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Generic keyof deduction does not use Interface #39612

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@kwasimensah

Description

@kwasimensah

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms:
generic deduction interface

Code

export type Param = number | string | boolean | Date | null;

export type ParamFields<T> = {
  [P in keyof Required<T>]: NonNullable<T[P]> extends Param ? P : never;
}[keyof T];

interface IHasId {
    id: number;
}

class HasId implements IHasId {
    id: number = 1;
}

function hasField<T>(x: T, field: ParamFields<T>) {
    return x[field];
}


hasField({ x: 5 }, "x");

function templateHasId<T extends IHasId > (x: T) {
    return hasField(x, "id"); // Doesn't realize id has to be a field of x.
}

Expected behavior:

hasId(x, "id"); compiles because it'd declared to have a field of x thats of type number.

Actual behavior:

Compains because it can't deduce "id" is a keyof T even though the interface says it is.

Playground Link:

https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=26&pc=1#code/KYDwDg9gTgLgBDAnmYcAKBDKGC2cC8cAdgK44BGwUcAPnAM4xQCWRA5rXORBADbAYinACIYYqOqV68A3ACg5oSLATJUmbDgBizYLwAm9ADwAVAHwE4Abzlw4AbTRxWcANbBEEAGZwASsABHEmYoYH1TMwBdAC44ADkIIjiSaQxyflNHSItQcSJDdCxcOAB+dDhYomAANyp5AF97d08fE0j5OVZxKC8MAGNUAEkACQx6Qf1rWztnfUqySigGhT7eMfo4UfHJ5hwwfhxgIhgNkbGJqZnZ+YoqSwBGZbkvEiI+mGZEuAALMZ09cLmAAUIFiJgANHAvLoDLENLh-gZjOYAJSXGahGAkKBCED2aEA9pyeoKOS-eiI-RAqxwUFwACscHqkIARCAWSiOi83h8vuI9mtxFsJqY4LkjgUzts4BYQWC0TYrpjsUJyZSQazmPoOTI4AB6PVwYQQYD0IgAcngoQwvGYAC9UFqfmMEBAuKgMFCYZNvLSAHTEhRAA

Related Issues:

#19211
#7294

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