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1. jcalz commented on Sep 20, 2021

   

   Joe Calzaretta (jcalz)

   on Sep 20, 2021

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   Assuming we want to support such assignments, I think you don't want

   • arg6: string | number
   • arg4: string

   If func3 is a Func then you should be able to call func3(1, 2), so arg4 should be string | number while arg6 should be string | number | undefined.
2. uhyo commented on Sep 21, 2021

   

   uhyo (uhyo)

   on Sep 21, 2021

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   Thank you for pointing out 😃 you're right, I updated my issue comment.
3. Andrew Branch (andrewbranch)

   added

   Awaiting More FeedbackThis means we'd like to hear from more people who would be helped by this featureThis means we'd like to hear from more people who would be helped by this feature

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   on Sep 23, 2021
4. graphemecluster commented on May 29, 2022

   

   graphemecluster (graphemecluster)

   on May 29, 2022

   Last edited by graphemecluster

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   Although we can wrap all the arguments inside a tuple with rest syntax as a workaround, the type of arg6 is inferred incorrectly to string | number, which lacks undefined:

   type Func = (...args: [...strs: string[], num1: number, num2: number]) => void
   
   const func: Func = () => {}
   
   const func1: Func = (...[arg1]) => {}
   
   const func2: Func = (...[arg2, arg3]) => {};
   
   const func3: Func = (...[arg4, arg5, arg6]) => {};

   Playground link with the above code

   ---

   Edit: The above code works because the arguments are being inferred as a whole. Adding types makes it break:

   type Func = (...args: [...strs: string[], num1: number, num2: number]) => void
   
   // OK
   const func: Func = () => {}
   // Error:
   // Type '(__0_0: string | number) => void' is not assignable to type 'Func'.
   //   Types of parameters '__0_0' and 'strs' are incompatible.
   //     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[string | number]'.
   //       Source has 2 element(s) but target allows only 1.
   const func1: Func = (...[arg1]: [string | number]) => {}
   // Error:
   // Type '(__0_0: string | number, __0_1: string | number) => void' is not assignable to type 'Func'.
   //   Types of parameters '__0_0' and 'strs' are incompatible.
   //     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[string | number, string | number]'.
   //       Target allows only 2 element(s) but source may have more.
   const func2: Func = (...[arg2, arg3]: [string | number, string | number]) => {};
   // Error:
   // Type '(__0_0: string | number, __0_1: string | number, __0_2: string | number | undefined) => void' is not assignable to type 'Func'.
   //   Types of parameters '__0_0' and 'strs' are incompatible.
   //     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[string | number, string | number, string | number | undefined]'.
   //       Target requires 3 element(s) but source may have fewer.
   const func3: Func = (...[arg4, arg5, arg6]: [string | number, string | number, string | number | undefined]) => {};

   Playground link with the above code

   In any case, this issue should be a bug, not a suggestion.

   👀React with 👀1Reacted by uhyo
5. Mateusz Burzyński (Andarist)

   added a commit that references this issue on Dec 28, 2022

   add tests from microsoft#45972

   beb241d
6. Andarist commented on Dec 29, 2022

   

   Mateusz Burzyński (Andarist)

   on Dec 29, 2022

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   I'm fixing the primary problem reported in this issue here: #49218 .

   That PR doesn't solve the missing | undefined problem though. I think this is a separate discussion and should be handled in a separate PR. In fact, this problem has its own dedicated issue: #46063 (also reported by uhyo (@uhyo) )