Type guard failures with negative condition #9861

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1. yortus commented on Jul 21, 2016

   

   Troy Gerwien (yortus)

   on Jul 21, 2016

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   I think the 'problem' here is that you assigned a new B<void> instance in the declaration of o. TypeScript has accordingly narrowed o to B<void> for the rest of the function body.

   To see the behaviour you are expecting, you can add an <any> cast to prevent the initial narrowing:

   class A<T> {
     private _: T;
   }
   class B<T> {
     private _: T;
   }
   function f() {
     var o: A<void> | B<void> = <any> new B<void>();            // <=====
     if (!(o instanceof B)) return o; // A<void>, should be A<void>
     o; // B<void>, should be B<boid>
   }}
2. mhegazy commented on Jul 22, 2016

   

   Mohamed Hegazy (mhegazy)

   on Jul 22, 2016

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   falsandtru (@falsandtru) i would appreciate it if you can limit the discussion to #9859 and avoid creating much noise.
3. mhegazy commented on Jul 22, 2016

   

   Mohamed Hegazy (mhegazy)

   on Jul 22, 2016

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   This is the same topic discussed in #9859. closing in favor of #9859
4. Mohamed Hegazy (mhegazy)

   closed this as completedon Jul 22, 2016
5. Mohamed Hegazy (mhegazy)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Jul 22, 2016
6. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018