typescript - 为什么这个打字稿模板永远不会返回？

Question

正如在这个打字稿游乐场中看到的那样：

打字稿游乐场

type FirstOrSecond<condition, T1, T2> = condition extends never ? T1 : T2

type foo = never extends never ? () => 'hi' : (arg1: never) => 'hi'
type wtf = FirstOrSecond<never, () => 'hi', (arg1: never) => 'hi'>

泛型FirstOrSecond分配never给wtf变量，而明确定义foo分配() => 'hi'为类型。

为什么泛型会发疯？

标签： typescript

Answer 1

Distributive conditional types. This is intentional.

Usually applied workaround is to wrap types into single-member tuple in the condition:

type FirstOrSecond<condition, T1, T2> = [condition] extends [never] ? T1 : T2