type Example<T extends Record<'a', string>> = T['a']

type Result1 = Example<{ a: "x" } | { a: "y" }> // type of Result1 is "x" | "y"
type Result2 = Example<{ a: "x" }> // type of Result2 is  "x"
type Result3 = Example<never> // type of Result3 is any 

type Workaround<T extends Record<'a', string>> = T[(keyof T) & 'a']
type Result4 = Workaround<never> // type of Result4 is never in typescript@next