Bug Report

🔎 Search Terms

infer contextual string union

🕗 Version & Regression Information

• This changed between versions 5.1.3 and 5.0.4

⏯ Playground Link

Playground link with relevant code

💻 Code

export interface UnionAltA {
  type?: 'text';
}

export interface UnionAltB {
  type?: 'image' | 'video' | 'document';
}

export type ValueUnion = UnionAltA | UnionAltB;

export type MapOrSingleton =
  | {
      [key: string]: ValueUnion;
    }
  | ValueUnion;

const withoutAsConst: MapOrSingleton = {
  1: {
    type: 'text' /*as const*/,
  },
};

const withAsConst: MapOrSingleton = {
  1: {
    type: 'text' as const,
  },
};

🙁 Actual behavior

The declaration of withoutAsConst fails, as type: "text" fails to be inferred as "text", and instead infers as string in this situation:

Type 'string' is not assignable to type '"text" | "image" | "video" | "document" | undefined'.

🙂 Expected behavior

The declaration of withoutAsConst should succeed. Typescript understands that the only valid values in this position are "text" | "image" | "video" | "document" | undefined from the contextual type, so the literal "text" should match that.