Missing reversed params comparison ops for C++17

[mcencora]

C++17 needs additional comparison operator overloads with reversed parameters. For C++20 and newer it is unnecessary due to "synthesized three-way comparison" feature.

[X1aomu]

The comparison operators of expected have not yet be adopt in STL,, refered in P0323R12 .

To avoid the possibly incompatible behaviour with STL, it's not perferred to implement this feature right now.

Free free to tell me if someday it comes :)

[mcencora]

I don't understand. The comparison ops for std::expected are available since the beginning, see:
https://wg21.link/std23
Chapter:
22.8.6.8 Equality operators

[X1aomu]

Hi, @mcencora

Equality is not all of the comparison. To acheive equality, we need only two functions, the operator== and operator!=, but six (plus operator<, operator>, operator<= and operator>=) for comparison.

As what you post,

https://wg21.link/std23
Chapter:
22.8.6.8 Equality operators

it only define three overloads of operator==. So this is the thing of equality, and zeus::expected has reached that.

[mcencora]

You are missing the point, I am talking here about support for C++17, and to achieve (non-)equality we need in 12 overloads in total.

These are defined in spec (for non-void T):
operator==(expected, expected<T2, E2>)
operator==(expected, T2)
operator==(expected, unexpected)

So for C++17 you need additionally following:
reversed param order:
operator==(expected<T2, E2>, expected)
operator==(T2, expected)
operator==(unexpected, expected)

not-equal:
operator!=(expected, expected<T2, E2>)
operator!=(expected, T2)
operator!=(expected, unexpected)

not-equal reversed param order:
operator!=(expected<T2, E2>, expected)
operator!=(T2, expected)
operator!=(unexpected, expected)

[X1aomu]

Ahh, I think I get the point now. Let me invastigate the scenario of reversed operands later.

Good catch!!!