I do not understand copy constructors

[Tejas]

On Thursday, 12 August 2021 at 15:39:40 UTC, Learner wrote:
> On Thursday, 12 August 2021 at 14:57:16 UTC, Steven 
> Schveighoffer wrote:
>> [...]
>
> It is not clear to me why the inout generated copy constructor 
> of the B structure is not able to copy the A structure.
>
> [...]

Why will copy constructor of ```struct B``` accept argument of 
type ```A```?