Added isPartiallyOrdered in std.traits

[Bolpat]

In short, isPartiallyOrdered!(S, T) compiles, iff the ordering exists, and it returns false iff the comparison s <= t || s >= t between values s and t of type S and T cannot be false from a static type analysis point of view. It is needed order to implement #6499 properly and can be useful in the future.

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[Bolpat]

I don't know why Jenkins fails. The error seems not to be related with this PR; it hardly is as this PR is addition-only. Can the error be ignored?

[wilzbach]

Yeah sadly the Ocean error seems to be appearing on a few other PRs too.

[wilzbach]

Disabled it temporarily -> https://github.com/dlang/ci/pull/208 (after the next push, Jenkins should be green again).

[Bolpat]

@ZombineDev maybe a quick review?

[ghost]

Is this assertion failure expected ?

unittest
{
    struct F
    {
        float f;
        alias f this;
    }
    static assert(isPartiallyOrdered!F == isPartiallyOrdered!float);
}

[Bolpat]

Thank you @BBasile, the answer is no. I missed that and it's fixed now.

[RazvanN7]

@Bolpat Is this addition still required?

cc @atilaneves

[Bolpat]

@RazvanN7 What do you mean by required? Has something similar been added and I didn't notice?

[RazvanN7]

I saw that PR #6499 was closed so I wanted to make sure that you are still pursuing this. Will merging this PR unblock #6499?

[Bolpat]

@RazvanN7, I cannot guarantee it, but it is likely to be unblocked by it. However, note that slice comparison is broken in the same way. I'll work on it soon (the next days).

Edit: This should probably go to core and std.traits should have a public import of it. That way, array comparison can use it.

[RazvanN7]

@Bolpat Have you made any progress on this?