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

[Paul Backus]

On Tuesday, 26 December 2023 at 19:05:08 UTC, Andrey Zherikov 
wrote:
> Here is the code:
> ```d
> struct A {}
> struct B { int[] i; }
>
> immutable s1 = SumType!(A,B)(B.init);    // (1) compiles
> immutable s2 = SumType!(B,A)(B.init);    // (2) compilation 
> error
> ```
[...]
> Questions:
> - Should (1) and (2) behave the same way?
> - Is there anything I can do with type `A` to make (1) fail to 
> compile the same way as (2)?

Yes, they should both behave the same way--either the conversion 
from mutable to immutable is valid, or it isn't.

I think this is probably a compiler bug, but it's hard to tell 
without a reduced example.