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.

Start with the linked TypeScript Playground reproduction and compare the inferred types of r1 through r5. Investigate how generic inference handles SomethingOne from BooleanMapToUnion versus the explicit union, then verify that the expected SomethingOne type is preserved for r1 and r5 without changing the behavior of the other examples.