identical conditional types with a callable constituent not assignable to each other #21823

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1. tvald commented on Feb 9, 2018

   

   Tony Valderrama (tvald)

   on Feb 9, 2018

   Author

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   Anders Hejlsberg (@ahejlsberg) Mohamed Hegazy (@mhegazy)

   This is still motivated in my particular case by the example from #21756:

   // lib/es5.d.ts
   declare type PromiseConstructorLike = new <T>(executor: (
     resolve: T extends void ? (value?: PromiseLike<void>) => void : (value: T | PromiseLike<T>) => void,
     reject: (reason?: any) => void) => void
   ) => PromiseLike<T>;
   
   // lib/es2015.promise.d.ts
   interface PromiseConstructor {
     new <T>(executor: (
       resolve: T extends void ? (value?: PromiseLike<void>) => void : (value: T | PromiseLike<T>) => void,
       reject: (reason?: any) => void) => void
     ): Promise<T>;
   }
2. lostfields commented on Feb 9, 2018

   

   lostfields

   on Feb 9, 2018

   Last edited by lostfields

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   Yeah, I think I have a similar issue in #21734 where generic constraints does not work well for anything but primary types.

   This work as it should and throws an error because of the generic constraint

   let first = <T extends Number>(items: Array<T>): T => {
       return items.shift();
   }
   
   let a = first([1,2,3]);
   let b = first(["a","b","c"]); // error TS2345: Argument of type 'string[]' is not assignable to parameter of type 'Number[]'

   This also works as it should, it inherits TType from the generic class

   class Item<TType> {
       public first<T extends Array<TType>>(items: T) {
           return items.slice(2,1).shift();
       }
   }
   
   let c = new Item<number>().first([1,2,3]);
   let d = new Item<number>().first(["a", "b", "c"]); //error TS2345: Argument of type 'string[]' is not assignable to parameter of type 'number[]'

   But, whenever I use more complex structures as constraint it never fails

   class Car {
   }
   
   let e = new Item<Car>().third([new Car(), new Car(), new Car()]);
   let f = new Item<Car>().third([1,2,3]); // no error because signature of third is third<number[]> instead of third<Car[]>

   The issue here is that the signature for e line is Item<Car>.third<Car[]>and for f line is Item<Car>.third<number[]>, the type TType is never assigned down to method first
3. Mohamed Hegazy (mhegazy)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Feb 9, 2018
4. Mohamed Hegazy (mhegazy)

   assigned

   Anders Hejlsberg (ahejlsberg)

   on Feb 9, 2018
5. Mohamed Hegazy (mhegazy)

   added this to the TypeScript 2.8 milestone on Feb 9, 2018
6. Anders Hejlsberg (ahejlsberg)

   added

   FixedA PR has been merged for this issueA PR has been merged for this issue

   on Feb 9, 2018
7. Anders Hejlsberg (ahejlsberg)

   closed this as completedin #21839on Feb 11, 2018
8. microsoft

   locked and limited conversation to collaborators on Jul 3, 2018