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.

Beginne mit dem verlinkten TypeScript Playground und vergleiche die Reproduktion in 4.7.4 und 4.8.4. Verfolge die von Context.check() durchgeführte Eingrenzung für das Beispiel FilterContext<Context, "one" | "two">. Als erledigt gilt, dass der Zugriff auf breaks akzeptiert und als number inferiert wird und die Regression durch einen Test abgedeckt ist.