Confusing error message: "'subscript(_:)' requires the types 'String.Index' and 'Int' be equivalent"

[Cyberbeni (Benedek Kozma)]

Subscripting in String extension to access substrings and characters with Int types

The reason that String doesn’t allow this today is that subscripting is expected to be a constant-time operation (and, in Collection, this is made explicit). The variable-width nature of Characters makes this infeasible for string without sacrificing efficiency in other areas.