Repro case:

class Repro {
  int data() { return 0; }
  int data() const { return 1; }
  int data() shared { return 2; }
  int data() immutable { return 3; }
  
  int opSlice() { return data(); }
  int opSlice() const { return data(); }
  int opSlice() shared { return data(); }
  int opSlice() immutable { return data(); }
}

The calls inside opSlice are ambugious accodring to the compiler.
If you replace the call with data() with this.data() everything works fine.