bpo-46018: Ensure that math.expm1 does not raise on underflow (GH-29997)

(cherry picked from commit 3363e1c)

Co-authored-by: Steve Dower <steve.dower@python.org>

Author: miss-islington

Misc/NEWS.d/next/Library/2021-12-09-00-44-42.bpo-46018.hkTI7v.rst

@@ -0,0 +1 @@
+Ensure that :func:`math.expm1` does not raise on underflow.

Modules/mathmodule.c

@@ -971,9 +971,13 @@ is_error(double x)
          * On some platforms (Ubuntu/ia64) it seems that errno can be
          * set to ERANGE for subnormal results that do *not* underflow
          * to zero.  So to be safe, we'll ignore ERANGE whenever the
-         * function result is less than one in absolute value.
+         * function result is less than 1.5 in absolute value.
+         *
+         * bpo-46018: Changed to 1.5 to ensure underflows in expm1()
+         * are correctly detected, since the function may underflow
+         * toward -1.0 rather than 0.0.
          */
-        if (fabs(x) < 1.0)
+        if (fabs(x) < 1.5)
             result = 0;
         else
             PyErr_SetString(PyExc_OverflowError,