[tuple.helper] paragraph one not universally true #6777
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[tuple.helper] p1 defines a general requirement that all specializations of
tuple_size
shall meet the Cpp17UnaryTypeTrait requirements but p4 actually defines a special situation where this requirement is not met ("Otherwise, it has no member value"). We have the same seemingly contradiction in [depr.tuple] p2. For clarity, we should point out that this provision is not universal.