I am getting the following message:
Warning: struct SumType has method toHash, however it cannot be called with const(SumType!(A,B,C)) this

Could someone point in the right direction to understand what I am doing that causes this?

On Sunday, 11 July 2021 at 09:20:23 UTC, JG wrote:

The two restrictions on toHash are

1. if you're compiling with betterC, it's not present. So you wouldn't get this message.
2. otherwise, the types all have to pass allSatisfy!(isHashable, Map!(ConstOf, Types)), where isHashable is defined in std.sumtype to private enum isHashable(T) = __traits(compiles, () nothrow @safe { hashOf(T.init); });

So the problem is one of

1. A, B, or C doesn't have a .toHash
2. A, B, or C has an improper .toHash that isn't nothrow and @safe
3. A, B, or C has a .toHash but it doesn't work for const(A) etc.