======================================================================
FAIL: test_specific_values (__main__.CMathTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/raymondhettinger/cpython/Lib/test/test_cmath.py", line 382, in test_specific_values
    msg=error_message)
  File "/Users/raymondhettinger/cpython/Lib/test/test_cmath.py", line 128, in rAssertAlmostEqual
    'got {!r}'.format(a, b))
AssertionError: rect1017: rect(complex(0.0, -0.0))
Expected: complex(0.0, -0.0)
Received: complex(0.0, 0.0)
Received value insufficiently close to expected value.

Hmm.  Looks similar to issue #15477, but that one was fixed (we added a local workaround and reported the OS bug to Apple).  And it was with clang rather than gcc.

Raymond:  from the title, can I assume that you don't see this failure on a debug build?  That would tend to point to a compiler optimization bug.

I can't reproduce this here on OS 10.6 (I don't have access to anything later), but I think I can guess what's happening:  I suspect that gcc 4.8 is optimizing the pair of 'cos' and 'sin' calls into a single call to 'cexp'.  And then the OS X library implementation of _cexp has buggy behaviour with respect to signs of zeros (this last part is something I've witnessed before, but I forget which version of OS X it was on).

It's easy to work around, by adding a special case for zeros in cmath_rect.  It's a bit annoying to have to do so, though.

New changeset ce771c2d0220 by Mark Dickinson in branch '3.3':
Issue #18513: Add workaround for OS X 10.8 cexp bug that leads to wrong cmath.rect(0.0,-0.0) results.
http://hg.python.org/cpython/rev/ce771c2d0220

New changeset ae769deb45b2 by Mark Dickinson in branch 'default':
Issue #18513: Add workaround for OS X 10.8 cexp bug that leads to wrong cmath.rect(0.0,-0.0) results.
http://hg.python.org/cpython/rev/ae769deb45b2

Should be fixed now.

New changeset 91374660355a by Raymond Hettinger in branch '2.7':
Issue #18513: Add workaround for OS X 10.8 cexp bug that leads to wrong cmath.rect(0.0,-0.0) results.
http://hg.python.org/cpython/rev/91374660355a

Thanks Mark.  test_cmath passed now.  I've also applied this to Python2.7.

Thanks, Raymond.

Postscript:  I had a feeling of deja vu about this, and I've just been searching through old emails.  It turns out I filed an OS X bug (#6466807  ) for this exact issue back in 2008;  cexp was broken in 10.5, fixed in 10.6, and clearly is now broken again in 10.8.  I've added a note to that Apple bug report, for what it's worth.