Annex D (normative) Compatibility features [depr]

D.3 Comma operator in subscript expressions [depr.comma.subscript]

1
#
A comma expression ([expr.comma]) appearing as the expr-or-braced-init-list of a subscripting expression ([expr.sub]) is deprecated.
[Note
:
A parenthesized comma expression is not deprecated.
— end note
]
[Example
: 
void f(int *a, int b, int c) {
    a[b,c];                     // deprecated
    a[(b,c)];                   // OK
}
— end example
]