Type narrowing for awaited values #30551

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1. MartinJohns commented on Mar 23, 2019

   

   Martin Johns (MartinJohns)

   on Mar 23, 2019

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   This is due to #9998.
2. DanielRosenwasser commented on Mar 25, 2019

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Mar 25, 2019

   Last edited by DanielRosenwasser

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   I think it's more that there's a limited set of syntactic constructs we can analyze because it would be inefficient otherwise. I guess the thing I want to know is whether there's a reason to write code like that.
3. Ryan Cavanaugh (RyanCavanaugh)

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   on Mar 26, 2019
4. jeremymeng commented on Jun 17, 2019

   

   Jeremy Meng (jeremymeng)

   on Jun 17, 2019

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   I guess this is the same issue that for-await-of doesn't narrow types?

   /// <reference lib="esnext.asynciterable"/>
   
   interface MyIterator<T> {
     next(): Promise<
       | {
           done: false;
           value: T;
         }
       | {
           done: true;
           value: undefined;
         }
     >;
     [Symbol.asyncIterator](): MyIterator<T>;
   }
   interface Item { name: string }
   
   async function m(iter: MyIterator<Item>) {
     for await (const it of iter) {
       console.log(it.name);           // Error here complaining about 'it': Object is possibly 'undefined'.ts(2532)
   
     }
   }
5. jeremymeng commented on Jun 18, 2019

   

   Jeremy Meng (jeremymeng)

   on Jun 18, 2019

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   hmm, actually it is not related to await. iterator has the same error

   interface MyIterator<T> {
       next():
         | {
             done: false;
             value: T;
           }
         | {
             done: true;
             value: undefined;
           }
       ;
       [Symbol.iterator](): MyIterator<T>;
   }
   
   interface Item { name: string }
   
   function m(iter: MyIterator<Item>) {
       for (const it of iter) {
           console.log(it.name);           // Error here complaining about 'it': Object is possibly 'undefined'.ts(2532)
   
       }
   }