Narrowing from `typeof x !== 'object'` incorrectly excludes functions.

Bug Report

🔎 Search Terms

typeof object function narrow, type narrowing

🕗 Version & Regression Information

Present in all recent versions from at least 3.3.3333 through the current nightly 4.4.0-dev.20210701

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "typeof" and "narrow"

⏯ Playground Link

Playground link

💻 Code

let x!: string|object; // |Function;
if (1 < 2) x = () => 1;

if (typeof x !== 'object') {
  useString(x); // should fail, but doesn't
}

declare function useString(s: string): void;

🙁 Actual behavior

The type narrowing for typeof x !== 'object' pruned the entire |object branch from x's type union, but typeof can return either "object" or "function" for values of type object, so this is overzealous. This causes x inside the conditional to be incorrectly narrowed to just string when we can clearly see from the earlier line that it's been assigned a function, which is incorrectly passed to useString with no complaint.

🙂 Expected behavior

Excluding "object" from typeof x should not remove |object from its type, in the same way that it doesn't narrow unknown any further. This would cause the appropriate type error from passing a function to useString.

Note that if an explicit |Function branch is added to to x's type union then the narrowing proceeds as expected.

Start with the linked TypeScript Playground and the provided narrowing example, then trace the type-narrowing logic for typeof x !== "object". The fix is done when a function assigned to x is not narrowed away and passing x to useString produces the expected type error; verify behavior against the stated versions.