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

TypeScript Playground の再現例と Code セクションの Array.filter 呼び出しから始め、predicate1 から predicate4 までを比較します。TruthyPredicate と EmptyInterface または ConstructorSignagure を交差させるとなぜ narrowing が失われるのかを調査します。影響を受ける交差型によって、追加で説明されているジェネリックのケースを壊すことなく、フィルタリングされた配列が期待どおりに絞り込まれれば完了です。