[DIP idea] out variables

[Q. Schroll]

On Thursday, 28 January 2021 at 09:17:47 UTC, Dukc wrote:
> int f(out int, int);
>
> int func()
> {  out int x;
>    if(someCond) x.f(0);
>    else if(someOtherCond) x.f(1);
>    return x;
> }
>
> What should the compiler do? It cannot know whether it's 
> possible x can be returned uninitialized. It can issue an error 
> just in case, and we hate to refactor code due to false alarms 
> like that.

Exactly this. Something similar is true for an immutable 
constructor:

struct S
{
     int a;
     this(int x) immutable
     {
         if (x < 0) { a = 1; }
         else if (x > 0) { a = 2; }
         if (x == 0) a = 3; // error
     }
}

This problem is inherent.