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From cppreference.com

Defined in header <memory>

struct allocator_arg_t { explicit allocator_arg_t() = default; };

 (1) (since C++11)

constexpr std::allocator_arg_t allocator_arg {};

 (2) (since C++11)
(inline since C++17)

1) std::allocator_arg_t is an empty class type used to disambiguate the overloads of constructors and member functions of allocator-aware objects, including std::tuple, std::function, std::packaged_task,(until C++17) and std::promise.

2) std::allocator_arg is an instance of (1) that can be passed to the constructors and member functions of such permitted types.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2510 C++11 the default constructor was non-explicit, which could lead to ambiguity made explicit

See also

checks if the specified type supports uses-allocator construction
(class template) [edit]