When the initial element of the tuple is a rest element, it is not possible to correctly infer the type of the initial element. #56885

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1. craigphicks commented on Dec 28, 2023

   

   Craig P Hicks (craigphicks)

   on Dec 28, 2023

   Last edited by craigphicks

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   Actually this is not a new feature. See #49138 which was fixed in 5.0.8.
   It is a bug - consider I am now assuming this is a feature request.

   type T1 = [...string[], number, boolean]
   
   const arr1: T1 = [2, true];
   const arr2: T1 = ["", 2, true];
   const arr3: T1 = [true]; // error
   //    ~~~~
   // Type '[true]' is not assignable to type 'T1'.
   //  Source has 1 element(s) but target requires 2.
   
   declare const arr: T1;
   const head = arr[0]
      // Actual: string | number | boolean
      // Expected: string | number

   https://www.typescriptlang.org/play?ts=5.0.4#code/FAFwngDgpgBAKgRhgXhgbQHRYM4gE4CWAdgOZoC6ANDEQK4C2ARlHtYwPbsA2UAhkeWDAAxuyK4YvPHgQAueElRoATNXy0o5ANwixEqXmXzEKdACIz1VTHWado8SEnSAzMcXpb2mAHofMFjx2PGA-GHCYAD9oyND-OEhYAHI0LySYAmwadidebGwCEiJeRh4bdhtEmCTEJIw48IBldlo8YVgACzyYJCgeeigiEAAKbABKGEZaJxApEignPCgAR1oCJazleuAAEyhhLilYB31pd3s9Jw6+HdMDNAAGQQiwgEFhEFpeLnlcQlIYAAfGgMZh4IGTTg8fjAF7+ACiAA9oB8oDtfvhiCQIXQmCwgA

   😕React with 😕1Reacted by Bruce Pascoe
2. craigphicks commented on Dec 28, 2023

   

   Craig P Hicks (craigphicks)

   on Dec 28, 2023

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   Bruce Pascoe (@fatcerberus)
   Let me clarify my statement.

   I am making two assumptions:

   1. that the current assignment behavior is correct
   2. that correct indexing behavior should follow exactly the results of assignment behavior

   Joe Calzaretta (@jcalz) asserts here that "Yes, that type (arr[0]) is wider than it should be, but that's not a bug, it's a design decision."

   In other words, that my assumption 2 is incorrect.

   Certainly there are times when economy of calculation dictates using wider types. Other times it may just be expedient coding.
   Either of those would enough to invalidate my assumption 2.

   Therefore I will withdraw my statement that this must be a bug, and instead assume it is a feature request.
3. craigphicks commented on Dec 28, 2023

   

   Craig P Hicks (craigphicks)

   on Dec 28, 2023

   Last edited by craigphicks

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   拾叁 (@13OnTheCode) - As a feature request, I believe more explanation is required.

   Currently, you have your motivating example code listed in the suggestion section, and the motivating example and use cases sections are empty.

   Here is an example:

   Suggestion

   • Tuple indexed access, start-relative or end-relative, by a literal number, should return the minimal possible type set.

   You could also give an algorithm that computes that.
4. fatcerberus commented on Dec 28, 2023

   

   Bruce Pascoe (fatcerberus)

   on Dec 28, 2023

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   ...that correct indexing behavior should follow exactly the results of assignment behavior

   Yes, this assumption is incorrect. The PR that implemented mid/lead rest elements (#41544) specifically calls out the current indexing behavior, suggesting it's intentional (or at least a known-upfront design limitation).

   From #41544:

   Also note that indexing a tuple type beyond its starting fixed elements (if any) yields a union type of all possible element types

   👍React with 👍2Reacted by Craig P Hicks and Andrea Simone Costa
5. Ryan Cavanaugh (RyanCavanaugh)

   added

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   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

   on Jan 2, 2024