Generic type gets widened in an unexpected way

Bug Report

🔎 Search Terms

widen

🕗 Version & Regression Information

4.9.4

⏯ Playground Link

Playground link with relevant code

💻 Code

enum One {
  A = 'a',
  B = 'b',
  C = 'c'
}

const isOneSomethingMap = {
  [One.A]: true,
  [One.B]: false,
  [One.C]: true
} as const satisfies Record<One, boolean>;

type BooleanMapToUnion<T extends Record<string, boolean>> = {
  [P in keyof T]: T[P] extends true ? P : never;
}[keyof T];

type SomethingOne = BooleanMapToUnion<typeof isOneSomethingMap>;

const a = <T>(value: T) => value;
const b = <T>(fn: (value: T) => T, v: T): T => fn(v);

const v: SomethingOne = One.A as SomethingOne;
const v2: One.A | One.C = One.A as One.A | One.C;

const r1 = b(a, v); // One and not SomethingOne - not expected
const r2 = a(v); // SomethingOne as expected
const r3 = b(a, v2); // Union as expected

const r4 = b(a, One.A as One.A | One.C); // Works
const r5 = b(a, One.A as SomethingOne); // Still doesn't work...

🙁 Actual behavior

The type gets widened when using BooleanMapToUnion... I guess it is the culprit.

🙂 Expected behavior

Both union and using BooleanMapToUnion should work equally.

Beginne mit der verlinkten TypeScript Playground-Reproduktion und vergleiche die abgeleiteten Typen von r1 bis r5. Untersuche, wie die generische Inferenz SomethingOne aus BooleanMapToUnion im Vergleich zur expliziten Union behandelt, und verifiziere anschließend, dass der erwartete SomethingOne-Typ für r1 und r5 erhalten bleibt, ohne das Verhalten der anderen Beispiele zu ändern.