[pair.piecewise]

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struct piecewise_construct_t {
  explicit piecewise_construct_t() = default;
};
inline constexpr piecewise_construct_t piecewise_construct{};

The struct piecewise_construct_t is an empty class type used as a unique type to disambiguate constructor and function overloading.

Specifically, pair has a constructor with piecewise_construct_t as the first argument, immediately followed by two tuple arguments used for piecewise construction of the elements of the pair object.

22 General utilities library [utilities]

22.3 Pairs [pairs]

22.3.5 Piecewise construction [pair.piecewise]