Broken type inference with a typeguard in a if statement

[JeanMeche]

Bug Report

🔎 Search Terms

• type guard
• inference
• if condition

🕗 Version & Regression Information

This is a regression.

Inference was OK on 4.7.4 but broken on 4.8.4 and 4.9.5 (tested on the playground).

⏯ Playground Link

Playground link with relevant code

💻 Code

export type Writable<T> = {
    -readonly [K in keyof T]: T[K];
};

class Foo {
    readonly foo = 'foo';
}

class Bar {
    readonly bar = 'bar';
}

type WritableFooBar = Writable<Foo | Bar>;

declare function isFoo(obj: any): obj is Foo;

function test(foobar: WritableFooBar): void {
    if (!isFoo(foobar)) {} // happens also with if(isFoo())

    foobar
    // ^? Foo | Writable<Bar>
}

🙁 Actual behavior

foobar is inference as Foo | Writable<Bar>

🙂 Expected behavior

foobar should be inferred as Writable<Foo|Bar> or WritableFooBar

[JeanMeche]

Actually, this has been fixed with 5.0 but could find the PR that fixed it !

[RyanCavanaugh]

#52282