Fix Issue 18374 - Add range functions to Nullable

[Herringway]

Let's get Nullable working with the rest of phobos. Is this overkill? Probably. Definitely. 100% yes.
I genuinely have no idea what sort of documentation to write for these functions.

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[PetarKirov]

I genuinely have no idea what sort of documentation to write for these functions.

I suggest grouping all of these functions under the name "Range interface functions" like this:

    /// $(MREF_ALTTEXT Range interface, std, range, primitives) functions.
    alias empty = isNull;

    /// ditto
    alias popFront = nullify;

    /// ditto
    alias popBack = nullify;

...

Here's an example in std.regex: https://dlang.org/phobos/std_regex.html#.Captures.front.

Also I suggest adding a documented unittest to showcase idiomatic range-based usage. You can borrow a few examples from the optional package: https://code.dlang.org/packages/optional

[PetarKirov]

@atilaneves @pbackus @dkorpel what are your thoughts on this addition?

[dkorpel]

I don't use Nullable, so I don't have a strong opinion, but I think it's fine to give it a range interface.

[dkorpel]

I get the range interface, but not sure why it needs slicing.

[pbackus]

See previous comment re: opSlice.

[Herringway]

I don't know why. I don't know why not, either. Either way, it was simple enough to support.

[dkorpel]

Prefer less code, it's easier to maintain.

[pbackus]

Satisfying hasSlicing!T allows more efficient branches to be taken in several Phobos algorithms:

• std.algorithm.mutation.copy
• std.algorithm.searching.find
• std.range.take
• etc.

It's also necessary if we want ranges of Nullables (e.g., chain(nullable(x), nullable(y), nullable(z))) to support slicing.

[pbackus]

See previous comment re: opSlice.

[Herringway]

Almost forgot to add a changelog entry.

[iK4tsu]

Very nice, a much welcome change 🎉