py/objint: Do not use fpclassify.

For combinations of certain versions of glibc and gcc the definition of
fpclassify always takes float as argument instead of adapting itself to
float/double/long double as required by the C99 standard.  At the time of
writing this happens for instance for glibc 2.27 with gcc 7.5.0 when
compiled with -Os and glibc 3.0.7 with gcc 9.3.0.  When calling fpclassify
with double as argument, as in objint.c, this results in an implicit
narrowing conversion which is not really correct plus results in a warning
when compiled with -Wfloat-conversion.  So fix this by spelling out the
logic manually.

py/objint.c

@@ -135,11 +135,15 @@ STATIC mp_fp_as_int_class_t mp_classify_fp_as_int(mp_float_t val) {
 #undef MP_FLOAT_EXP_SHIFT_I32
 
 mp_obj_t mp_obj_new_int_from_float(mp_float_t val) {
-    int cl = fpclassify(val);
-    if (cl == FP_INFINITE) {
-        mp_raise_msg(&mp_type_OverflowError, MP_ERROR_TEXT("can't convert inf to int"));
-    } else if (cl == FP_NAN) {
-        mp_raise_ValueError(MP_ERROR_TEXT("can't convert NaN to int"));
+    mp_float_union_t u = {val};
+    // IEEE-754: if biased exponent is all 1 bits...
+    if (u.p.exp == ((1 << MP_FLOAT_EXP_BITS) - 1)) {
+        // ...then number is Inf (positive or negative) if fraction is 0, else NaN.
+        if (u.p.frc == 0) {
+            mp_raise_msg(&mp_type_OverflowError, MP_ERROR_TEXT("can't convert inf to int"));
+        } else {
+            mp_raise_ValueError(MP_ERROR_TEXT("can't convert NaN to int"));
+        }
     } else {
         mp_fp_as_int_class_t icl = mp_classify_fp_as_int(val);
         if (icl == MP_FP_CLASS_FIT_SMALLINT) {

tests/float/float2int_doubleprec_intbig.py

@@ -96,6 +96,7 @@ def fp2int_test(num, name, should_fail):
     fp2int_test(1.9999999999999981 * 2.0 ** 1023.0, "large pos", False)
 
 fp2int_test(float("inf"), "inf test", True)
+fp2int_test(float("-inf"), "inf test", True)
 fp2int_test(float("nan"), "NaN test", True)
 
 # test numbers < 1 (this used to fail; see issue #1044)

tests/float/float2int_fp30_intbig.py

@@ -93,6 +93,7 @@ def fp2int_test(num, name, should_fail):
     fp2int_test(1.999999879 * 2.0 ** 126.0, "large pos", False)
 
 fp2int_test(float("inf"), "inf test", True)
+fp2int_test(float("-inf"), "inf test", True)
 fp2int_test(float("nan"), "NaN test", True)
 
 # test numbers < 1 (this used to fail; see issue #1044)