Push correct enclosing class while deferring a method (#4073)

Fixes #4069

This pushes the correct enclosing class while deferring a method instead 
of just the top class.

Author: ilevkivskyi

mypy/checker.py

@@ -261,16 +261,15 @@ def check_top_level(self, node: MypyFile) -> None:
 
     def handle_cannot_determine_type(self, name: str, context: Context) -> None:
         node = self.scope.top_function()
-        if (self.pass_num < LAST_PASS and node is not None
-                and isinstance(node, (FuncDef, LambdaExpr))):
+        if self.pass_num < LAST_PASS and isinstance(node, (FuncDef, LambdaExpr)):
             # Don't report an error yet. Just defer.
             if self.errors.type_name:
                 type_name = self.errors.type_name[-1]
             else:
                 type_name = None
             # Shouldn't we freeze the entire scope?
-            active_class = self.scope.active_class()
-            self.deferred_nodes.append(DeferredNode(node, type_name, active_class))
+            enclosing_class = self.scope.enclosing_class()
+            self.deferred_nodes.append(DeferredNode(node, type_name, enclosing_class))
             # Set a marker so that we won't infer additional types in this
             # function. Any inferred types could be bogus, because there's at
             # least one type that we don't know.
@@ -3403,6 +3402,16 @@ def active_class(self) -> Optional[TypeInfo]:
             return self.stack[-1]
         return None
 
+    def enclosing_class(self) -> Optional[TypeInfo]:
+        top = self.top_function()
+        assert top, "This method must be called from inside a function"
+        index = self.stack.index(top)
+        assert index, "Scope stack must always start with a module"
+        enclosing = self.stack[index - 1]
+        if isinstance(enclosing, TypeInfo):
+            return enclosing
+        return None
+
     def active_self_type(self) -> Optional[Union[Instance, TupleType]]:
         info = self.active_class()
         if info:

test-data/unit/check-classes.test

@@ -3996,6 +3996,41 @@ class F(six.with_metaclass(t.M)): pass
 @six.add_metaclass(t.M)
 class G: pass
 
+[case testCorrectEnclosingClassPushedInDeferred]
+class C:
+    def __getattr__(self, attr: str) -> int:
+        x: F
+        return x.f
+
+class F:
+    def __init__(self, f: int) -> None:
+        self.f = f
+[out]
+
+[case testCorrectEnclosingClassPushedInDeferred2]
+from typing import TypeVar
+T = TypeVar('T', bound=C)
+class C:
+    def m(self: T) -> T:
+        class Inner:
+            x: F
+            f = x.f
+        return self
+
+class F:
+    def __init__(self, f: int) -> None:
+        self.f = f
+[out]
+
+[case testCorrectEnclosingClassPushedInDeferred3]
+class A:
+    def f(self) -> None:
+        def g(x: int) -> int:
+            return y
+
+y = int()
+[out]
+
 [case testMetaclassMemberAccessViaType]
 from typing import Type
 class M(type):