Implement opCmp for Nullable objects

[DeterminedSage]

Fixes : #10743

Summary

This PR adds a opCmp implementation for Nullable, enabling it to participate in standard algorithms like sort, uniq, and allowing direct comparison with raw values or other nullable-like types.

Root Cause

Previously, 'Nullable' lacked a flexible 'opCmp' method that allowed it to be compared with another 'Nullable' or 'Nullable'-like objects with '.isNull' and '.get' .

This limitation meant that generic code and algorithms relying on opCmp (e.g., sort, uniq, associative containers) could not work with Nullable values out of the box.

For example :

import std.algorithm : sort;

auto arr = [Nullable!int(5), Nullable!int(), Nullable!int(10)];
sort(arr); // compile error before

would fail to compile due to missing comparison support between Nullable instances .

Fix

This patch introduces a generic opCmp(this This, Rhs) function with appropriate constraints to ensure type safety and correct behavior .

int opCmp(this This, Rhs)(auto ref Rhs rhs) const
if (is(typeof(_value.payload < rhs.get)) && is(typeof(_value.payload > rhs.get)))

The implementation handles :

Nullable vs Nullable: compares null-ness, then payload.

Nullable vs nullable-like: uses .isNull and .get.

Nullable vs raw value: directly compares to rhs.

Null is considered less than any non-null value, and two nulls are considered equal.

Example Reproducer

import std.algorithm : sort;
import std.typecons : Nullable;

void main() {
    auto arr = [Nullable!int(10), Nullable!int(), Nullable!int(5)];
    sort(arr);
    assert(arr[0].isNull);
    assert(arr[1].get == 5);
    assert(arr[2].get == 10);
}

This now compiles and runs as expected. Previously, this would result in a compile-time error.

Notes

Unittest addition for basic comparison between null and non-null values

Verified locally using custom Phobos build and test suite.

This patch fully resolves the issue described in #10743.

[0xEAB]

Buildkite failure looks unrelated: gecko0307/dagon#106

[DeterminedSage]

Buildkite failure looks unrelated: gecko0307/dagon#106

@PetarKirov went for a round 2 of Testing without making any changes but fails again due to same reason

[rikkimax]

Use typeof(this) instead of this This if you need the this pointers type.

[rikkimax]

Since it is no longer needed, you can remove this template parameter.

[rikkimax]

Suggested change

	int opCmp(this This, Rhs)(auto ref Rhs rhs) const
	int opCmp(Rhs)(auto ref Rhs rhs) const

[pbackus]

The DDoc comment says that this opCmp function is intended to compare two Nullable values, but the actual code allows values that are not Nullable. The code should be fixed to match the documentation.

[pbackus]

This test will return true for any type with a get method that returns a value comparable to the payload, even if it is not a Nullable type. The correct way to check whether Rhs is a Nullable type whose payload is comparable to this Nullable's payload is like this:

    if (is(Rhs == Nullable!U, U) && is(typeof(this.get < rhs.get)))

(Strictly speaking, it doesn't matter whether you use payload or get in the typeof(...) expression, but you should use the same on both sides for consistency.)

[pbackus]

Once you've fixed the template constraint, you can get rid of the static if (is(Rhs == Nullable)) statement and combine these two branches.

[pbackus]

This branch is unreachable and should be deleted.