Sum types with never in typescript

sumTypesWithNever.ts

// never is dropped from sum types
let x: string | never // string

let y: Exclude<'a' | 'b', 'b'> // y has the type 'a'
/* 
 * Explanation:
 * Given that Exclude is defined as: type Exclude<T, U> = T extends U ? never : T
 * Exclude<'a' | 'b', 'b'> checks if 'a' extends 'b'. 
 * It doesn't and hence 'a' is returned
 * Type system then checks if 'b' extends 'b'. 
 * It does and hence `never` is returned.
 * The resultant type is 'a' | never which resolves to just 'a'
 * This is why variable `y` has the type 'a'
 * /

  // https://til.mudit.xyz/12