In your example, after you manually change the return type at the caller to Bar from Foo, the usage of Foo.length and Bar.length remains and nothing will help y

In your example, after you manually change the return type at the caller to Bar from Foo, the usage of Foo.length and Bar.length remains and nothing will help you if they changed semantics with neither inferred nor explicit types. The access of the length field is still valid in either case.