How are extra copy constructor parameters used?

[Tejas]

On Thursday, 30 December 2021 at 01:04:10 UTC, Ali Çehreli wrote:
> The second item in the documentation mentions "any number of 
> default parameters" when describing copy constructor syntax:
>
>   https://dlang.org/spec/struct.html#struct-copy-constructor
>
> 1) I can't figure out how to use those extra parameters. For 
> example, I can't find a special function name to call 
> explicitly:
>
>   S.__cctor(a, 42);  // No go
>


Behold the pathetic hack!
```d
import std.stdio:writeln;

struct A
{
     this(ref return scope A rhs) inout {}                        
// copy constructor
     this(ref A rhs, int b = 7) inout {        					 // copy 
constructor with default parameter
         if (b != 7) {
             rhs.b = b;//yes, modify the parameter, not the this 
object :(
             rhs.a = rhs.a;
         }        	
         else
             foreach (i, ref field; rhs.tupleof)
         		field = this.tupleof[i] ;


     }
     this(this) @disable;
     int a=4;
     int b=3;
}

void main()
{
     A a = A();
     A b = void;
     a.__ctor(b, 9); // because writing A b = A(a, 9); like a sane 
human is giving errors D:
     writeln(b.b);
}
```

Replacing `a.__ctor(b, 9);` with `A b = A(a, 9);` is yielding:

```d
onlineapp.d(26): Error: cannot implicitly convert expression `a` 
of type `A` to `int`
```