9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.9 Type specifiers [dcl.type] Pack indexing specifier [dcl.type.pack.index]

The typedef-name P in a pack-index-specifier shall denote 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-specifier is a pack expansion ([temp.variadic]).
[Note 1: 
The pack-index-specifier denotes the type of the element of the pack.
— end note]