Predicates break arbitrarily when intersected with some types. (Probably when the predicate is generic).

[miguel-leon]

🔎 Search Terms

predicate intersection

🕗 Version & Regression Information

latest version v5.6.2

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.2#code/JYOwLgpgTgZghgYwgAgCpQK5gBYE8AKUEAJsAnJMgN4CwAUMo8gDyoB8AFAPYBcaAlHy7JgAZzTIAZNQC+Abnoz69UJFiIUAUQC2ABzC4AkuGjwk1RcroHdKAMJcQosJgRguUAMrAA5iDg+GETIALwWDEwgEADuyBwAdIlwUD6ifHAguADaALqCyBm4CnTyVsQQCAA2ySgIjs4FUFB8HM5QoD7IAD7IGCDlMKAk-LnF9OVVNch1TmDIukSk5JAAjHzoWHiEJGQUEMUzDYOVaiQroY1Q8cdqHAs7yxAr-HLIAPRvyNEeANbibR1clZxhVqsFDnN7ks9gAmdaYHAERa7SjSHT6IwmdRIA71OY3aAkGEXZJXAlQO7Ix4wl7vT7ELgQUQgADkc2+UB+wLoEzBtTx8ypewAzPDNkiHnspLJcbNkOSSMKSU1rsATtBKZLIMLaR8vr9-i5ATlubyphDBVqIAAWMWI7bQ1HIByzVzuLy+fyBIiyo5q07Ea3Ksn+jVQlE23X0xnMtn6zn0IA

💻 Code

interface TruthyPredicate {
    <T>(o: T): o is T & {};
}

interface EmptyInterface {
}

type ConstructorSignagure = {
    new (...args: any[]): any;
};

declare const arr: (string | undefined)[];

declare const predicate1: TruthyPredicate;
const filtered1 = arr.filter(predicate1); // works string[]


declare const predicate2: TruthyPredicate & EmptyInterface;
const filtered2 = arr.filter(predicate2); // doesn't work


declare const predicate3: TruthyPredicate & {} /* Empty object type instead of interface */;
const filtered3 = arr.filter(predicate3); // works string[]


declare const predicate4: TruthyPredicate & ConstructorSignagure;
const filtered4 = arr.filter(predicate4); // doesn't work

🙁 Actual behavior

Predicate stops working

🙂 Expected behavior

Predicate works

Additional information about the issue

I tried other scenarios

• truthy variation
• generic without undefined
• non-generic, always works, but won't be able to use the parameter type
• this is why it needs to support generics