XXX correct LambertW assumption helpers

LambertW._eval_is_extended_real now more conservative.

diofant/functions/elementary/exponential.py

@@ -513,12 +513,9 @@ def _eval_is_extended_real(self):
             elif (x + 1/S.Exp1).is_nonpositive:
                 return False
         elif (k + 1).is_zero:
-            if x.is_negative and (x + 1/S.Exp1).is_positive:
+            if x.is_negative and (x + 1/S.Exp1).is_nonnegative:
                 return True
-            elif x.is_nonpositive or (x + 1/S.Exp1).is_nonnegative:
-                return False
-        elif k.is_nonzero and (k + 1).is_nonzero:
-            if x.is_extended_real:
+            elif x.is_nonpositive or (x + 1/S.Exp1).is_positive:
                 return False
 
     def _eval_is_algebraic(self):

diofant/functions/elementary/tests/test_exponential.py

@@ -364,13 +364,14 @@ def test_lambertw():
     assert LambertW(-p - 2/S.Exp1, evaluate=False).is_extended_real is False
     assert LambertW(S.Half, -1, evaluate=False).is_extended_real is False
     assert LambertW(-S.One/10, -1, evaluate=False).is_extended_real
-    assert LambertW(-10, -1, evaluate=False).is_extended_real is False
-    assert LambertW(-2, 2, evaluate=False).is_extended_real is False
 
     assert LambertW(0, evaluate=False).is_algebraic
     na = Symbol('na', nonzero=True, algebraic=True)
     assert LambertW(na).is_algebraic is False
 
+    assert LambertW(x, -1).is_extended_real is None
+    assert LambertW(x, 2).is_extended_real is None
+
     # See sympy/sympy#7259:
     assert LambertW(x).series(x) == x - x**2 + 3*x**3/2 - 8*x**4/3 + \
         125*x**5/24 + O(x**6)