I do not understand copy constructors

[Learner]

On Thursday, 12 August 2021 at 13:56:17 UTC, Paul Backus wrote:
> On Thursday, 12 August 2021 at 12:10:49 UTC, Learner wrote:
>>
>> That worked fine, but the codebase is @safe:
>>
>> ```d
>> cast from `int[]` to `inout(int[])` not allowed in safe code
>> ```
>>
>> So copy constructors force me to introduce trusted methods, 
>> while that was not necessary with postblits?
>
> A postblit would simply ignore the type qualifier--which can 
> lead to undefined behavior. (Scroll down to the paragraph that 
> begins "An unqualified postblit..." under ["Struct 
> Postblits"][1] in the spec.) The copy constructor merely forces 
> you to be honest about the safety of your code.
>
> In your case, I would recommend encapsulating the unsafe cast 
> in a function like the following:
>
> ```d
> T[] dupWithQualifiers(T[] array)
> {
>     auto copy = array.dup;
>     return (() @trusted => cast(T[]) copy)();
> }
> ```
>
> You can then use this function in place of `dup` in your copy 
> constructor.
>
> [1]: https://dlang.org/spec/struct.html#struct-postblit

Thank you, now everything is more clear.

A last question, if you do not mind, just to better understand 
inout. It seems a shortcut to avoid repeating the same function 
body for mutable, const, and immutable. Why the following code is 
not equal to the single inout constructor?

     struct A {
         int[] data;

         //this(ref return scope inout A rhs) inout                
{ /*body*/ }

         this(ref return scope           Timestamp rhs)            
{ /*body*/ }
         this(ref return scope const     Timestamp rhs) const      
{ /*body*/ }
         this(ref return scope immutable Timestamp rhs) immutable  
{ /*body*/ }
     }
     Error: Generating an `inout` copy constructor for `struct B` 
failed, therefore instances of it are uncopyable

Inout is compatible only with inout, and not with the unrolled 
code it implies?