[Issue 21087] New: refRange() does not work with ranges which have internal const fields

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21087

          Issue ID: 21087
           Summary: refRange() does not work with ranges which have
                    internal const fields
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: andrej.mitrovich at gmail.com

```
import std.range;

public static auto byRange ()
{
    static struct Range
    {
        size_t lbound = 0;
        const size_t hbound = 100;

        size_t length () const { return 0; }
        bool empty () const { return false; }
        int front () const { return 0; }
        void popFront () { }
    }

    return Range(0);
}

void main ()
{
    auto keys = byRange();
    auto r = refRange(&keys);
}
```

$ dmd test.d
> /Library/D/dmd/src/phobos/std/range/package.d(11504,13): Error: cannot modify struct instance `*this._range` of type `Range` because it contains `const` or `immutable` members
> /Library/D/dmd/src/phobos/std/range/package.d(12215,16): Error: template instance `std.range.RefRange!(Range)` error instantiating
test.d(22,22):        instantiated from here: `refRange!(Range)`

As a workaround `hbound` can be declared an enum here.

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