7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.4 Names [expr.prim.id]

7.5.4.4 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
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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
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[Note 1: 
A pack-index-expression denotes the element of the pack.
— end note]