Hello folks, I recently observed a failure on the s390x fedora rawhide buildbot, on the clang builds, when clang got updated to version 10: https://bugs.python.org/issue39689 The call: struct.unpack('>?', b'\xf0') means to unpack a "native bool", i.e. native size and alignment. Internally, this does: static PyObject * nu_bool(const char *p, const formatdef *f) { _Bool x; memcpy((char *)&x, p, sizeof x); return PyBool_FromLong(x != 0); } i.e., copies "sizeof x" (1 byte) of memory to a temporary buffer x, and then treats that as _Bool. While I don't have access to the C standard, I believe it says that assignment of a true value to _Bool can coerce to a unique "true" value. It seems that if a char doesn't have the exact bit pattern for true or false, casting to _Bool is undefined behavior. Is that correct? Clang 10 on s390x seems to take advantage of this: it probably only looks at the last bit(s) so a _Bool with a bit pattern of 0xf0 turns out false. But the tests assume that 0xf0 should unpack to True. -- Regards, Charalampos Stratakis Software Engineer Python Maintenance Team, Red Hat