gh-158942: restore constant-expression support in Py_MIN, Py_MAX, and Py_ABS

[lpyu001]

• Issue: Py_MIN(), Py_MAX() and Py_ABS() cause C compatibility regressions #158942

[ZeroIntensity]

It's not clear to me that this is the right fix. We should first try to fix the macros, not just jump to reverting #157496.

[bedevere-app]

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[picnixz]

It's not clear to me that this is the right fix. We should first try to fix the macros, not just jump to reverting #157496.

To me it's the only possible fix. We can't have issues for:

/* bitfield.c: compile with -c */
#include <Python.h>

struct S { unsigned bf : 4; };
int f(struct S *s, int k) { return Py_MAX(s->bf, k); }

this specific one:

=== clang -std=c11 -c bitfield.c
bitfield.c:4:36: error: invalid application of 'typeof' to bit-field

and the correctness issue here is concerning:

/* wrong_result.c */
#include <Python.h>
#include <stdio.h>

int main(void)
{
    int a = 1, _x = 5;
    printf("Py_MAX(a, _x) = %d (expected 5)\n", Py_MAX(a, _x));

    int b = 9, _y = 2;
    printf("Py_MIN(b, _y) = %d (expected 2)\n", Py_MIN(b, _y));

    int c = -7;
    {
        int _x = c;
        printf("Py_ABS(_x) = %d (expected 7)\n", Py_ABS(_x));
    }
    return 0;
}

Note: I didn't run the code but having different (wrong) results depending on -O flag and compiler is really not good.

[vstinner]

I will merge #158969 instead, PR which should fix most issues that you reported.