The fact that the compiler cannot enforce it is a big deal since it prevents automatic refactorings, often impairs performance and overall, hinders readability.

The fact that the compiler cannot enforce it is a big deal since it prevents automatic refactorings, often impairs performance and overall, hinders readability. And yes, you really have no idea what the type of that object is unless you look at the source of the function. Which is obviously suboptimal, especially if you don't have access to that source.