Description

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...

Contributor guide

Tech stack

typescript

Domain

developer experience

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

4

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

over 1 week

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

active

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Understanding of TypeScript genericsFamiliarity with conditional types

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

15

Research direction

Investigate the TypeScript type checker's handling of conditional types when used with generic functions. The issue occurs when a union type from a conditional type is passed to a generic function that expects a single type, causing the type to widen to the enum base type. Look at the code in src/compiler/checker.ts, particularly the parts dealing with type inference and conditional type evaluation. Consider experimenting with the provided playground link to understand the behavior. Check if any existing tests cover similar scenarios.