Inconsistent reachability with different type annotations

[kanwren]

TypeScript Version: Nightly

Search Terms: control flow analysis, reachability analysis, never, type annotation

Code

function impossible1(): never {
    throw new Error();
}

const impossible2: () => never = () => {
    throw new Error();
};

const impossible3 = (): never => {
    throw new Error();
}

function foo(): never {
    // impossible1 and impossible2 work here, but not impossible3
    impossible3();
}

// but all seem to have the same type
const x: typeof impossible2 = impossible1;
const y: typeof impossible3 = impossible2;
const z: typeof impossible1 = impossible3;

Expected behavior:

The three functions with the same type should all behave the same under control flow/reachability analysis.

Actual behavior:

Using impossible3, which removes the type annotation from the LHS, does not compile, while using impossible1 or impossible2 does.

Playground Link: Playground Link

Related Issues: N/A

Other issues exist related to reachability not working as expected, but removing the type annotation like this is a strange way to run into it.

[RyanCavanaugh]

See implementation notes in #32695 - the declaration of a never-returning function needs to have an explicit type annotation to participate in CFA, and impossible3's declaration does not have a type annotation (its initializer does, which is different).