[Issue 17771] New: foreach over const input range fails

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=17771

          Issue ID: 17771
           Summary: foreach over const input range fails
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: alex.goltman at gmail.com

```
    static struct NumRange {
        int front = 0;
        int end = 0;

        @property auto length() const { return end - front; }
        @property bool empty() const { return length == 0; }
        void popFront() { front++; }
    }
    const r = NumRange(0, 5);
    foreach (x; r) {
        writeln(x);
    }
```
results in
```
Error: mutable method test.main.NumRange.popFront is not callable using a const
object
```
even though foreach copies the input range it gets (as evident from running
foreach on same range twice), so it can at least try to copy it to an
non-const.

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