Do not warn on returning any from deferred node (#4295)

Fix #4296.

The access to member defined in __init__ causes the current node to be deferred, 
so mypy assumes expressions are Any but should not warn about it.

This is not the only conceivable fix - for example, I'm not sure why mypy should 
continue checking the function in the first place.

Author: elazarg

mypy/checker.py

@@ -2133,7 +2133,7 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
                 if isinstance(typ, AnyType):
                     # (Unless you asked to be warned in that case, and the
                     # function is not declared to return Any)
-                    if (self.options.warn_return_any and
+                    if (self.options.warn_return_any and not self.current_node_deferred and
                             not is_proper_subtype(AnyType(TypeOfAny.special_form), return_type)):
                         self.msg.incorrectly_returning_any(return_type, s)
                     return

test-data/unit/check-warnings.test

@@ -197,3 +197,15 @@ class Test(object):
             return None
 [builtins fixtures/list.pyi]
 [out]
+
+[case testReturnAnyDeferred]
+# flags: --warn-return-any
+def foo(a1: A) -> int:
+    if a1._x:
+        return 1
+    n = 1
+    return n
+
+class A:
+    def __init__(self, x: int) -> None:
+        self._x = x