py/modmath: Fix two-argument math function domain check.

Prior to this fix, pow(1.5, inf) and pow(0.5, -inf) (among other things)
would incorrectly raise a ValueError, because the result is inf with the
first argument being finite.  This commit fixes this by allowing the result
to be infinite if the first or second (or both) argument is infinite.

This fix doesn't affect the other three math functions that have two
arguments:
- atan2 never returns inf, so always fails isinf(ans)
- copysign returns inf only if the first argument x is inf, so will never
  reach the isinf(y) check
- fmod never returns inf, so always fails isinf(ans)

Signed-off-by: Damien George <damien@micropython.org>

py/modmath.c

@@ -54,7 +54,7 @@ STATIC mp_obj_t math_generic_2(mp_obj_t x_obj, mp_obj_t y_obj, mp_float_t (*f)(m
     mp_float_t x = mp_obj_get_float(x_obj);
     mp_float_t y = mp_obj_get_float(y_obj);
     mp_float_t ans = f(x, y);
-    if ((isnan(ans) && !isnan(x) && !isnan(y)) || (isinf(ans) && !isinf(x))) {
+    if ((isnan(ans) && !isnan(x) && !isnan(y)) || (isinf(ans) && !isinf(x) && !isinf(y))) {
         math_error();
     }
     return mp_obj_new_float(ans);

tests/float/math_domain.py

@@ -39,7 +39,26 @@
 
 # double argument functions
 for name, f, args in (
-    ("pow", math.pow, ((0, 2), (-1, 2), (0, -1), (-1, 2.3), (nan, 0), (1, nan))),
+    (
+        "pow",
+        math.pow,
+        (
+            (0, 2),
+            (-1, 2),
+            (0, -1),
+            (-1, 2.3),
+            (0.5, inf),
+            (-0.5, inf),
+            (0.5, -inf),
+            (-0.5, -inf),
+            (1.5, inf),
+            (-1.5, inf),
+            (1.5, -inf),
+            (-1.5, -inf),
+            (nan, 0),
+            (1, nan),
+        ),
+    ),
     ("log", math.log, ()),
     ("fmod", math.fmod, ((1.2, inf), (1.2, -inf), (1.2, 0), (inf, 1.2))),
     ("atan2", math.atan2, ((0, 0), (-inf, inf), (-inf, -inf), (inf, -inf))),