gh-123836: workaround fmod(x, y) bug on Windows

Misc/NEWS.d/next/Library/2024-09-17-18-06-42.gh-issue-124171.PHCvRJ.rst

@@ -0,0 +1,2 @@
+Add workaround for broken :c:func:`!fmod()` implementations (e.g. on
+Windows), that loose zero sign.  Patch by Sergey B Kirpichev.

Modules/mathmodule.c

@@ -2348,6 +2348,13 @@ math_fmod_impl(PyObject *module, double x, double y)
         return PyFloat_FromDouble(x);
     errno = 0;
     r = fmod(x, y);
+    /* Some platforms (e.g. Windows 10 with MSC v.1916) loose sign
+       for zero result.  But C99+ says: "if y is nonzero, the result
+       has the same sign as x".
+     */
+    if (r == 0.0 && y != 0.0) {
+        r = copysign(r, x);
+    }
     if (isnan(r)) {
         if (!isnan(x) && !isnan(y))
             errno = EDOM;