assert only functions are properly inferred as returning None

Close #668

ChangeLog

@@ -25,6 +25,10 @@ Release Date: TBA
   Close PyCQA/pylint#2326
   Close PyCQA/pylint#2021
 
+* ``assert`` only functions are properly inferred as returning ``None``
+
+  Close #668
+
 * Add support for Python 3.8's `NamedExpr` nodes, which is part of assignment expressions.
 
   Close #674

astroid/scoped_nodes.py

@@ -1656,7 +1656,13 @@ def infer_call_result(self, caller=None, context=None):
 
         first_return = next(returns, None)
         if not first_return:
-            raise exceptions.InferenceError("Empty return iterator")
+            if self.body and isinstance(self.body[-1], node_classes.Assert):
+                yield node_classes.Const(None)
+                return
+
+            raise exceptions.InferenceError(
+                "The function does not have any return statements"
+            )
 
         for returnnode in itertools.chain((first_return,), returns):
             if returnnode.value is None:

astroid/tests/unittest_inference.py

@@ -5289,5 +5289,19 @@ def both_branches():
     assert [third[0].value, third[1].value] == [1, 2]
 
 
+def test_assert_last_function_returns_none_on_inference():
+    code = """
+    def check_equal(a, b):
+        res = do_something_with_these(a, b)
+        assert a == b == res
+
+    check_equal(a, b)
+    """
+    node = extract_node(code)
+    inferred = next(node.infer())
+    assert isinstance(inferred, nodes.Const)
+    assert inferred.value is None
+
+
 if __name__ == "__main__":
     unittest.main()