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1. Ryan Cavanaugh (RyanCavanaugh)

   added

   QuestionAn issue which isn't directly actionable in codeAn issue which isn't directly actionable in code

   on May 14, 2021
2. RyanCavanaugh commented on May 14, 2021

   

   Ryan Cavanaugh (RyanCavanaugh)

   on May 14, 2021

   Member

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   instanceof won't allow arbitrary cross-type coercions. If this is the behavior you want, write

   class Base {
     f(this: Base | D1) {
       if (this instanceof D1) {
         const s: D1 = this;
         s.f1();
         this.f1();
       }
     }
   }

   Note that this, the type, does not change meaning depending on the narrowing context it's in

   ❤️React with ❤️1Reacted by Zzzen
3. Zzzen commented on May 14, 2021

   

   Zzzen (Zzzen)

   on May 14, 2021

   ContributorAuthor

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   Thanks! Now I get it. Type is somehow immutable. If this is used as an expression in typeof (which I'm trying to implement in #43898), it should get narrowed. right?

   class Base {
     f(this: Base | D1) {
       if (this instanceof D1) {
         const s: typeof this = this;
         s.f1();
       }
     }
   }

   👍React with 👍1Reacted by Ryan Cavanaugh
4. Zzzen (Zzzen)

   closed this as completedon May 14, 2021
5. RyanCavanaugh commented on May 14, 2021

   

   Ryan Cavanaugh (RyanCavanaugh)

   on May 14, 2021

   Member

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   Yeah, typeof x is a "value position" for x and all value logic should apply there
6. microsoft

   locked as resolved and limited conversation to collaborators on Oct 21, 2025