keyof.ts

// how keyof works
type P1 = keyof ({ a: string } | { b: string }) // => "a" & "b" (aka never)
type P2 = keyof ({ a: string } & { b: string }) // => "a" | "b"
type P3 = ({ a: string } | { b: string }) extends infer P ? P extends object ? keyof P : never : never
// => "a" | "b"
// in P3, the conditional type forces branching so that expression keyof P is applied for each part of the union rather than the whole

// the reason why this works as it does:
//   a function is given as argument { a: string } | { b: string }, then keyof cannot be statically determined
//   infact at runtime, we cannot know what all the keys of { a: string } | { b: string } are, because we can
//   always only inspect one case