I do not understand copy constructors

[Learner]

On Thursday, 12 August 2021 at 16:12:39 UTC, Paul Backus wrote:
> On Thursday, 12 August 2021 at 15:39:40 UTC, Learner wrote:
>> [...]
>
> You have forgotten to add a member variable of type `A` to your 
> `B` struct. If you add one, you will see the following error 
> message:
>
> [...]

"implicit conversions are not allowed for arguments passed to 
`ref` parameters" was the point missing to me.

Good to know, and thank yo to everyone!