Fix issue 20850 - Can't assign enum of Tuple

[Biotronic]

areCompatibleTuples has to check OriginalType to account for enums. Also, swap is confused, so we're not using that for these assignments. Casting to OriginalType would work, but breaks @safe.

[dlang-bot]

Thanks for your pull request and interest in making D better, @Biotronic! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20850	normal	Can't assign enum of Tuple

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7489"

[schveiguy]

I actually just envisioned that assignment to the same type would be supported. For instance, a type that alias this' itself to a Tuple should be assignable to that type.

[Biotronic]

If you mean this should work, it does:

import std.typecons;

struct S {
    alias t this;
    Tuple!int t;
}

unittest {
    Tuple!int a;
    S s;
    a = s;
}

Assigning s = a; doesn't work, but I'm pretty sure that can't be fixed in Tuple.

Strangely, a = S(); also doesn't work. That seems like a DMD issue though, since the above code works.

I don't think there's a way to allow this to work:

import std.typecons;

enum T1 : Tuple!(int*) { a = T1(null) }
enum T2 : Tuple!(int*) { a = T2(null) }

unittest {
    T1 t1;
    T2 t2;
    // Allow assignment from T1 to T1:
    t1 = T1.a;
    // But not T2 to T1:
    static assert(!__traits(compiles, t1 = T2.a));
}

[schveiguy]

Well, that's not what I expected. That seems like an odd difference between enum and alias this.

[schveiguy]

https://issues.dlang.org/show_bug.cgi?id=20854

[schveiguy]

This workaround is OK for now. But I don't love the extra isTuple inside the static if. I get why you need to do it, it's unfortunate we can't figure out how to make the enum work with the existing operator overloads.