Optional variance annotations by ahejlsberg · Pull Request #48240 · microsoft/TypeScript

With this PR we introduce optional declaration site variance annotations for type parameters of classes, interfaces and type aliases. Annotations take the form of an in and/or out keyword immediately preceding the type parameter name in a type parameter declaration. An out annotation indicates that a type parameter is covariant. An in annotation indicates that a type parameter is contravariant. An

[efcl]

型がcontravariant/共変(`in`)、covariant/反変(`out`)、invariant/不変(`in out`)なのかを指定できるアノテーションを追加。

[gfx]

TypeScript 4.7 で type parameter に covariant/contravariant/invariant を宣言する out / in / in out キーワードが導入されそう。