REF replace impossible cases by corresponding assertions

(cherry picked from commit de4cc96)

sympy/core/mul.py

@@ -1284,12 +1284,12 @@ def _eval_is_integer(self):
                 if not b.is_integer or not e.is_integer: return
                 if e.is_negative:
                     denominators.append(b)
-                elif e.is_positive:
-                    denominators.append(b)
-                elif e.is_zero:
-                    if b.is_zero: return
                 else:
-                    return
+                    # for integer b and positive integer e: a = b**e would be integer
+                    assert not e.is_positive
+                    # for self being rational and e equal to zero: a = b**e would be 1
+                    assert not e.is_zero
+                    return # sign of e unknown -> self.is_integer cannot be decided
             else:
                 return
 

sympy/core/tests/test_arit.py

@@ -388,6 +388,8 @@ def test_Mul_is_integer():
     assert (x*k*n).is_integer is None
     assert (e/2).is_integer is True
     assert (e**2/2).is_integer is True
+    assert (2/k).is_integer is None
+    assert (2/k**2).is_integer is None
     assert ((-1)**k*n).is_integer is True
     assert (3*k*e/2).is_integer is True
     assert (2*k*e/3).is_integer is None