Is it valid in D to write an opSlice overload that takes no arguments?

[solidstate1991 via Digitalmars-d-learn]

Here's this code:

```d
auto opSlice()
{
    struct Range
    {
        Attr currentAttr;
        auto front() { return currentAttr; }
        void popFront() { currentAttr = currentAttr._nextAttr; }
        bool empty() { return currentAttr is null; }
    }
    return Range(firstAttr);
}
```

This was originally in std.experimental.xml in `domimpl.d`, and 
it causes LDC to segfault, but compiles fine under DMD.