[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 class 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 meets the Cpp17Allocator requirements ([allocator.requirements.general]).

20 Memory management library [mem]

20.2 Memory [memory]

20.2.7 Allocator argument tag [allocator.tag]