Fail to infer Indexed access to the intersection type when it includes an union derived from a generic input

🔎 Search Terms

"indexed access on union", "generic index"

🕗 Version & Regression Information

5.6.2, and it seems never worked for every ts versions.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.2#code/C4TwDgpgBAKhDOwA8MoQB7AgOwCbykQCcBLbAcwD4oBeKAbwG0BpKM2AXQC4GBrCED2YBfYYxgcoAMgbAAhkXIRgPYmXLCAUJoDGAe2yIoAMz17aUFGkw58hYKQqUAFAvI84iFJQCUtavSaUMFQ+obAUABucgA2AK4QHgjIMNR0bgDcQSFhRtHxEABMSV6pjADk-CDlkumKFVU1WSGhBnmxCQDMJSmUFfKKyjUWbv1uQxwZUAD001AAokREekSawkA

💻 Code

type Test<T extends string> = {[K in T]: {key: K}}[T] & {target: string}

const foo = <T extends string>(arg: Test<T>) => {
    const value: Test<T> = arg; 
    const value2: Test<T>['key'] = arg['key']; 
    const value3: Test<T>['target'] = arg['target']; // Error
}

🙁 Actual behavior

The declaration of value and value2 works well, but ts emit an error saying Type 'string' is not assignable to type '{ [K in T]: { key: K; }; }[T]["target"] & string' for value3.

🙂 Expected behavior

The Test<T>['target'] should equal to string and assume the type of '{ [K in T]: { key: K; }; }[T]["target"] as any before intersection, as it does when I give non-generic type in the same place. It seems even weird because it well infers the type of Test<T>['key'].

Additional information about the issue

With additional experiment, I found that it always happens when I generate an union type with generic input. For instance,

type Test<T> = (T extends string ? {key: number} : {key: string}) & {target: string}

const foo = <T extends string>(arg: Test<T>) => {
    const value: Test<T> = arg;
    const value2: Test<T>['key'] = arg['key'];
    const value3: Test<T>['target'] = arg['target']; // Error
}

it also does not work in the case above

Beginne mit der verlinkten TypeScript Playground-Reproduktion und vergleiche die im Issue gezeigten generischen und nicht-generischen Indexed-Access-Fälle. Als abgeschlossen gilt die Änderung, wenn der generische Indexed Access für den Intersection-Typ den String-Wert für die Ziel-Property akzeptiert, ohne den gemeldeten Typfehler zu erzeugen, während die bestehende Key-Inferenz korrekt bleibt.