \grammarterm{template-id}, or \grammarterm{decltype-specifier}
+If a name, \grammarterm{template-id}, or \grammarterm{computed-type-specifier}
 is followed by a \tcode{::},
 it shall designate a namespace, class, enumeration, or dependent type,
 and the \tcode{::} is never interpreted as

source/classes.tex

@@ -2191,7 +2191,7 @@
 In an explicit destructor call, the destructor is specified by a
 \tcode{\~{}}
 followed by a
-\grammarterm{type-name} or \grammarterm{decltype-specifier}
+\grammarterm{type-name} or \grammarterm{computed-type-specifier}
 that denotes the destructor's class type.
 The invocation of a destructor is subject to the usual rules for member
 functions\iref{class.mfct};
@@ -3403,16 +3403,16 @@
 
 \begin{bnf}
 \nontermdef{base-specifier}\br
-    \opt{attribute-specifier-seq} class-or-decltype\br
-    \opt{attribute-specifier-seq} \keyword{virtual} \opt{access-specifier} class-or-decltype\br
-    \opt{attribute-specifier-seq} access-specifier \opt{\keyword{virtual}} class-or-decltype
+    \opt{attribute-specifier-seq} class-or-computed-type-specifier\br
+    \opt{attribute-specifier-seq} \keyword{virtual} \opt{access-specifier} class-or-computed-type-specifier\br
+    \opt{attribute-specifier-seq} access-specifier \opt{\keyword{virtual}} class-or-computed-type-specifier
 \end{bnf}
 
 \begin{bnf}
-\nontermdef{class-or-decltype}\br
+\nontermdef{class-or-computed-type-specifier}\br
     \opt{nested-name-specifier} type-name\br
     nested-name-specifier \keyword{template} simple-template-id\br
-    decltype-specifier
+    computed-type-specifier
 \end{bnf}
 
 \indextext{specifier access|see{access specifier}}%

source/compatibility.tex

@@ -31,6 +31,29 @@
 bool b = ne({2,3}, {1,2,3});    // unspecified result; previously \tcode{false}
 \end{codeblock}
 
+\rSec2[diff.cpp23.dcl.dcl]{\ref{dcl.dcl}: Declarations}
+
+\diffref{dcl.array}
+\change
+Previously, \tcode{T...[n]} would declare a pack of function parameters.
+\tcode{T...[n]} is now a \grammarterm{pack-index-specifier}.
+\rationale
+Improve the handling of packs.
+\effect
+Valid \CppXXIII{} code that declares a pack of parameters
+without specifying a \grammarterm{declarator-id} becomes ill-formed.
+For example:
+\begin{codeblock}
+template <typename... T>
+void f(T... [1]);
+template <typename... T>
+void g(T... ptr[1]);
+int main() {
+  f<int, double>(nullptr, nullptr);     // ill-formed, previously \tcode{void f<int, double>(int [1], double [1])}
+  g<int, double>(nullptr, nullptr);     // ok
+}
+\end{codeblock}
+
 \rSec2[diff.cpp23.library]{\ref{library}: library introduction}
 
 \diffref{headers}

source/declarations.tex

@@ -1286,7 +1286,7 @@
 \nontermdef{simple-type-specifier}\br
     \opt{nested-name-specifier} type-name\br
     nested-name-specifier \keyword{template} simple-template-id\br
-    decltype-specifier\br
+    computed-type-specifier\br
     placeholder-type-specifier\br
     \opt{nested-name-specifier} template-name\br
     \keyword{char}\br
@@ -1312,6 +1312,12 @@
     typedef-name
 \end{bnf}
 
+\begin{bnf}
+\nontermdef{computed-type-specifier}\br
+    decltype-specifier\br
+    pack-index-specifier
+\end{bnf}
+
 \pnum
 \indextext{component name}
 The component names of a \grammarterm{simple-type-specifier} are those of its
@@ -1383,6 +1389,7 @@
 \grammarterm{type-name}           & the type named                    \\
 \grammarterm{simple-template-id}  & the type as defined in~\ref{temp.names}\\
 \grammarterm{decltype-specifier}  & the type as defined in~\ref{dcl.type.decltype}\\
+\grammarterm{pack-index-specifier} & the type as defined in~\ref{dcl.type.pack.index}\\
 \grammarterm{placeholder-type-specifier}
                                   & the type as defined in~\ref{dcl.spec.auto}\\
 \grammarterm{template-name}       & the type as defined in~\ref{dcl.type.class.deduct}\\
@@ -1432,6 +1439,32 @@
 forces \tcode{char} objects to be signed; it is redundant in other contexts.
 \end{note}
 
+\rSec3[dcl.type.pack.index]{Pack indexing specifier}
+
+\begin{bnf}
+\nontermdef{pack-index-specifier}\br
+    typedef-name \terminal{...} \terminal{[} constant-expression \terminal{]}
+\end{bnf}
+
+\pnum
+The \grammarterm{typedef-name} $P$ in a \grammarterm{pack-index-specifier}
+shall denote a pack.
+
+\pnum
+The \grammarterm{constant-expression} shall be
+a converted constant expression\iref{expr.const} of type \tcode{std::size_t}
+whose value $V$, termed the index,
+is such that $0 \le V < \tcode{sizeof...($P$)}$.
+
+\pnum
+A \grammarterm{pack-index-specifier} is a pack expansion\iref{temp.variadic}.
+
+\pnum
+\begin{note}
+The \grammarterm{pack-index-specifier} denotes
+the type of the $V^\text{th}$ element of the pack.
+\end{note}
+
 \rSec3[dcl.type.elab]{Elaborated type specifiers}%
 \indextext{type specifier!elaborated}%
 \indextext{\idxcode{typename}}%
@@ -1605,6 +1638,13 @@
 decltype(i) x2;                 // type is \tcode{int}
 decltype(a->x) x3;              // type is \tcode{double}
 decltype((a->x)) x4 = x3;       // type is \tcode{const double\&}
+
+void f() {
+  [](auto ...pack) {
+    decltype(pack...[0]) x5;    // type is \tcode{int}
+    decltype((pack...[0])) x6;  // type is \tcode{int\&}
+  }(0);
+}
 \end{codeblock}
 \end{example}
 \begin{note}
@@ -2367,7 +2407,6 @@
 \begin{bnf}
 \nontermdef{noptr-abstract-pack-declarator}\br
     noptr-abstract-pack-declarator parameters-and-qualifiers\br
-    noptr-abstract-pack-declarator \terminal{[} \opt{constant-expression} \terminal{]} \opt{attribute-specifier-seq}\br
     \terminal{...}
 \end{bnf}
 

source/expressions.tex

@@ -1325,7 +1325,8 @@
 \begin{bnf}
 \nontermdef{id-expression}\br
     unqualified-id\br
-    qualified-id
+    qualified-id\br
+    pack-index-expression
 \end{bnf}
 
 \pnum
@@ -1430,7 +1431,7 @@
     conversion-function-id\br
     literal-operator-id\br
     \terminal{\~} type-name\br
-    \terminal{\~} decltype-specifier\br
+    \terminal{\~} computed-type-specifier\br
     template-id
 \end{bnf}
 
@@ -1447,7 +1448,7 @@
 \grammarterm{conversion-function-id}{s}, see~\ref{class.conv.fct}; for
 \grammarterm{literal-operator-id}{s}, see~\ref{over.literal}; for
 \grammarterm{template-id}{s}, see~\ref{temp.names}.
-A \grammarterm{type-name} or \grammarterm{decltype-specifier}
+A \grammarterm{type-name} or \grammarterm{computed-type-specifier}
 prefixed by \tcode{\~} denotes the destructor of the type so named;
 see~\ref{expr.prim.id.dtor}.
 Within the definition of a non-static member function, an
@@ -1595,7 +1596,7 @@
     \terminal{::}\br
     type-name \terminal{::}\br
     namespace-name \terminal{::}\br
-    decltype-specifier \terminal{::}\br
+    computed-type-specifier \terminal{::}\br
     nested-name-specifier identifier \terminal{::}\br
     nested-name-specifier \opt{\keyword{template}} simple-template-id \terminal{::}
 \end{bnf}
@@ -1633,8 +1634,8 @@
 \pnum
 The \grammarterm{nested-name-specifier} \tcode{::} nominates
 the global namespace.
-A \grammarterm{nested-name-specifier} with a \grammarterm{decltype-specifier}
-nominates the type denoted by the \grammarterm{decltype-specifier},
+A \grammarterm{nested-name-specifier} with a \grammarterm{computed-type-specifier}
+nominates the type denoted by the \grammarterm{computed-type-specifier},
 which shall be a class or enumeration type.
 If a \grammarterm{nested-name-specifier} $N$
 is declarative and
@@ -1666,9 +1667,9 @@
 \pnum
 A \grammarterm{qualified-id} shall not be of the form
 \grammarterm{nested-name-specifier} \opt{\keyword{template}} \tcode{\~}
-\grammarterm{decltype-specifier}
+\grammarterm{computed-type-specifier}
 nor of the form
-\grammarterm{decltype-specifier} \tcode{::} \tcode{\~} \grammarterm{type-name}.
+\grammarterm{computed-type-specifier} \tcode{::} \tcode{\~} \grammarterm{type-name}.
 
 \pnum
 The result of a \grammarterm{qualified-id} $Q$ is
@@ -1690,6 +1691,32 @@
 \end{itemize}
 and a prvalue otherwise.
 
+\rSec3[expr.prim.pack.index]{Pack indexing expression}
+
+\begin{bnf}
+\nontermdef{pack-index-expression}\br
+    id-expression \terminal{...} \terminal{[} constant-expression \terminal{]}
+\end{bnf}
+
+\pnum
+The \grammarterm{id-expression} $P$ in a \grammarterm{pack-index-expression}
+shall be an \grammarterm{identifier} that denotes a pack.
+
+\pnum
+The \grammarterm{constant-expression} shall be
+a converted constant expression\iref{expr.const} of type \tcode{std::size_t}
+whose value $V$, termed the index,
+is such that $0 \le V < \tcode{sizeof...($P$)}$.
+
+\pnum
+A \grammarterm{pack-index-expression} is a pack expansion\iref{temp.variadic}.
+
+\pnum
+\begin{note}
+A \grammarterm{pack-index-expression} denotes
+the $V^\text{th}$ element of the pack.
+\end{note}
+
 \rSec3[expr.prim.id.dtor]{Destruction}
 
 \pnum
@@ -4696,14 +4723,14 @@
 \end{note}
 There is an ambiguity
 in the grammar when \tcode{\~{}} is followed by
-a \grammarterm{type-name} or \grammarterm{decltype-specifier}.
+a \grammarterm{type-name} or \grammarterm{computed-type-specifier}.
 The ambiguity is resolved by treating \tcode{\~{}} as the
 operator rather than as the start of an \grammarterm{unqualified-id}
 naming a destructor.
 \begin{note}
 Because the grammar does not permit an operator to follow the
 \tcode{.}, \tcode{->}, or \tcode{::} tokens, a \tcode{\~{}} followed by
-a \grammarterm{type-name} or \grammarterm{decltype-specifier} in a
+a \grammarterm{type-name} or \grammarterm{computed-type-specifier} in a
 member access expression or \grammarterm{qualified-id} is
 unambiguously parsed as a destructor name.
 \end{note}

source/preprocessor.tex

@@ -1851,6 +1851,7 @@
 \defnxname{cpp_nontype_template_args}             & \tcode{201911L} \\ \rowsep
 \defnxname{cpp_nontype_template_parameter_auto}   & \tcode{201606L} \\ \rowsep
 \defnxname{cpp_nsdmi}                             & \tcode{200809L} \\ \rowsep
+\defnxname{cpp_pack_indexing}                     & \tcode{202311L} \\ \rowsep
 \defnxname{cpp_placeholder_variables}             & \tcode{202306L} \\ \rowsep
 \defnxname{cpp_range_based_for}                   & \tcode{202211L} \\ \rowsep
 \defnxname{cpp_raw_strings}                       & \tcode{200710L} \\ \rowsep

source/templates.tex

@@ -2164,6 +2164,19 @@
 e.g., by a \grammarterm{typedef-name}.
 \end{note}
 
+\pnum
+For a type template parameter pack \tcode{T},
+\tcode{T...[}\grammarterm{constant-expression}\tcode{]} denotes
+a unique dependent type.
+
+\pnum
+If the \grammarterm{constant-expression} of a \grammarterm{pack-index-specifier}
+is value-dependent,
+two such \grammarterm{pack-index-specifier}s refer to the same type
+only if their \grammarterm{constant-expression}s are equivalent\iref{temp.over.link}.
+Otherwise, two such \grammarterm{pack-index-specifier}s refer to the same type
+only if their indexes have the same value.
+
 \rSec1[temp.decls]{Template declarations}
 
 \rSec2[temp.decls.general]{General}
@@ -2763,6 +2776,12 @@
 \item In a \tcode{sizeof...} expression\iref{expr.sizeof}; the pattern is an
 \grammarterm{identifier}.
 
+\item In a \grammarterm{pack-index-expression};
+the pattern is an \grammarterm{identifier}.
+
+\item In a \grammarterm{pack-index-specifier};
+the pattern is a \grammarterm{typedef-name}.
+
 \item In a \grammarterm{fold-expression}\iref{expr.prim.fold};
 the pattern is the \grammarterm{cast-expression}
 that contains an unexpanded pack.
@@ -2865,6 +2884,16 @@
 The instantiation of a \tcode{sizeof...} expression\iref{expr.sizeof} produces
 an integral constant with value $N$.
 
+\pnum
+When instantiating a \grammarterm{pack-index-expression} $P$,
+let $K$ be the index of $P$.
+The instantiation of $P$ is the \grammarterm{id-expression} $\tcode{E}_K$.
+
+\pnum
+When instantiating a \grammarterm{pack-index-specifier} $P$,
+let $K$ be the index of $P$.
+The instantiation of $P$ is the \grammarterm{typedef-name} $\tcode{E}_K$.
+
 \pnum
 The instantiation of an \grammarterm{alignment-specifier} with an ellipsis
 produces $\tcode{E}_1$ $\tcode{E}_2$ $\dotsc$ $\tcode{E}_N$.
@@ -5110,7 +5139,7 @@
 This includes an injected-class-name\iref{class.pre} of a class template
 used without a \grammarterm{template-argument-list}.
 \end{footnote}
-or
+\item a \grammarterm{pack-index-specifier}, or
 \item denoted by \tcode{decltype(}\grammarterm{expression}{}\tcode{)},
 where \grammarterm{expression} is type-dependent\iref{temp.dep.expr}.
 \end{itemize}
@@ -5258,6 +5287,10 @@
 \pnum
 A \grammarterm{fold-expression} is type-dependent.
 
+\pnum
+A \grammarterm{pack-index-expression} is type-dependent
+if its \grammarterm{id-expression} is type-dependent.
+
 \rSec3[temp.dep.constexpr]{Value-dependent expressions}
 
 \pnum
@@ -8197,6 +8230,8 @@
 of a type that was specified using a
 \grammarterm{qualified-id}.
 \item
+A \grammarterm{pack-index-specifier} or a \grammarterm{pack-index-expression}.
+\item
 The \grammarterm{expression} of a \grammarterm{decltype-specifier}.
 \item
 A non-type template argument or an array bound in which a subexpression