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The issue is related to the definition of PyCArgObject:
typedef struct tagPyCArgObject PyCArgObject;
struct tagPyCArgObject {
 PyObject_HEAD
 ffi_type *pffi_type;
 char tag;
 union {
 char c;
 char b;
 short h;
 int i;
 long l;
 long long q;
 long double D;
 double d;
 float f;
 void *p;
 } value;
 PyObject *obj;
 Py_ssize_t size; /* for the 'V' tag */
};
This object must be allocated with suitable alignment (which is 16 on many platforms), and the default Python allocator apparently provides 8-byte alignment only on 64-bit platforms. In short, using PyObject_New with PyCArgObject results in undefined behavior.
This issue potentially affects all compilers, not just Clang.