Actually raising an exception in PyErr_WarnEx() is not an error, but a standard behavior with corresponding configuration.

For now running tests with -We in debug build crashes.

$ ./python -We -m test test_gc
Run tests sequentially
0:00:00 load avg: 0.33 [1/1] test_gc
Fatal Python error: a function returned a result with an error set
RuntimeWarning: Garbage collector enabled while another thread is inside gc.ensure_enabled

The above exception was the direct cause of the following exception:

SystemError: <built-in method __exit__ of gc.ensure_disabled object at 0x7fb1b62a08f8> returned a result with an error set

Thread 0x00007fb1b1773700 (most recent call first):
  File "/home/serhiy/py/cpython/Lib/test/test_gc.py", line 1050 in disabling_thread
  File "/home/serhiy/py/cpython/Lib/threading.py", line 865 in run
  File "/home/serhiy/py/cpython/Lib/threading.py", line 917 in _bootstrap_inner
  File "/home/serhiy/py/cpython/Lib/threading.py", line 885 in _bootstrap

Current thread 0x00007fb1b824f040 (most recent call first):
  File "/home/serhiy/py/cpython/Lib/test/test_gc.py", line 1061 in test_ensure_disabled_thread
  File "/home/serhiy/py/cpython/Lib/test/support/__init__.py", line 2083 in decorator
  File "/home/serhiy/py/cpython/Lib/unittest/case.py", line 615 in run
  File "/home/serhiy/py/cpython/Lib/unittest/case.py", line 663 in __call__
  File "/home/serhiy/py/cpython/Lib/unittest/suite.py", line 122 in run
  File "/home/serhiy/py/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/home/serhiy/py/cpython/Lib/unittest/suite.py", line 122 in run
  File "/home/serhiy/py/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/home/serhiy/py/cpython/Lib/test/support/__init__.py", line 1760 in run
  File "/home/serhiy/py/cpython/Lib/test/support/__init__.py", line 1861 in _run_suite
  File "/home/serhiy/py/cpython/Lib/test/support/__init__.py", line 1951 in run_unittest
  File "/home/serhiy/py/cpython/Lib/test/test_gc.py", line 1088 in test_main
  File "/home/serhiy/py/cpython/Lib/test/libregrtest/runtest.py", line 176 in runtest_inner
  File "/home/serhiy/py/cpython/Lib/test/libregrtest/runtest.py", line 140 in runtest
  File "/home/serhiy/py/cpython/Lib/test/libregrtest/main.py", line 379 in run_tests_sequential
  File "/home/serhiy/py/cpython/Lib/test/libregrtest/main.py", line 458 in run_tests
  File "/home/serhiy/py/cpython/Lib/test/libregrtest/main.py", line 536 in _main
  File "/home/serhiy/py/cpython/Lib/test/libregrtest/main.py", line 510 in main
  File "/home/serhiy/py/cpython/Lib/test/libregrtest/main.py", line 585 in main
  File "/home/serhiy/py/cpython/Lib/test/__main__.py", line 2 in <module>
  File "/home/serhiy/py/cpython/Lib/runpy.py", line 85 in _run_code
  File "/home/serhiy/py/cpython/Lib/runpy.py", line 193 in _run_module_as_main
Aborted (core dumped)