Merge 2021-10 CWG Motion 9

P2128R6 Multidimensional subscript operator

Author: tkoeppe

source/compatibility.tex

@@ -40,6 +40,21 @@
 \end{codeblock}
 \end{example}
 
+\rSec2[diff.cpp20.expr]{\ref{expr}: expressions}
+
+\diffref{expr.sub}
+\change
+Change the meaning of comma in subscript expressions.
+\rationale
+Enable repurposing a deprecated syntax to support multidimensional indexing.
+\effect
+Valid \CppXX{} code that uses a comma expression within a
+subscript expression may fail to compile. For example:
+\begin{codeblock}
+arr[1, 2]               // was equivalent to \tcode{arr[(1, 2)]},
+                        // now equivalent to \tcode{arr.operator[](1, 2)} or ill-formed
+\end{codeblock}
+
 \rSec2[diff.cpp20.utilities]{\ref{utilities}: general utilities library}
 
 \diffref{format}

source/expressions.tex

@@ -2982,7 +2982,7 @@
 \begin{bnf}
 \nontermdef{postfix-expression}\br
     primary-expression\br
-    postfix-expression \terminal{[} expr-or-braced-init-list \terminal{]}\br
+    postfix-expression \terminal{[} \opt{expression-list} \terminal{]}\br
     postfix-expression \terminal{(} \opt{expression-list} \terminal{)}\br
     simple-type-specifier \terminal{(} \opt{expression-list} \terminal{)}\br
     typename-specifier \terminal{(} \opt{expression-list} \terminal{)}\br
@@ -3020,8 +3020,18 @@
 \pnum
 \indextext{operator!subscripting}%
 \indextext{\idxcode{[]}|see{operator, subscripting}}%
-A postfix expression followed by an expression in square brackets is a
-postfix expression. One of the expressions shall be a glvalue of type ``array of
+A \defnadj{subscript}{expression} is a postfix expression
+followed by square brackets containing
+a possibly empty, comma-separated list of \grammarterm{initializer-clause}s
+which constitute the arguments to the subscript operator.
+The \grammarterm{postfix-expression} is sequenced before
+each expression in the \grammarterm{expression-list}.
+
+\pnum
+With the built-in subscript operator,
+an \grammarterm{expression-list} shall be present,
+consisting of a single \grammarterm{assignment-expression}.
+One of the expressions shall be a glvalue of type ``array of
 \tcode{T}'' or a prvalue of type ``pointer
 to \tcode{T}'' and the other shall be a prvalue of unscoped enumeration or integral type.
 The result is of type ``\tcode{T}''.
@@ -3036,13 +3046,6 @@
 \tcode{*((E1)+(E2))},
 except that in the case of an array operand, the result is an lvalue
 if that operand is an lvalue and an xvalue otherwise.
-The expression \tcode{E1} is sequenced before the expression \tcode{E2}.
-
-\pnum
-A comma expression\iref{expr.comma}
-appearing as the \grammarterm{expr-or-braced-init-list}
-of a subscripting expression is deprecated;
-see \ref{depr.comma.subscript}.
 
 \pnum
 \begin{note}
@@ -3052,9 +3055,6 @@
 \tcode{+} and~\ref{dcl.array} for details of array types.
 \end{note}
 
-\pnum
-A \grammarterm{braced-init-list} shall not be used with the built-in subscript operator.
-
 \rSec3[expr.call]{Function call}
 
 \pnum
@@ -7067,7 +7067,9 @@
 \pnum
 \begin{note}
 In contexts where the comma token is given special meaning
-(e.g. function calls\iref{expr.call}, lists of initializers\iref{dcl.init},
+(e.g. function calls\iref{expr.call},
+subscript expressions\iref{expr.sub},
+lists of initializers\iref{dcl.init},
 or \grammarterm{template-argument-list}{s}\iref{temp.names}),
 the comma operator as described in this subclause can appear only in parentheses.
 \begin{example}
@@ -7079,14 +7081,6 @@
 \end{example}
 \end{note}
 
-\pnum
-\begin{note}
-A comma expression
-appearing as the \grammarterm{expr-or-braced-init-list}
-of a subscripting expression\iref{expr.sub} is deprecated;
-see \ref{depr.comma.subscript}.
-\end{note}
-
 \rSec1[expr.const]{Constant expressions}%
 \indextext{expression!constant}
 

source/future.tex

@@ -58,24 +58,6 @@
 \end{codeblock}
 \end{example}
 
-\rSec1[depr.comma.subscript]{Comma operator in subscript expressions}
-
-\pnum
-A comma expression\iref{expr.comma}
-appearing as the \grammarterm{expr-or-braced-init-list}
-of a subscripting expression\iref{expr.sub} is deprecated.
-\begin{note}
-A parenthesized comma expression is not deprecated.
-\end{note}
-\begin{example}
-\begin{codeblock}
-void f(int *a, int b, int c) {
-    a[b,c];                     // deprecated
-    a[(b,c)];                   // OK
-}
-\end{codeblock}
-\end{example}
-
 \rSec1[depr.array.comp]{Array comparisons}
 
 \pnum

source/overloading.tex

@@ -3297,29 +3297,32 @@
 \pnum
 A \defnadj{subscripting}{operator function}
 is a function named \tcode{\keyword{operator}[]}
-that is a non-static member function with exactly one non-object parameter.
+that is a non-static member function.
 For an expression of the form
 \begin{ncsimplebnf}
-postfix-expression \terminal{[} expr-or-braced-init-list \terminal{]}
+postfix-expression \terminal{[} \opt{expression-list} \terminal{]}
 \end{ncsimplebnf}
 the operator function
 is selected by overload resolution\iref{over.match.oper}.
 If a member function is selected,
 the expression is interpreted as
 \begin{ncsimplebnf}
-postfix-expression . \keyword{operator} \terminal{[}\terminal{]} \terminal{(} expr-or-braced-init-list \terminal{)}
+postfix-expression . \keyword{operator} \terminal{[}\terminal{]} \terminal{(} \opt{expression-list} \terminal{)}
 \end{ncsimplebnf}
 
 \pnum
 \begin{example}
 \begin{codeblock}
 struct X {
   Z operator[](std::initializer_list<int>);
+  Z oeprator[](auto...);
 };
 X x;
-x[{1,2,3}] = 7;                 // OK: meaning \tcode{x.\keyword{operator}[](\{1,2,3\})}
+x[{1,2,3}] = 7;                 // OK, meaning \tcode{x.\keyword{operator}[](\{1,2,3\})}
+x[1,2,3] = 7;                   // OK, meaning \tcode{x.\keyword{operator}[](1,2,3)}
 int a[10];
 a[{1,2,3}] = 7;                 // error: built-in subscript operator
+a[1,2,3] = 7;                   // error: built-in subscript operator
 \end{codeblock}
 \end{example}
 

source/preprocessor.tex

@@ -1777,6 +1777,7 @@
 \defnxname{cpp_inline_variables}                  & \tcode{201606L} \\ \rowsep
 \defnxname{cpp_lambdas}                           & \tcode{200907L} \\ \rowsep
 \defnxname{cpp_modules}                           & \tcode{201907L} \\ \rowsep
+\defnxname{cpp_multidimensional_subscript}        & \tcode{202110L} \\ \rowsep
 \defnxname{cpp_namespace_attributes}              & \tcode{201411L} \\ \rowsep
 \defnxname{cpp_noexcept_function_type}            & \tcode{201510L} \\ \rowsep
 \defnxname{cpp_nontype_template_args}             & \tcode{201911L} \\ \rowsep

source/xrefdelta.tex

@@ -69,5 +69,8 @@
 \movedxref{range.split.outer.value}{range.lazy.split.outer.value}
 \movedxref{range.split.inner}{range.lazy.split.inner}
 
+% P2128R6 Multidimensional subscript operator
+\removedxref{depr.comma.subscript}
+
 % Deprecated features.
 %\deprxref{old.label}  (if moved to depr.old.label, otherwise use \movedxref)