Type predicate covariant comparison is unsound

[gabritto]

🔎 Search Terms

type predicate comparison, type predicate variance

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240416#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwygGsQBhACgA8AueAIjDoEpaA3HLYAbgCgfRIsBBBAZ4WAM4BBAEK0qtOlDrwAPvQBGK9Q2bwAvAD54lcRPrK1murwHQ48EWMlT5NC9utXdTA8dOSHrw8LjIGZlLBdkKOoiaKljpa3oy8WIjw5KFUTL4A3jzwRSZc8AD0ZR7eWgA08BrIYmBQ+FCYyFAQEACe8Hg99QhKdDwAvjwgEBIIBcUl5ZW6dQ1NLfBtGB1dvRpDjNUjc4QkFJRMpRXwACoAFoGBGlDAY3xAA

💻 Code

declare function takeC(x: "c"): void;

declare let isAB: (x: "a" | "b" | "c") => x is "a" | "b";
declare let isA: (x: "a" | "b" | "c") => x is "a";

isAB = isA;

declare let x: "a" | "b" | "c";
if (isAB(x)) {
    x; // "a" | "b", but can actually only be "a"
}
else {
    x; // "c", but can actually be "c" | "b"
    takeC(x); // This is bad
}

🙁 Actual behavior

Assignment isAB = isA doesn't error.

🙂 Expected behavior

Assignment isAB = isA errors because it is unsound.

Additional information about the issue

No response

[jcalz]

duplicate of #26981