Type narrowing does not extend to return type

[shark-admin]

🔎 Search Terms

"Type narrowing", "generic dependent return type", "GADT", "GADT expression evaluation"

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about "narrow", "dependent". I also looked through all the headings and didn't see any relevant sections.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250405#code/C4TwDgpgBAcgrgWwgJwJYGMDSqB2ATKAXigCJdgSoAfUgMwBsB7AQwurtQA8I8SBuAFChIUbPgAqjcQGdx4CAB5MUCJ2AR802IhQYxeAHxFRKtRrxaAFGRxsaJBiwoBKKAH4oORACMUUAFxQ0sBoOADmgkLyUACinGDIENLSqIw4CgBKRsRxCUkpaQCC3tIAqjip6VmRqmCMyMBQwtC5icmVxWUVaZnZUADeAlBQANrKuNpIaFi4eAC6ga35leXMyCCZADSkzCUk25gGggC+I-BTerNzNfH1jc2x8W0FOKvrWyaq6ppBIbhh2wAYqZvhZJroZvg+oNhgBrWaBTCCYZgAAWzFsjAQgQyI2uQygbTg9GA+kCgORUEYkGQGLwiyeyx6WSgADIBoSksTSQioMDjicBAJancoLQ4Dh0MBKioAG7MehwVgQACMvUstWQDLy7WZBm2yEYjGAOOciNmkhkckgCmajFopgSADoiST9EYYVBUA6NU8nfD8ERiCRdtJKKz2ZqXVy3bMg6RyCRXJ7hvQII1OOaJFJZPJbfJ7Y7kE7qSg6dHpNz3ZThgB6WtQTherR4CDoehrHhQdD1RJS+ggKDMLReBC+ZBC4bDHs4YJUml04wQeWK5Uq33O0u0-AGo3AZw1qD1+dlwOoFttjuJAi4HvIPvAAdDqzBUJhdij8fOSdT48ASUac8vUlXs20fQccDbdB8jWQdgEYKA4GkaBmCaaJh2QhoZXgqAwAwppUWgV1GkLZcFSVdR1y3OlXBwgBZVhUSdUN-AJYZEmAOBkBwKAGOAJjQ0sajA2HTwfBQA82KPBtxEIzlOO4qAVzgaBCweT8-EIxJm27UD+0HZh0GgsB1DwAljgJfjDQAd08CBbJie96msGBjS9BAwDTJBbB4JNBWFW4GjFCUpRlMjV3UAAmD4YhBcwtCWXUqgMAwNy1WJd2NU0szwS1cxtGIRhIYj9BIOYPQJb0oDS-040IYNQ3DSM-RKurg0TZMpLTDMcry61FDtB0o1aqFD2PJsgNbdtOwIO8HyfUSNInKSZznYSCGIcKKIgSKavWzL9zGht1p0qary7OawIWkdxOQTxGGsn86xkuSOK4njlNUh11NunTbz08DOQAKzArtrNQfjGDge50OSFBpTSVip3k97eMY5iSiEhd8FcUT9D6vNCuKmMeXwMqjnMyzURsuyHKc5AXLc1APK8jRTL8gQLKAA

💻 Code

type NumericKind = "int" | "float" | "fixed";
type KindToTsType<K extends NumericKind> = K extends ("int" | "float") ? number : string;

type Expression<R> = ExpressionAbsUnion<R>;

export type ExpressionAbsUnion<R> = {
  [K in NumericKind]: ExpressionUnary<R, "abs", K>;
}[NumericKind];

export type ExpressionUnary<R, K extends string, F extends NumericKind> = {
  kind: K;
  phantom: R[];
  resultKind: F;
  operand: Expression<R> & { resultKind: F };
}

export function evaluate2<R, E extends Expression<R>>(expr: E, root: R): KindToTsType<E["resultKind"]> {
  if (expr.kind == "abs" && expr.resultKind == "int") {
    let x: KindToTsType<typeof expr.resultKind>;
    // x is declared correctly as number

    const operand = evaluate2(expr.operand, root);
    // operand is declared correctly as number

    // ----------------------------------------------
    // ------------ THE ERROR IS HERE: ------------
    // ----------------------------------------------
    // The return value of type number is incorrectly rejected without type assertion:
    return Math.abs(operand) as KindToTsType<E["resultKind"]>;
  }
  throw new Error("Not implemented");
}

🙁 Actual behavior

The declared return type of KindToTsType<E["resultKind"]> and the actual return type of number in the narrowed branch are not compatible.

🙂 Expected behavior

E should be proven to be ExpressionUnary<R, "abs", "int"> in the branch and therefore the compiler should know that KindToTsType<E["resultKind"]> is equal to number

Additional information about the issue

[shark-admin]

I just realized this is not yet possible in TS and is a duplicate of #33014