Type guard affects type of variable in surprising way

Bug Report

🔎 Search Terms

type guard / fall through / narrow / change / lazy / evaluate

🕗 Version & Regression Information

• This changed between versions v4.7.4 and v4.8.2

⏯ Playground Link

Playground link with relevant code

💻 Code

type Identity<T> = {[K in keyof T]: T[K]};

type Self<T> = T extends unknown ? Identity<T> : never;

function is<T>(value: T): value is Self<T> {
    return true;
}

type Union =  {a: number} | {b: number} | {c: number};

function example(x: Union) {
    if (is(x)) {}
    if (is(x)) {}
    if (is(x)) {}
    if (is(x)) {}
    if (is(x)) {}
    if (is(x)) {}
    if (is(x)) {}
    if (is(x)) {}

    return x;
    //     ^?
}

🙁 Actual behavior

The type of x is “narrowed” to Identity<Identity<Identity<Identity<Identity<Identity<Identity<Identity<{a: number;}>>>>>>>> | Identity<Identity<Identity<Identity<Identity<Identity<Identity<Identity<{b: number;}>>>>>>>> | Identity<Identity<Identity<Identity<Identity<Identity<Identity<Identity<{c: number;}>>>>>>>>

It’s as if a variable gets narrowed to the union of the types of both sides of the type predicate, e.g.

if (isA(aOrB)) {
  // `aOrB` gets narrowed to `A`
} else {
  // `aOrB` gets narrowed to `Exclude<typeof aOrB, A>`
}

// `aOrB` gets narrowed to `A | Exclude<typeof aOrB, A>` but it should just be left alone

🙂 Expected behavior

The type of x doesn’t change.

[ahejlsberg]

Agreed about the footgun. We’ll continue to think about ways to improve this.

[ahejlsberg]

This is an effect of #50044. The issue here is that the argument type and the asserted type are subtypes of each other, and therefore appear interchangeable in control flow analysis. From the PR:

Note that one issue with favoring the asserted type is that CFA continues with that type after the conditional block. Though undesired, that’s an effect of how CFA works in the face of mutual subtypes. This behavior was already present for singleton types, but now also extends to union types.

So this is effectively a design limitation, but we’ll continue to think of ways in which to improve it.