7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.4 Names [expr.prim.id] Pack indexing expression [expr.prim.pack.index]

The id-expression P in a pack-index-expression shall be an identifier that denotes a pack.
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 .
A pack-index-expression is a pack expansion ([temp.variadic]).
[Note 1: 
A pack-index-expression denotes the element of the pack.
— end note]