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deduction guides for std::reference_wrapper

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Defined in header <functional>
template<typename T>
reference_wrapper(T&) -> reference_wrapper<T>;
(since C++17)

One deduction guide is provided for std::reference_wrapper to support deduction of the sole class template parameter.

[edit] Example

[edit] 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 2981 C++17 a redundant deduction guide from reference_wrapper<T> was provided removed
LWG 2993 C++17 defect resolution removed a constructor used for class template argument deduction added deduction guide to compensate