https://www.typescriptlang.org/docs/handbook/type-compatibility.html#a-note-on-soundness https://www.typescriptlang.org/docs/handbook/type-compatibil... For ex

https://www.typescriptlang.org/docs/handbook/type-compatibility.html#a-note-on-soundness https://www.typescriptlang.org/docs/handbook/type-compatibil... For example: https://www.typescriptlang.org/docs/handbook/type-compatibility.html#function-parameter-bivariance https://www.typescriptlang.org/docs/handbook/type-compatibil... https://www.typescriptlang.org/docs/handbook/type-compatibility.html?#optional-parameters-and-rest-parameters https://www.typescriptlang.org/docs/handbook/type-compatibil... Or the type of an array and its elements: const arr : number[] = [1] but const this_is_actually_undefined : number = arr[456] so it should be const correct_type : number | undefined = arr[idx] And there are way more, I'm too lazy to search for them or think about them.