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Check for integer overflow in PyObject_GC_Resize() and PyObject_GC_NewVar(). Add _PyObject_GC_VAR_SIZE() which is similar to _PyObject_VAR_SIZE() but checks for integer overflow.

Remove redundant (and incomplete) integer overflow from tuple_alloc() and _PyTuple_Resize().

• Issue: [C API] Add PyTupleWriter API #161694

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It makes me grumpy that _PyObject_GC_NewVar() and _PyObject_GC_Resize() code is basically copied-pasted (with minor changes!) between Python/gc.c and Python/gc_free_threading.c.

The problem is that gc_alloc() code is different and gc_alloc() is a private function. It's uneasy to share code between Python/gc.c and Python/gc_free_threading.c. I tried adding Python/gc_common.c, but I got stuck at gc_alloc(). I tried moving _PyObject_GC_VAR_SIZE() to Include/internal/pycore_gc.c, but _PyType_PreHeaderSize() is a static inline function declared in pycore_object.h which is not included by pycore_gc.h (and including pycore_object.h in pycore_gc.h would create a inter-dependency mess).

So it seems like the best we can do for now is to copy/paste code :-(

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I searched for _PyObject_VAR_SIZE in PyPI top 15,000 projects (at 2026-09-17). I found 7 projects using _PyObject_VAR_SIZE():

• cython (3.3.0)
• jpype1 (1.7.1)
• nuitka (4.2.1)
• numpy (2.5.3)
• orjson (3.12.0)
• pywinpty (3.0.5)
• recordclass (0.24.1)

Hum. A more generic approach would be to change _PyObject_VAR_SIZE() instead of changing PyObject_GC_NewVar() and PyObject_GC_Resize().

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I close this PR in favor of the more generic #161911.