AA with class keys compared with identity instead of opEquals

[Per Nordlöw]

On Sunday, 27 September 2020 at 18:39:10 UTC, Per Nordlöw wrote:
> How do I defined an AA with class as key where keys are 
> compared using `is` instead of `opEquals`? Do I have to store 
> the key as a `void*`?

I got a good answer at 
https://dlang.slack.com/archives/C1ZDHBB2S/p1601234030016700