[allocator.tag]

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namespace std {
  struct allocator_arg_t { explicit allocator_arg_t() = default; };
  inline constexpr allocator_arg_t allocator_arg{};
}

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

Specifically, several types (see tuple [tuple]) have constructors with allocator_arg_t as the first argument, immediately followed by an argument of a type that satisfies the Cpp17Allocator requirements (Table 33).

19 General utilities library [utilities]

19.10 Memory [memory]

19.10.7 Allocator argument tag [allocator.tag]