error casting interface union field #20889

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1. jcalz commented on Dec 26, 2017

   

   Joe Calzaretta (jcalz)

   on Dec 26, 2017

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   Duplicate of #12052 ?

   👍React with 👍1Reacted by Daniel Rosenwasser
2. DanielRosenwasser commented on Dec 26, 2017

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Dec 26, 2017

   Last edited by DanielRosenwasser

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   Using a type assertion actually does work here.

   interface A1 {
       type: "A1";
   }
   interface A2 {
       type: "A2";
   }
   
   type AU = A1 | A2;
   
   function foo(obj1: AU) {
       const obj2 = {
           type: obj1.type
       } as AU;
       // ^^^^
       // Here
   }

   👍React with 👍1Reacted by Joe Graham
3. Daniel Rosenwasser (DanielRosenwasser)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Dec 26, 2017
4. codemau5 commented on Dec 26, 2017

   

   Joe Graham (codemau5)

   on Dec 26, 2017

   Last edited by codemau5

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   Daniel Rosenwasser (@DanielRosenwasser) the issue you labeled this as a duplicate of, is labeled as "intended behavior",
   because { type: "A1" | "A2"; } !== { type: "A1"; } | { type: "A1" ; },
   otherwise if we had more fields, we can get an unstable behaviour.

   looking into it, I found that you do get unstable behavior using the fix you commented!!

   interface A1 {
       typeA: "A1";
       typeB: "B1";
   }
   interface A2 {
       typeA: "A2";
       typeB: "B2";
   }
   type AU = A1 | A2;
   
   function foo(obj1: AU, obj2: AU): AU {
       const obj3: AU = <AU>{
           typeA: obj1.typeA,
           typeB: obj2.typeB
       };
       return obj3;
   }
   
   const a1: A1 = {
       "typeA": "A1",
       "typeB": "B1"
   }
   const a2: A2 = {
       "typeA": "A2",
       "typeB": "B2"
   }
   
   const a3: AU = foo(a1, a2);
   
   console.log(a3.typeA, a3.typeB);
   // A1 B2

   a3 is of type AU, but is in an invalid state!!

   if you would try to create such an object, you would normally get this error:

   const au: AU = {
       typeA: "A1",
       typeB: "B2"
   };
   /*
   Type '{ typeA: "A1"; typeB: "B2"; }' is not assignable to type 'AU'.
     Type '{ typeA: "A1"; typeB: "B2"; }' is not assignable to type 'A2'.
       Types of property 'typeA' are incompatible.
         Type '"A1"' is not assignable to type '"A2"'.
   */

   EDIT: I have issued a new issue for this new bug - #20896
5. typescript-bot commented on Jan 9, 2018

   

   TypeScript Bot (typescript-bot)

   on Jan 9, 2018

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   Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
6. TypeScript Bot (typescript-bot)

   closed this as completedon Jan 9, 2018
7. microsoft

   locked and limited conversation to collaborators on Jul 3, 2018