type predicates don't narrow the type of the parent object

[DetachHead]

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

4.3.0-dev.20210406

⏯ Playground Link

Playground link with relevant code

💻 Code

declare function isBar(object: unknown): object is Bar

type Bar = {
    qux: number
}

type Foo<T extends Bar | {}> = {
    bar: T
    baz: number
}

declare const foo: Foo<Bar | {}>

declare function fooBar(value: Foo<Bar>): void

if (isBar(foo.bar)) {
    //foo.bar correctly narrowed to Bar:
    const a = foo.bar
    const b = foo.bar.qux

    //foo doesn't get narrowed and is still Foo<{} | Bar>, even though we know it's a Foo<Bar>
    fooBar(foo)
}

🙁 Actual behavior

foo.bar is narrowed, but foo remains the same type

🙂 Expected behavior

foo is narrowed to Foo

[MartinJohns]

Duplicate of #42384. Used search terms: narrow parent