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deduction guides for std::weak_ptr

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std::weak_ptr
Defined in header <memory>
template < class T >
weak_ptr ( std:: shared_ptr < T > ) - > weak_ptr < T > ;
(since C++17)

One deduction guide is provided for std::weak_ptr to account for the edge case missed by the implicit deduction guides.

Example

Run this code 
#include <memory>
int main()
{
    auto p = std::make_shared<int>(42);
    std::weak_ptr w{p}; // explicit deduction guide is used in this case
}