SumType!(A,B) != SumType!(B,A) - a bug?

[Nickolay Bukreyev]

On Tuesday, 26 December 2023 at 19:19:08 UTC, Paul Backus wrote:
> I think this is probably a compiler bug, but it's hard to tell 
> without a reduced example.

```d
union A {
     int x;
     int[] arr;
}

union B {
     int[] arr;
     int x;
}

A createA() /+pure+/ { return A.init; }
B createB() /+pure+/ { return B.init; }

void main() {
     immutable a = createA(); // OK
     immutable b = createB(); // cannot implicitly convert 
expression `createB()` of type `B` to `immutable(B)`
}
```

If we mark `createB` pure, it will compile successfully. This 
behaviour is described in the spec: `createB` becomes a [pure 
factory 
function](https://dlang.org/spec/function.html#pure-factory-functions).

I believe DMD should issue an error for `A` as well.