Object union member functions that return never are incorrectly narrowed

🔎 Search Terms

"never", "class union", "narrowing unions", "never union"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about never

⏯ Playground Link

Playground Link

💻 Code

class Foo {
    defoo(): never {
        throw new Error("foo error")
    }
}

class Bar {
    defoo() { }
}

type Baz = Foo | Bar;

function defooer(baz: Baz) {
    baz.defoo() 
    // type = void
    // aka (never | void)

    return baz
    // type = Baz
    // should narrow to Bar
}

🙁 Actual behavior

baz doesn't get narrowed after calling .defoo(), even though it's impossible for any value after that point to be Foo. This seems to happen because the type of Baz["defoo"] is equal to Foo["defoo"] | Bar["defoo"] or () => never | void.

🙂 Expected behavior

baz should narrow to Bar after calling .defoo() because the return type of Foo.defoo is never.

Additional information about the issue

No response

Reproduce the behavior using the linked TypeScript Playground and the defooer example. Trace TypeScript's control-flow narrowing for calls whose union return type includes never; done means the value is narrowed from Foo | Bar to Bar after baz.defoo() without breaking related narrowing behavior.