This has not been adopted by LEWG/LWG yet, but was submitted
as a proposed resolution for a new LWG issue; optional<const T> can't
current be compared to a T, and an optional<T> can't be compared
to something non-T that is comparable to a T. This approach fixes
both of those problems, allowing optional's comparisons to actually
work like the comparisons of the underlying type.

Tested on Linux-x64.

2017-02-19  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Make optional's comparisons be two-parameter templates.
    * include/std/optional
    (operator==(const optional<_Tp>&, const optional<_Tp>&)):
    Turn into operator==(const optional<_Tp>&, const optional<_Up>&).
    (operator!=(const optional<_Tp>&, const optional<_Tp>&)):
    Turn into operator!=(const optional<_Tp>&, const optional<_Up>&).
    (operator<(const optional<_Tp>&, const optional<_Tp>&)):
    Turn into operator<(const optional<_Tp>&, const optional<_Up>&.
    (operator>(const optional<_Tp>&, const optional<_Tp>&)):
    Turn into operator>(const optional<_Tp>&, const optional<_Up>&.
    (operator<=(const optional<_Tp>&, const optional<_Tp>&)):
    Turn into operator<=(const optional<_Tp>&, const optional<_Up>&).
    (operator>=(const optional<_Tp>&, const optional<_Tp>&)):
    Turn into operator>=(const optional<_Tp>&, const optional<_Up>&).
    (operator==(const optional<_Tp>&, const _Tp&)):
    Turn into operator==(const optional<_Tp>&, const _Up&).
    (operator==(const _Tp&, const optional<_Tp>&)):
    Turn into operator==(const _Up&, const optional<_Tp>&).
    (operator!=(const optional<_Tp>&, const _Tp&)):
    Turn into operator!=(const optional<_Tp>&, const _Up&).
    (operator!=(const _Tp&, const optional<_Tp>&)):
    Turn into operator!=(const _Up&, const optional<_Tp>&).
    (operator<(const optional<_Tp>&, const _Tp&)):
    Turn into operator<(const optional<_Tp>&, const _Up&).
    (operator<(const _Tp&, const optional<_Tp>&)):
    Turn into operator<(const _Up&, const optional<_Tp>&).
    (operator>(const optional<_Tp>&, const _Tp&)):
    Turn into operator>(const optional<_Tp>&, const _Up&).
    (operator>(const _Tp&, const optional<_Tp>&)):
    Turn into operator>(const _Up&, const optional<_Tp>&).
    (operator<=(const optional<_Tp>&, const _Tp&)):
    Turn into operator<=(const optional<_Tp>&, const _Up&).
    (operator<=(const _Tp&, const optional<_Tp>&)):
    Turn into operator<=(const _Up&, const optional<_Tp>&).
    (operator>=(const optional<_Tp>&, const _Tp&)):
    Turn into operator>=(const optional<_Tp>&, const _Up&).
    (operator>=(const _Tp&, const optional<_Tp>&)):
    Turn into operator>=(const _Up&, const optional<_Tp>&).
    * testsuite/20_util/optional/relops/7.cc: New.