Fix Mod._eval_is_integer helper

When one cannot determine it is an integer, it should
return None, and not False.  This closes issue sympy/sympy#10024.

Signed-off-by: Sergey B Kirpichev <skirpichev@gmail.com>

sympy/core/mod.py

@@ -141,7 +141,8 @@ def doit(p, q):
     def _eval_is_integer(self):
         from sympy.core.logic import fuzzy_and
         p, q = self.args
-        return fuzzy_and([p.is_integer, q.is_integer, q.is_nonzero])
+        if fuzzy_and([p.is_integer, q.is_integer, q.is_nonzero]):
+            return True
 
     def _eval_is_nonnegative(self):
         if self.args[1].is_positive:

sympy/core/tests/test_assumptions.py

@@ -1,7 +1,8 @@
 import pytest
 
 from sympy import I, sqrt, log, exp, sin, asin
-from sympy.core import Symbol, S, Rational, Integer, Dummy, Wild, Pow, Float
+from sympy.core import (Symbol, S, Rational, Integer, Dummy,
+                        Wild, Pow, Float, Mod, pi)
 from sympy.core.facts import InconsistentAssumptions
 from sympy import simplify
 
@@ -918,3 +919,8 @@ def test_issue_9165():
     assert 0/z == S.NaN
     assert 0*(1/z) == S.NaN
     assert 0*f == S.NaN
+
+
+def test_issue_10024():
+    x = Dummy('x')
+    assert Mod(x, 2*pi).is_zero is None