Checked out Python-3.5 from mercurial. Compiled with Clang 3.4, added -fsanitzie=undefined to CFLAGS. Ran 'make' and 'make check'. A few issues during `make`:

Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
Objects/listobject.c:2046:22: runtime error: index 623 out of bounds for type 'PyObject *[256]'
Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'

Similar issues with `make test`, with the addition of some mis-aligned pointers. A digest is below, and the tagCDataObject alignment issue dominates the issues. The tests ended early due to a segfault.

$ make test
...
[ 19/389/1] test_struct
Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
/home/jwalton/Desktop/cpython-checkout/Objects/floatobject.c:2028: runtime error: value 3.40282e+38 is outside the range of representable values of type 'float'
/home/jwalton/Desktop/cpython-checkout/Modules/_struct.c:792:15: runtime error: left shift of 72057594037927935 by 8 places cannot be represented in type 'long long'
...
[ 43/389/2] test_cmd_line
Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
/home/jwalton/Desktop/cpython-checkout/Modules/_ctypes/_ctypes.c:2890:10: runtime error: member access within misaligned address 0x2b54033f1cc8 for type 'CDataObject' (aka 'struct tagCDataObject'), which requires 16 byte alignment
0x2b54033f1cc8: note: pointer points here
 ff ff ff ff  01 00 00 00 00 00 00 00  68 06 43 03 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^ 
test test_cmd_line failed -- multiple errors occurred; run in verbose mode for details
...
[ 45/389/2] test_datetime
Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
/home/jwalton/Desktop/cpython-checkout/Python/pytime.c:218: runtime error: value -1e+200 is outside the range of representable values of type 'long'
/home/jwalton/Desktop/cpython-checkout/Python/pytime.c:218: runtime error: value 1e+200 is outside the range of representable values of type 'long'
...
test test_traceback failed -- Traceback (most recent call last):
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/test_traceback.py", line 155, in test_encoded_file
    do_test("", "foo", "ascii", 3)
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/test_traceback.py", line 132, in do_test
    stdout = stdout.decode(output_encoding).splitlines()
LookupError: unknown encoding: Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
...
[130/389/5] test_capi
Objects/longobject.c:40:42: runtime error: index -3 out of bounds for type 'PyLongObject [262]'
Fatal Python error: Segmentation fault

Current thread 0x00002b6eaf8c7b20 (most recent call first):
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/test_capi.py", line 466 in test__testcapi
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/case.py", line 574 in run
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/case.py", line 622 in __call__
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/suite.py", line 125 in run
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/suite.py", line 87 in __call__
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/suite.py", line 125 in run
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/suite.py", line 87 in __call__
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/suite.py", line 125 in run
  File "/home/jwalton/Desktop/cpython-checkout/Lib/unittest/suite.py", line 87 in __call__
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/support/__init__.py", line 1584 in run
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/support/__init__.py", line 1685 in _run_suite
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/support/__init__.py", line 1719 in run_unittest
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 1277 in <lambda>
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 1278 in runtest_inner
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 978 in runtest
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 532 in main
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 1562 in main_in_temp_cwd
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 1587 in <module>
  File "/home/jwalton/Desktop/cpython-checkout/Lib/runpy.py", line 86 in _run_code
  File "/home/jwalton/Desktop/cpython-checkout/Lib/runpy.py", line 171 in _run_module_as_main
Traceback (most recent call last):
  File "/home/jwalton/Desktop/cpython-checkout/Lib/runpy.py", line 171, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/jwalton/Desktop/cpython-checkout/Lib/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/__main__.py", line 3, in <module>
    regrtest.main_in_temp_cwd()
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 1562, in main_in_temp_cwd
    main()
  File "/home/jwalton/Desktop/cpython-checkout/Lib/test/regrtest.py", line 738, in main
    raise Exception("Child error on {}: {}".format(test, result[1]))
Exception: Child error on test_capi: Exit code -11
make: *** [test] Error 1