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

🔎 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

Start with the TypeScript Playground repro and the Array.filter calls in the Code section, comparing predicate1 through predicate4. Investigate why intersecting the generic TruthyPredicate with EmptyInterface or ConstructorSignagure loses narrowing; done when the affected intersections narrow the filtered array as expected without breaking the additional generic cases described.