OOI, how do you propose to emit a compiler warning for the ma_verison_tag field without creating lots of warnings when building CPython?

OOI, how do you propose to emit a compiler warning for the ma_verison_tag field without creating lots of warnings when building CPython?

We would have to move the dict watchers data to some other field. Then we can remove all uses of ma_version_tag in CPython, which should hopefully mean no warnings when we compile CPython.

Until we remove the field, we need to update it, otherwise we should just remove it.
Leaving it, but not updating it, breaks any code using it.

I was wondering: don't dict watchers break it already? Or at least makes it backwards incompatible to a certain extent?

dict watchers use the low 8 bits. The only change is that the dict-version could wrap in a few years, rather than centuries.

Hmm ok, this clearly needs more discussion. Mark do you mind responding on the discourse thread with your opinions on why we should remove it immediately?

My thinking is that the ma_version_tag cannot be deprecated with a compiler warning, because we need to update the value every time we update the dict.

Is it possible (in GCC or some other compiler) to deprecate the attribute, and then suppress the warnings only for the places we use it?

Ohh right I get what you mean. Hmmm this is a pickle. I need to think about it.

You could try replacing uint64_t ma_version_tag with this:

union {
    uint64_t ma_version_tag  __attribute__ ((deprecated));
    uint64_t internal_vt; /* Internal. Do not use. May be removed without warning! */
};

And replacing all uses of ma_version_tag with internal_vt.

And replacing all uses of ma_version_tag with internal_vt.

But we still need to maintain the semantics of ma_version_tag until we remove it right? That would mean internally we still need to write to it and update it, which would trigger cpython build warnings?

Is it possible (in GCC or some other compiler) to deprecate the attribute, and then suppress the warnings only for the places we use it?

We can deprecate the attribute for extension modules but not for core code and continue to use it as usual. The warning will only be emitted for extension modules. See my implementation of this idea kumaraditya303/cpython#15

But we still need to maintain the semantics of ma_version_tag until we remove it right? That would mean internally we still need to write to it and update it, which would trigger cpython build warnings?

We update it without warning through internal_vt. That's what the union is for.

But we still need to maintain the semantics of ma_version_tag until we remove it right? That would mean internally we still need to write to it and update it, which would trigger cpython build warnings?

We update it without warning through internal_vt. That's what the union is for.

Sorry, I didn't read it properly on my phone. I see what you mean now. Thanks.

OK, I think both solutions are viable. Thanks Mark and Kumar! I've thanked both of you in the PEP. I think this is ready now.