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1. MartinJohns commented on Apr 13, 2022

   

   Martin Johns (MartinJohns)

   on Apr 13, 2022

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   Duplicate of #45972. Used search terms: `source has but target allows only"
2. jcalz commented on Apr 13, 2022

   

   Joe Calzaretta (jcalz)

   on Apr 13, 2022

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   I'm... not sure this is a duplicate of that, since there are no leading rest elements here to make the analysis weirder. Maybe the same underlying issue is happening in both places?

   Anyway this one feels more like a bug to me, since the following is fine:

   const f1: (x: string | number) => void = x => { }; 
   const f2: (x: string | number, y: string | number) => void = f1; // okay
   const f3: (...args: [number, string] | [string, number]) => void = f2; // okay

   And while transitivity of assignability isn't the be-all and end-all for TypeScript, it's nice to be able to reason about things this way when possible.

   👀React with 👀2Reacted by Martin Johns and Ovidijus Parsiunas
3. Ryan Cavanaugh (RyanCavanaugh)

   added

   BugA bug in TypeScriptA bug in TypeScript

   Help WantedYou can do thisYou can do this

   on Apr 13, 2022
4. Ryan Cavanaugh (RyanCavanaugh)

   added this to the Backlog milestone on Apr 13, 2022
5. graphemecluster commented on May 29, 2022

   

   graphemecluster (graphemecluster)

   on May 29, 2022

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   Sorry to copy my #49218 (comment) here:

   I think the easiest way to solve all the related issues is to wrap all the arguments into a tuple with rest syntax, i.e. item => {} should become (...[item]) => {} and ([first, ...rest], foo, ...args) => {} should work just like (...[[first, ...rest], foo, ...args]) => {}.
6. cdauth commented on Jun 25, 2024

   

   Candid Dauth (cdauth)

   on Jun 25, 2024

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   Not exactly the same issue, but probably the same root cause:

   const func: (arg1: string, arg2?: string) => void = (...args: [string] | [string, string]) => undefined; // Error

   Also:

   type Func = (...args: [string] | [string, string]) => void;
   type Test = Func extends (arg1: string, ...args: any[]) => any ? true : false; // false
   
   // Workaround:
   type Test = Func extends (...args: infer Args) => any ? (Args extends [string, ...any[]] ? true : false) : false; // true
7. Ryan Cavanaugh (RyanCavanaugh)

   added

   Domain: check: Variance RelationshipsThe issue relates to variance relationships between typesThe issue relates to variance relationships between types

   on Oct 16, 2025