deduction guides for std::optional

Defined in header <optional> 
template< class T >
optional(T) -> optional<T>;
 (since C++17)

One deduction guide is provided for std::optional to account for the edge cases missed by the implicit deduction guides, in particular, non-copyable arguments and array to pointer conversion.

Example

#include <optional>
#include <type_traits>
 
int main()
{
    int a[2];
    std::optional oa{a}; // uses explicit deduction guide
    static_assert(std::is_same_v<decltype(oa), std::optional<int*>> == true);
}

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