> If the implementation uses C UB, which causes it to be out of compliance with the Python specification, then it is both undefined behavior for C and a failure to follow defined behavior for Python.
That's true, but it's not what you said. You said "the C specification is irrelevant" when I brought up bugs in the C code in CPython, and you said they were implementation bugs but not UB, because the Python spec doesn't define them as UB -- even though the code we were talking about was C! In other words, case 2 in my previous comment.
Now you say "When run in CPython, the implementation follows the C language specification," and "If the implementation uses C UB, which causes it to be out of compliance with the Python specification, then it is both undefined behavior for C and a failure to follow defined behavior for Python."
Those arguments contradict each other. Which one do you believe?
Programs written in Python follow the Python language specification.
When run in CPython, the implementation follows the C language specification.
If the implementation uses C UB, which causes it to be out of compliance with the Python specification, then it is both undefined behavior for C and a failure to follow defined behavior for Python.
It is not undefined behavior for Python.
I have said multiple times that I don't like how your non-standard definition mixed the two together. It is no longer interesting to come up with new ways to restate my statement.