Regression of `this is` type predicate for intersection of unions from 4.7 to 4.8

🔎 Search Terms

"this is" "type predicate" "intersection" "union"

🕗 Version & Regression Information

• This changed between versions 4.7.4 and 4.8.4

⏯ Playground Link

https://www.typescriptlang.org/play/?ssl=40&ssc=1&pln=1&pc=1#code/PTAEF5NBlBRAVAqgBQFABcCeAHApqRbAEwEN19xQBvUAV2LNwH0BLIgLlADtaBbAI1wAnUAF8A3KgDGAGxIBneaADCAey7kAHumqpQ+0FPXz0Q2lPSqhACmy1+MllLoNynQqXIBKaqL0GpAAtcKQBray9OdECWJViVYLDcIl0DNNAhXHRaIS5QU1pcSTS-Pyw8BJDQ5IhqF09cThoAJVgAWQB5ADVYJngACQBJaD6OpgAxQYANAH5OLlwAN2ExMUlUEAgoeABNZFhQNoBBAHFB5QwcfHGWGXIhNQ1cbQAeZVBn8i4iJUetdAANKAAIofbS4b5KExCFhcADmAD5ashhAAzKy8YGFISYF7+fTKAH40DNWhcLHCXHAhGoBGSTYkbDYGSYUAkUAAR2xrMy8lod1A-FZsPu8hC6FhcNALB0AHcZYECK5cECSN98sE8rCAFbiyXSjSqFSXCoooTooSY7lvMFfH4qdT-IGIW0Q+2qfi6ixIyguz5upRk0JcVSyrjEmYqUAAMjq9Aa7mVMYIYmJ8yWwkk5XwpPJ1tB-shoGhkp9DoEsNwLyOimE6AA0rgqQigdSs1dQDWxUIG02XgXwUWS-CywO7YGuMHQ+G0pHmiErER+0CAHLqFf8uQOKtBkNhhE0tLp5ZCdabIi4VGVtlEIgyljqEgyUBX3Ayd15F3y6KgRZPwqgAABmSF5XgsRCASa+BqBWCwvIgQL1q6Q6mKWtR+oO9q7tOEYprGyAkD2LBPi885GEIS6wJosi0BeLz1kC1SYKoqIEC2dDfJelZEAeabcBmp6oBsYCQJQ-QdAA6qAgzwKAABCrRHPW0CoBesiEfgRhcCYhjoJonA3Hcwh-J8eJpIZ9wma8VmAqAABE6i4HZoAAD72egsqqHZLbEnZHlebSkhaTpnlCKESiUBYmgAHTxowsXKqwRDthUElWOFtTWNmLG6ZoybKIk1REF4ADadlxeQdkALplRVzBsNVkgsKx1hRdFQRVBEPhUMSwU6PwmQkBlkV6QlDRjYwSWSH4QA

💻 Code

// === SETUP
type Update = { update_id: number };
class Context {
    constructor(public update: Update) {}
    check(): this is Checked {
        return true;
    }
}
type Checked = { update: { REMOVE_THIS_TO_FIX?: never } };

// === TYPE MAGIC
type FilterContext<C extends Context, Q extends string> = PerformQuery<
    C,
    RunQuery<Q>
>;
// apply a query result by intersecting it with Update, and then injecting into C
type PerformQuery<C extends Context, U extends object> = U extends unknown
    ? C & { update: Update & U }
    : never;
type RunQuery<Q extends string> = Combine<AssertKey<Q>, Q>;
type AssertKey<Q extends string> = Q extends unknown
    ? Record<Q, NonNullable<unknown>>
    : never;

// define additional fields on U with value `undefined`
type Combine<U, K extends string> = U extends unknown
    ? U & Partial<Record<Exclude<K, keyof U>, undefined>>
    : never;

// === HOW IT BREAKS
declare const ctx: FilterContext<
    FilterContext<Context, "one" | "two">,
    "two"
>;
const works = ctx.update.update_id;
type Works = (typeof ctx & Checked)["update"]["update_id"];
if (ctx.check()) {
    const breaks = ctx.update.update_id;
}

🙁 Actual behavior

In the line where the variable breaks is defined, ctx.update is never

🙂 Expected behavior

The variable breaks can be defined and it has type number

Additional information about the issue

If you change a seemingly unrelated piece in the code, the error goes away!

// Replace
type Checked = { update: { REMOVE_THIS_TO_FIX?: never } };
// by
type Checked = { update: { } };

and then everything works as expected.

Bắt đầu với TypeScript Playground được liên kết và so sánh việc tái hiện trên 4.7.4 và 4.8.4. Theo dõi quá trình thu hẹp kiểu do Context.check() thực hiện đối với ví dụ FilterContext<Context, "one" | "two">. Được xem là hoàn tất khi việc truy cập breaks được chấp nhận và được suy luận là number, đồng thời hồi quy được bao phủ bằng một test.