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deduction guides for std::tuple

Defined in header <tuple> 
template<class... UTypes>
tuple(UTypes...) -> tuple<UTypes...>;
 (1) (since C++17) 
template<class T1, class T2>
tuple(std::pair<T1, T2>) -> tuple<T1, T2>;
 (2) (since C++17) 
template<class Alloc, class... UTypes>
tuple(std::allocator_arg_t, Alloc, UTypes...) -> tuple<UTypes...>;
 (3) (since C++17) 
template<class Alloc, class T1, class T2>
tuple(std::allocator_arg_t, Alloc, std::pair<T1, T2>) -> tuple<T1, T2>;
 (4) (since C++17) 
template<class Alloc, class... UTypes>
tuple(std::allocator_arg_t, Alloc, tuple<UTypes...>) -> tuple<UTypes...>;
 (5) (since C++17)

These deduction guides are provided for std::tuple to account for the edge cases missed by the implicit deduction guides, in particular, non-copyable arguments and array to pointer conversion.

Example

#include <tuple>
int main()
{
    int a[2], b[3], c[4];
    std::tuple t1{a, b, c}; // explicit deduction guide is used in this case
}

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