Now one can write Dict[T, int][str], while Callable[[T], int][str] raises an error. However, most of the arguments for allowing indexable generics also apply for Union, Tuple, Callable, etc. Actually, these types are also generic in the sense that they could depend on other types or type variables (for example Callable takes two types str and int, and returns a new type Callable[[str], int]).

@gvanrossum If you agree, then you can assign this issue to me.