Narrowing union types with array's indexOf #9842

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1. DanielRosenwasser commented on Jul 20, 2016

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Jul 20, 2016

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   There's a lot going on there that would need to be formalized. Specifically:

   1. The expression of a property access becomes a literal type location.
   2. An array literal becomes a literal type location if its parent is a literal type location.
   3. indexOf would need to have a way of expressing that any value >= 0 implies that its type is related to its array's numeric index signature.

   (1) and (2) are very easy to implement, but I think that (3) is too complex to express right now. While we have type predicate functions, we'd need a more powerful mechanism for expressing relationships between produced values and their arguments.
2. Mohamed Hegazy (mhegazy)

   added

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   Too ComplexAn issue which adding support for may be too complex for the value it addsAn issue which adding support for may be too complex for the value it adds

   on Sep 29, 2016
3. Mohamed Hegazy (mhegazy)

   closed this as completedon Sep 29, 2016
4. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018