Merge 2019-07 CWG Motion 2

P1161R3 Deprecate uses of the comma operator in subscripting expressions

Fixes #2981.

Author: zygoloid

source/expressions.tex

@@ -2862,6 +2862,14 @@
 if that operand is an lvalue and an xvalue otherwise.
 The expression \tcode{E1} is sequenced before the expression \tcode{E2}.
 
+\pnum
+\begin{note}
+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}.
+\end{note}
+
 \pnum
 \begin{note}
 Despite its asymmetric appearance, subscripting is a commutative
@@ -6793,6 +6801,14 @@
 \tcode{5}.
 \end{example}
 
+\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

@@ -54,6 +54,24 @@
 \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