Greg, I think you should apply your change to eliminate the regression and get us back to the state of all stdlib context managers being well-behaved in this regard (we unfortunately missed the 3.5.2 release, but that will ensure it's fixed for 3.5.3).

Given the challenges I had trying to reproduce the hang with a plain context manager rather than a generator, I now the problem is specifically just #25782 - in the absence of that, ExitStack() should misbehave in the same way actual nested with statements would, rather than trying to implicitly "fix" the misbehaving context managers.