As far as I understand it, every sound language eventually has to fall back to runtime checks to maintain soundness in tricky cases. For array covariance I know that Java/Dart eventually use runtime checks on array accesses to verify the types work out.
However, it's kind of against the spirit of TypeScript to insert runtime checks. For example because the type system models 'undefined', to model out of bounds array accesses you'd either need to make every array access have type T|undefined or insert bounds checks that throw. (I believe Dart does the latter.)
However, it's kind of against the spirit of TypeScript to insert runtime checks. For example because the type system models 'undefined', to model out of bounds array accesses you'd either need to make every array access have type T|undefined or insert bounds checks that throw. (I believe Dart does the latter.)