Providing implicit conversion of - memory-safety

[Danilo]

On Tuesday, 23 January 2024 at 17:54:25 UTC, bachmeier wrote:
> Here's a reduced version of one of the most bizarre bugs I've 
> dealt with in any language. The only reason I didn't move on to 
> another language was because I was too busy at the time.
>
> The code allows for initial values if the index is less than 0, 
> otherwise it returns the element.
>
> ```
> import std;
>
> double value(T)(T index, double * x) {
>   if (index - 5 < 0) {
>     return 0.0;
>   } else {
>     return x[index-5];
>   }
> }
>
> void main() {
>   double[] v = [1.1, 2.2, 3.3];
>   // Works
>   writeln(value(3, v.ptr));
>   // Lucky: program segfaults
>   writeln(value(v.length, v.ptr));
> }
> ```
>
> I noticed this behavior only because the program crashes. Once 
> I figured out what was going on, I realized that the thousands 
> of lines of code I had already written needed to be checked and 
> possibly rewritten. If only I had a compiler to do that for me.

How did you make it correct?

Write 2 different versions for `signed` and `unsigned` types?
Or could you utilize `core.checkedint` somehow for checking 
overflow?

```d
double value(T)(T index, double * x) {
     bool overflow;
     subu(index, 5, overflow);

     if (overflow) {
         return 0.0;
     } else {
         return x[index-5];
     }
}
```
This is probably only correct for `unsigned` types.