Description

falsandtru (falsandtru)

opened on May 11, 2016

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TypeScript Version:

master

Code

function f<T extends string[] | {}>(a: T) {
  if (a instanceof Array) {
    a = a;
  }
}

Expected behavior:

$ node built/local/tsc.js index.ts

Actual behavior:

$ node built/local/tsc.js index.ts
index.ts(3,5): error TS2322: Type 'any[]' is not assignable to type 'T'.

Activity

1. ahejlsberg commented on May 11, 2016

   

   Anders Hejlsberg (ahejlsberg)

   on May 11, 2016

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   Hmm. This one is tricky. Currently the only type we consider assignable to a type parameter is that same type parameter (and any of course). In order for the above to work we would basically need to create a new type parameter that is a subtype of T and has a narrowed constraint string[]. It's possible, but it isn't a simple fix.
2. Anders Hejlsberg (ahejlsberg)

   added

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   on May 11, 2016
3. DanielRosenwasser commented on May 13, 2016

   

   Daniel Rosenwasser (DanielRosenwasser)

   on May 13, 2016

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   I would think that #7422 would have allowed this. Care to weigh in Nathan Shively-Sanders (@sandersn)?
4. sandersn commented on May 13, 2016

   

   Nathan Shively-Sanders (sandersn)

   on May 13, 2016

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   Yeah, it did basically what Anders Hejlsberg (@ahejlsberg) said -- but (1) only locally, when getting the narrowed type, and (2) not updated for control-flow rewrite of narrowing. So it can't be used as-is.
5. ahejlsberg commented on May 13, 2016

   

   Anders Hejlsberg (ahejlsberg)

   on May 13, 2016

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   I don't think the solution in #7422 works. The thing that makes this issue complicated is that you can't just clone type parameters without also being prepared to treat the clones as aliases for the original type parameter in the type instantiation logic. Otherwise you'll "leak" type parameters.
6. ahejlsberg commented on May 3, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on May 3, 2017

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   This has worked correctly for a while now. The type guard produces type T & any[] which is assignable to T so there is no error. I'm closing this issue.

   Reacted by falsandtru
7. Anders Hejlsberg (ahejlsberg)

   closed this as completedon May 3, 2017
8. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018

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