7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.5 Names [expr.prim.id]

7.5.5.4 Pack indexing expression [expr.prim.pack.index]

1
#
The id-expression P in a pack-index-expression shall be an identifier that denotes a pack.
2
#
The constant-expression shall be a converted constant expression ([expr.const]) of type std​::​size_t whose value V, termed the index, is such that .
3
#
A pack-index-expression is a pack expansion ([temp.variadic]).
4
#
[Note 1: 
A pack-index-expression denotes the element of the pack.
— end note]