Aliased discriminants with declarations in binding elements within parameters are not narrowing

[Andarist]

🔎 Search Terms

discriminant alias condition binding element parameter narrowing narrow

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20231109#code/C4TwDgpgBAqgzhAigVwgJxAJQnZAbYAHgBUA+KAXgCgooAfKAbxttYEs4BlZAYx5zgAuKADMAhngQBuFqygATMcDHDkAO3kQRbNRHky5AXxYNmc2h258Bw4GlQHzi5cOKPahmVRHqewNgD2alDICCjoIAAUAJTC8EioGNi4BIRqyAC2AEbo5GZQaBDAyGjB+RZcvPxwQqISCAA0sgpKKiEaWjp6TR4yxlQ8QXDATC0uY2IAjA1QllU2s5XWNZNQhpQhYYlR0V4+an6BwcA4wJGME8LOUzNzy7V31XCrhnFbEcn4ROnZudFMLEGamGo2uV1aACZbksnsJHgIIWsNqEEhEYl5aECQWJClYnht4SsoAAyYmLPEIxxsERQSI4iAUmr-coTSYAOmAAQAogAPMBBCBqfwSdFQAD0YqgcAAFgF8PIoDkoAEANbNa4QjncvkCoVsEW7cWSjjKtUeKjGIA

💻 Code

type UseQueryResult<T> =
  | {
      isSuccess: false;
      data: undefined;
    }
  | {
      isSuccess: true;
      data: T;
    };

function useQuery(): UseQueryResult<number> {
  return {
    isSuccess: false,
    data: undefined,
  };
}

const { data: data1, isSuccess: isSuccess1 } = useQuery();

function test({ data: data1, isSuccess: isSuccess1 }: UseQueryResult<number>) {
  const { data: data2, isSuccess: isSuccess2 } = useQuery();

  const areSuccess = isSuccess1 && isSuccess2;
  if (areSuccess) {
    data1.toExponential(); // should be ok
    data2.toExponential(); // is ok
  }
}

🙁 Actual behavior

data1 is not narrowed down

🙂 Expected behavior

data1 should be narrowed down just like data2 is narrowed down

Additional information about the issue

The fix for this would be an extension to the recently landed #56173 . However, it's not straightforward to fix this right now. isConstantReference can't properly assess if the related parameter symbol is "const" or not and the code operates there on a pseudo reference of the whole binding pattern and not on the individual symbol. It would become much more straightforward if this would land: #56313