type guards strangeness #7224

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1. yortus commented on Feb 25, 2016

   

   Troy Gerwien (yortus)

   on Feb 25, 2016

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   Aleksey-Bykov I'm also finding that user-defined type guards often don't work the way I intuitively expect. Your example makes intuitive sense to me and I would have expected it to work.

   However, since you haven't started with a union type, I suspect you are working outside the bounds of what tsc can currently do with type guards. I got your example to work by changing the declaration of value to:

   let value: string | (string & AsNonBlank);

   This seems to satisfy the compiler, and the narrowing happens as expected in both the if and else branches.
2. yortus commented on Feb 25, 2016

   

   Troy Gerwien (yortus)

   on Feb 25, 2016

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   Interestingly, the following version of your code does work as you expect:

   const enum AsUnaryFunc {}
   export function isUnaryFunc(value: Function) : value is Function & AsUnaryFunc {
       return value.length === 1;
   }
   let value: Function;
   if (isUnaryFunc(value)) {
       doThis(value); // expected value to be Function & AsUnaryFunc, AND IT IS
   } else {
       doThat(value);
   }
   function doThis(value: Function & AsUnaryFunc): void {
   }
   function doThat(value: Function) : void {
   }

   It's the same code as yours, just with Function/AsUnaryFunction instead of string/AsNonBlank.
3. zpdDG4gta8XKpMCd commented on Feb 25, 2016

   

   ZpdDG4gta (zpdDG4gta8XKpMCd)

   on Feb 25, 2016

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   Troy Gerwien (@yortus) thanks!
4. zpdDG4gta8XKpMCd commented on Feb 26, 2016

   

   ZpdDG4gta (zpdDG4gta8XKpMCd)

   on Feb 26, 2016

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   Troy Gerwien (@yortus) just checked with the union, it doesn't work either

   const enum AsNonBlank {}
   export function isNonBlank(value: string | string & AsNonBlank) : value is string & AsNonBlank {
       return !/^\s*$/g.test(value);
   }
   let value: string;
   if (isNonBlank(value)) {
       doThis(value); // expected value to be string & AsNonBlank, actual is *STILL* a string
   } else {
       doThat(value);
   }
   function doThis(value: string & AsNonBlank): void {
   }
   function doThat(value: string) : void {
   }
5. yortus commented on Feb 26, 2016

   

   Troy Gerwien (yortus)

   on Feb 26, 2016

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   Sorry I wasn't clear - this one works for me:

   const enum AsNonBlank {}
   export function isNonBlank(value: string) : value is string & AsNonBlank {
       return !/^\s*$/g.test(value);
   }
   let value: string | (string & AsNonBlank);    // <-- union here
   if (isNonBlank(value)) {
       doThis(value); // expected value to be string & AsNonBlank
   } else {
       doThat(value);
   }
   function doThis(value: string & AsNonBlank): void {
   }
   function doThat(value: string) : void {
   }
6. Nathan Shively-Sanders (sandersn)

   closed this as completedin #7235on Jun 2, 2016
7. Nathan Shively-Sanders (sandersn)

   added a commit that references this issue on Jun 2, 2016

   Merge pull request #7235 from weswigham/narrow-all-types

   ...

   This issue will close once commit ef0f6c8 is merged into the 'main' branch.

   ef0f6c8
8. zpdDG4gta8XKpMCd commented on Jun 2, 2016

   

   ZpdDG4gta (zpdDG4gta8XKpMCd)

   on Jun 2, 2016

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   Thanks! That's a great help!
9. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018