Fix issue 4130

[thewilsonator]

This fixes the issue, I'm not sure if the correct thing to do is to make DMD not emit bogus ASTs to begin with.

fixes #4130

[JohanEngelen]

add log output about this strange case?

[JohanEngelen]

does this actually produce correct code now?

[thewilsonator]

It doesn't crash, which is what it did before. As for the correctness of the code,I'm not really sure, but I don't know what its supposed to do other than iterate through the linked list of classes. @ZILtoid1991 could you provide a reduced test case of this code in action?

[ZILtoid1991]

class DOMImplementation {
class Attr  {  Attr  _nextAttr; }
class Map  {
    Attr firstAttr;
    auto opSlice()
    {
         struct Range
         {
             Attr currentAttr;

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

[kinke]

Thx, somewhat further reduced:

class Outer {
    class Inner {
        auto opSlice() {
            struct Range {
                void foo() {}
            }
            return Range();
        }
    }
}

2 outer aggregates are apparently required, and both need to be classes (works with any one of them being a struct).

[JohanEngelen]

It doesn't crash, which is what it did before. As for the correctness of the code,I'm not really sure, but I don't know what its supposed to do other than iterate through the linked list of classes.

If unsure about the correct behavior, then better to exit gracefully instead of silently continuing.