[_fbpt]

In my experience with type inference (quite a bit of mypy, now learning rust), inferring local variable types based on what's assigned to it is not very confusing, as long as me or the IDE can figure out what type is being assigned. But when a function return value or variable's generic argument is determined by where the value is used, I tend to have more trouble (since one type can propagate from one function, to its argument, to the function which generated the argument).