nsPromiseSubstring( const nsReadingIterator<CharT>& aStart, const nsReadingIterator<CharT>& aEnd ) : mString(*aStart.mOwningString)
nsPromiseSubstring( const basic_nsAReadableString<CharT>& aString, const nsReadingIterator<CharT>& aStart, const nsReadingIterator<CharT>& aEnd ) : mString(aString)
      nsPromiseSubstring( const nsReadingIterator<CharT>& aStart, const nsReadingIterator<CharT>& aEnd )          : mString(*aStart.mOwningString)        {          nsReadingIterator<CharT> zeroPoint;          mOwningString.BeginReading(zeroPoint);          mStartPos = Distance(zeroPoint, aStart);          mLength = Distance(aStart, aEnd);        }