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digitalmars.D.bugs - [Issue 20964] New: poor CTFE support for backward pointer (iterator)

https://issues.dlang.org/show_bug.cgi?id=20964

          Issue ID: 20964
           Summary: poor CTFE support for backward pointer (iterator)
                    iteration
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: blocker
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: ilyayaroshenko gmail.com

Pointer can refer to the next right element after memory block.
However, referring to the next left element before the memory block is
prohibited.

```
uint foo(uint[3] m)
{
        auto p = m.ptr;
    p += 3; // ok, p refers to the next element after block!
    p -= 4; // fails: `cannot assign pointer to index -1 inside memory block
[0..3]`
    p++;
    return *p;
}

static assert(foo([3, 2, 4]) == 3);
```

The use case:

uint* retroStart; // refers to the last element
uint length;

foreach (i; 0 .. length)
{
    // use *retroStart
    retroStart--;
}

This case is a reduced use case for generic retro iterators of Mir. Ranges
can't be used for this case.

--
Jun 20