TypeScript doesn't do proper occurrence typing: https://www.typescriptlang.org/play?ts=4.4.0-beta#code/GYVwdgxgLglg9mABMOcAUBDAXIgzlAJxjAHMAfMEAGyoBpEAjHfI0i6q

TypeScript doesn't do proper occurrence typing: https://www.typescriptlang.org/play?ts=4.4.0-beta#code/GYVwdgxgLglg9mABMOcAUBDAXIgzlAJxjAHMAfMEAGyoBpEAjHfI0i6qgSmcOJMQDeAWABQicYhjBEmRAF4FiSjUQAyVY3mLlXQaIkHEBAKZQQBJAHIAJseAZqUSwG59EgL6i34qTIyIAQm0OTj0xQ3ETMwtEDFdw8U8RbyNTcyQGePcgA https://www.typescriptlang.org/play?ts=4.4.0-beta#code/GYVwd... Compare to Typed Racket: #lang typed/racket (: foo (-> (U String Null) (U String Null) String)) (define (foo a b) (cond [(and (null? a) (null? b)) "default"] [(not (null? a)) a] [#t b])) Or Psalm, a PHP typechecker I created with occurrence typing: https://psalm.dev/r/60ef4c299e https://psalm.dev/r/60ef4c299e