[issue17589] Make documentation about macros in C API explicit about rvalue vs statement

[Larry Hastings]

Larry Hastings added the comment:

Oh, and, yes, it's true that Py_RETURN_NONE currently takes advantage of Py_INCREF being an rvalue, and changing Py_INCREF to a statement would break the existing implementation.  But Py_RETURN_NONE itself is of necessity a statement.   We would simply change Py_RETURN_NONE's implementation to multiple statements, probably with the do { ... } while(0) trick, so it worked again.  I'd be shocked if that change broke any existing code.  So that's no big deal.

Having external code that depends on Py_INCREF being an rvalue is my concern, and what I hoped you'd bring up on bug #17206.

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