Fix crash related to propagating type narrowing to nested functions (#15181)

See #15133 (comment)
for context.

Author: JukkaL

mypy/checker.py

@@ -1222,6 +1222,7 @@ def check_func_def(
                             if new_frame is None:
                                 new_frame = self.binder.push_frame()
                             new_frame.types[key] = narrowed_type
+                            self.binder.declarations[key] = old_binder.declarations[key]
                 with self.scope.push_function(defn):
                     # We suppress reachability warnings when we use TypeVars with value
                     # restrictions: we only want to report a warning if a certain statement is

test-data/unit/check-optional.test

@@ -1329,3 +1329,17 @@ def narrow_with_multi_assign_3(x: Optional[str]) -> None:
     y, x = None, None
 
 [builtins fixtures/isinstance.pyi]
+
+[case testNestedFunctionSpecialCase]
+class C:
+    def __enter__(self, *args): ...
+    def __exit__(self, *args) -> bool: ...
+
+def f(x: object) -> None:
+    if x is not None:
+        pass
+
+    def nested() -> None:
+        with C():
+            pass
+[builtins fixtures/tuple.pyi]