What is the Variance annotation in TypeScript 4.7?

TypeScript 4.7 introduced explicit variance annotations with in and out modifiers on generic type parameters, giving you direct control over how the type-checker handles subtype relationships. out T marks T as covariant — T only appears in output positions (return types): interface Getter<out T> { get(): T; }. in T marks T as contravariant — T only appears in input positions (parameters): interface Setter<in T> { set(value: T): void; }. in out T marks as invariant — T appears in both positions. Benefits: (1) TypeScript can skip expensive variance inference for complex types, significantly improving compilation performance. (2) You explicitly document the intended variance, catching bugs when you accidentally use T in the wrong position. (3) Better error messages. TypeScript still infers variance automatically, but adding annotations enforces the intent and speeds up type-checking for deeply parameterized generic types in large codebases.