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The order of values ​​in a union affects the correct type inference #59729

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@dartess

Description

@dartess

🔎 Search Terms

union order type infer

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about union / order

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.4#code/C4TwDgpgBAshDO8CGBzaBeKBvKB7MwAlrgHbwD8AXFAEoQDGuATgCYA88wThJKANFACuJANYlcAdxIA+KAF8A3ACgloSFAASSEiwA2EJmwAqszAAoAtgmRpqRgJRR0sgG65CLZUpYNdSJtAAZsL0RKRQ8IIARvD03FEQxlAQAB7AEDrwsNaoENJmSlBFUGBIILq4SCyUhcVFAD7YUFaIuXbyUABkTQAW2noG1Fo6+oYm8rV1jTgtNhDUZo7OUEYd3Th9I4Oa-aPGsnK19tRuHl6RMXGECWZYtbNtUItOsrd4BMRk1DP+IhDA1AA5BZfv9AR05PY+LVNgMmAscPgwmR5EtZHc6pjigB6bFQAB65FqcmhkOUQA

💻 Code

type Message = { options?: Record<string, unknown> };

type Handler<T> = (message: T) => void;

declare function subscribe<T extends Message>(
    payload:
      | { message: T } & { handler: Handler<T> }
      | { message: () => T } & { handler: Handler<T> }
  ): void;

subscribe({
  message: () => ({ options: { market: 'market' } }),
  handler: ({ options }) => {
                  // ^? Record<string, unknown>
  },
});

🙁 Actual behavior

options type is Record<string, unknown>

🙂 Expected behavior

options type should be inferred as { market: string }

Additional information about the issue

if you reverse the order of values in the union:

declare function subscribe<T extends Message>(
    payload:
      | { message: () => T } & { handler: Handler<T> }
      | { message: T } & { handler: Handler<T> }
  ): void;

then the type will be inferred correctly

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    Help WantedYou can do thisPossible ImprovementThe current behavior isn't wrong, but it's possible to see that it might be better in some casesUnion Order DependenceTS behavior is not supposed to depend on union order, but sometimes does

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