Rest elements in generic tuple #26113

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1. AlCalzone commented on Aug 1, 2018

   

   AlCalzone

   on Aug 1, 2018

   Last edited by AlCalzone

   Contributor

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   Interstingly, adding angled brackets works (but requires an array of arrays following the number):

   interface Foo<ArgsT extends any[]> {
   	arr: ArgsT;
   }
   
   interface Bar<SubArgsT extends any[]> extends Foo<[number, ...SubArgsT[]]>{ }
   
   var f: Foo<number[]> = {
   	arr: [1,2,3],
   };
   
   var b1: Bar<string[]> = {
   	arr: [1, [""], ["1", "2"]], // not what OP meant, but it works
   }
   var b2: Bar<string[]> = {
   	arr: [1, "", "1", "2"], // error: [number, string, string] cannot be assigned to [number, ...string[][]]
   }

   And this correctly captures the type OP wants to create

   type Fooify<SubArgsT extends any[]> = 
   	((arg1: number, ...rest: SubArgsT) => any) extends ((...args: infer R) => any)
   	? Foo<R> : never;

   but an interface cannot extend that.
2. Mohamed Hegazy (mhegazy)

   added

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   In DiscussionNot yet reached consensusNot yet reached consensus

   on Aug 2, 2018
3. McPrescott commented on Sep 10, 2018

   

   Jesse McCarthy (McPrescott)

   on Sep 10, 2018

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   Similarly, it seems that following the last example of AlCalzone that rest element types should be able to be inferred without having to use a functions parameter list like so:

   type dropFirst<T extends any[]> = 
       T extends [any, ...(infer U)] ? U : T;
   // Instead of
   type dropFirst<T extends any[]> = 
       ((...args: T) => any) extends (arg: any, ...rest: infer U) => any ? U : T;

   But the same error occurs: A rest element type must be an array type.

   👍React with 👍13Reacted by csha, Eric Simonton, Ran Lottem, Augusto Moura, Micah Zoltu, James Conkling, Salathiel Genese, ExE Boss, Stepan Zubashev, Amine Ben hammou and 3 more
4. cshaa commented on Oct 16, 2018

   

   csha (cshaa)

   on Oct 16, 2018

   Last edited by cshaa

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   A minimal example showing the non-symmetricity of function arguments vs. tuple types.

   declare function foo<T0, T extends any[]>(a: T0, ...b: T); // works
   type foo<T0, T extends any[]> = [T0, ...T]; // rest element type must be an array type

   I think this should be supported and it seems as an oversight to me that it isn't in 3.1 already.
   Is there a specific reason for it to be disallowed, or is it just that it wasn't top priority?

   ---

   Also, it took me quite a long time to find this issue, so I'm adding a few keywords.
   Keywords: spread a generic tuple in a tuple type, generic tuple rest parameter in a tuple type, spreading array type in type declaration, T extends any[] does not work in tuple literal

   👍React with 👍18Reacted by Vladimir Rindevich, Jake Teton-Landis, Dylan C. Kendal, Colin Kahn, Ran Lottem, Calvin, Linus Unnebäck, Paul Maréchal, ExE Boss, Amin Pakseresht and 8 more
5. paul-marechal commented on Aug 25, 2019

   

   Paul Maréchal (paul-marechal)

   on Aug 25, 2019

   Last edited by paul-marechal

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   I would like to extend this issue for code looking like the following:

   function meh<T>(...args: Extract<T, any[]>) {} // works
   type Meh<T> = [...Extract<T, any[]>] // does not work (bug?)

   Hopefully this all boils down to the same actual bug.

   👍React with 👍2Reacted by Amin Pakseresht and csha
7. Anders Hejlsberg (ahejlsberg)

   added

   Fix AvailableA PR has been opened for this issueA PR has been opened for this issue

   on Jun 17, 2020
8. Anders Hejlsberg (ahejlsberg)

   added this to the TypeScript 4.0 milestone on Jun 17, 2020
9. ahejlsberg commented on Jun 17, 2020

   

   Anders Hejlsberg (ahejlsberg)

   on Jun 17, 2020

   Member

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   This is now implemented in #39094.

   🎉React with 🎉8Reacted by Lauren Yim, Paul Maréchal, Denis Bendrikov, Daisy Leigh Brenecki, Roger Li, Nick, Jules Sam. Randolph and mwalkerr

   ❤️React with ❤️7Reacted by csha, Lauren Yim, Paul Maréchal, Denis Bendrikov, Roger Li, Jules Sam. Randolph and mwalkerr
10. Anders Hejlsberg (ahejlsberg)

    closed this as completedin #39094on Jun 23, 2020