I have been following this bug in its various form for 30 years, and it is covered in pages 55-62 of The End of Error: Unum Computing. It is the "hidden scratchpad" bug where designers of a computing environment try to be helpful by providing more precision than what is explicitly requested by the programmer. If you read the blog postings of William Kahan about this issue, you will find he speaks fervently on both sides, sometimes defending it and other times vehemently deriding it. Thus the confusion. Kahan favors any technique that improves accuracy, even if it creates behavior that can only be explained through careful numerical analysis (which very few programmers have the time or skill set to perform).

Unums and posits, incidentally, are designed to eliminate all sources of irreproducibility and inexplicable behavior like this gcc bug. Hardware support for posits is on the way, maybe before the year is out. There is a way out of this mess, but it will take a few years to become mainstream.