gh-138580: add sys.float_info.stdc_iec_559 boolean flag

This value indicating support the IEC 60559 floating-point standard (the
Annex F of C99).  If enabled, the float type characteristics matches the
IEC 60559 double format and exceptional cases for the math's functions
follow to the section F.10 of the C99 standard.

Author: skirpichev

Doc/library/sys.rst

@@ -694,15 +694,15 @@ always available. Unless explicitly noted otherwise, all variables are read-only
    A :term:`named tuple` holding information about the float type. It
    contains low level information about the precision and internal
    representation.  The values correspond to the various floating-point
-   constants defined in the standard header file :file:`float.h` for the 'C'
-   programming language; see section 5.2.4.2.2 of the 1999 ISO/IEC C standard
+   constants defined by C implementation and in the standard header file :file:`float.h` for the 'C'
+   programming language; see Annex F and section 5.2.4.2.2 of the 1999 ISO/IEC C standard
    [C99]_, 'Characteristics of floating types', for details.
 
    .. list-table:: Attributes of the :data:`!float_info` :term:`named tuple`
       :header-rows: 1
 
       * - attribute
-        - float.h macro
+        - C macro
         - explanation
 
       * - .. attribute:: float_info.epsilon
@@ -771,6 +771,15 @@ always available. Unless explicitly noted otherwise, all variables are read-only
           All other values for :c:macro:`!FLT_ROUNDS` characterize
           implementation-defined rounding behavior.
 
+      * - .. attribute:: float_info.stdc_iec_559
+        - :c:macro:`!__STDC_IEC_559__`
+        - A boolean, indicating support the IEC 60559 floating-point
+          standard (the Annex F of C99).  If enabled, the
+          :class:`float` type characteristics matches the IEC
+          60559 double format and exceptional cases for
+          the :mod:`math` functions follow to the section F.10
+          of the C99 standard.
+
    The attribute :attr:`sys.float_info.dig` needs further explanation.  If
    ``s`` is any string representing a decimal number with at most
    :attr:`!sys.float_info.dig` significant digits, then converting ``s`` to a

Misc/NEWS.d/next/Library/2025-09-12-07-30-13.gh-issue-138580.Qr_fSH.rst

@@ -0,0 +1,2 @@
+Add a boolean flag, indicating support the IEC 60559 floating-point standard
+(the Annex F of C99).  Patch by Sergey B Kirpichev.

Objects/floatobject.c

@@ -68,14 +68,16 @@ static PyStructSequence_Field floatinfo_fields[] = {
     {"radix",           "FLT_RADIX -- radix of exponent"},
     {"rounds",          "FLT_ROUNDS -- rounding mode used for arithmetic "
                     "operations"},
+    {"stdc_iec_559", "test if implementation supports the IEC 60559"
+                     " floating-point standard"},
     {0}
 };
 
 static PyStructSequence_Desc floatinfo_desc = {
     "sys.float_info",           /* name */
     floatinfo__doc__,           /* doc */
     floatinfo_fields,           /* fields */
-    11
+    12
 };
 
 PyObject *
@@ -113,6 +115,7 @@ PyFloat_GetInfo(void)
     SetDblFlag(DBL_EPSILON);
     SetIntFlag(FLT_RADIX);
     SetIntFlag(FLT_ROUNDS);
+    SetFlag(PyBool_FromLong(__STDC_IEC_559__));
 #undef SetIntFlag
 #undef SetDblFlag
 #undef SetFlag