Brief absl::optional description

optional.h

#if __cplusplus >= 201703 && __has_include(<optional>)

#include <optional>

namespace absl {
using std::bad_optional_access;
using std::optional;
using std::make_optional;
using std::nullopt_t;
using std::nullopt;
}  // namespace absl

#else  // __cplusplus >= 201703 && __has_include(<optional>)

// Internal abseil implementation.
// Under define not to generate more code to a compiler because std::optional
// and absl::optional claim to be compatible.

#endif