Incorrect generic inference inside a discriminated union

Bug Report

🔎 Search Terms

discrimated union generic extra property

🕗 Version & Regression Information

• This changed between versions 4.1.5 and 4.2.3

⏯ Playground Link

Playground link with relevant code

💻 Code

type Loadable<TValue> =
    | {
          state: "loading"
      }
    | {
          state: "complete"
          value: TValue
      }


function mapLoadable<TValue>(loadable: Loadable<TValue>, map: (value: TValue) => void): void {
}

type AugmentedLoadable =
    | {
          state: "loading"
          value: number
      }
    | {
          state: "complete"
          value: string
      }

function mapAugmented(augmented: AugmentedLoadable) {
    mapLoadable(augmented, () => {}) // ❌ Errors with "Argument of type 'AugmentedLoadable' is not assignable to parameter of type 'Loadable<number>'"
    mapLoadable<string>(augmented, () => {}) // ✅ Works now that type is generic is specified
}

🙁 Actual behavior

TypeScript incorrectly infers that augmented should be assignable to Loadable<number> rather than a Loadable<string>, because we've added an unrelated value: number property to it. It can be nudged into working by writing out the generic type.

🙂 Expected behavior

TypeScript to infer the correct generic of string.