Discriminated union types lose their narrowed type inside nested function scopes #9258

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1. yortus commented on Jun 20, 2016

   

   Troy Gerwien (yortus)

   on Jun 20, 2016

   Contributor

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   I believe this is by design, and applies to all type guards, not just those for discriminated unions. The problem is that the compiler cannot in general be sure when the nested function will be called, so it's not safe to assume the narrowed type will still apply by the time the function is actually called. The variable may have been re-assigned by then.

   There are a couple of special cases involving IIFEs and consts where the narrowing can safely cross into the nested function body, and the compiler handles these cases correctly. See #8849.
2. ryanatkn commented on Jun 20, 2016

   

   Ryan Atkinson (ryanatkn)

   on Jun 20, 2016

   Last edited by ryanatkn

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   Makes sense, thank you!

   So the workaround would look like this:

   interface Rectangle {
       kind: "rectangle";
       width: number;
       height: number;
   }
   
   interface Circle {
       kind: "circle";
       radius: number;
   }
   
   type Shape = Rectangle | Circle;
   
   function test1(s: Shape) {
       if (s.kind === "rectangle") {
           const rect = s;
           s;  // Rectangle
           setTimeout(() => {
             s; // Rectangle | Circle
             rect; // Rectangle
           });
       }
   }

   👍React with 👍2Reacted by Ryan Cavanaugh and Leon Adler
3. Ryan Atkinson (ryanatkn)

   closed this as completedon Jun 20, 2016
4. Ryan Cavanaugh (RyanCavanaugh)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Jun 20, 2016
5. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018