Type argument inference does not take call's contextual type into account #10245

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1. DanielRosenwasser commented on Aug 10, 2016

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Aug 10, 2016

   Last edited by DanielRosenwasser

   Member

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   What's happening here is that the call to array.map doesn't try to infer its return type from its contextual type. It only tries to infer from its arguments.

   A workaround would be to give [number, number] as a type argument to map:

   function map(array: number[]) {
       return array.map<[number, number]>(x => [x, x * x]);
   }
2. Daniel Rosenwasser (DanielRosenwasser)

   added

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   Needs ProposalThis issue needs a plan that clarifies the finer details of how it could be implemented.This issue needs a plan that clarifies the finer details of how it could be implemented.

   on Aug 10, 2016
3. Daniel Rosenwasser (DanielRosenwasser)

   changed the title [-]Incorrect contextual type inferred[/-] [+]Type argument inference does not take call's contextual type into account[/+] on Aug 10, 2016
4. manueledones commented on Jan 16, 2019

   

   Manuele Dones (manueledones)

   on Jan 16, 2019

   Last edited by manueledones

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   Daniel Rosenwasser (@DanielRosenwasser) Here some other examples:

   export interface DataModel {
     prop1: string;
     prop2: number;
   }
   
   const array = [1,2,3,4];
   
   // NO ERROR
   const datamodels2: DataModel[] = array.map(i => ({
     prop1: 'asdf',
     prop2: 1234,
     extra: 'extra'
   }))
   
   // NO ERROR
   const datamodels4: DataModel[] = array.map(i => {
     return {
       prop1: 'asdf',
       prop2: 1234,
       extra: 'extra'
     } 
   });
   
   // NO ERROR
   const datamodels5: DataModel[] = array.map(i => {
       return {
           prop1: 'asdf',
           prop2: 1234,
           extra: 'extra'
       }  as DataModel
   });
   
   // NO ERROR
   const datamodels6: DataModel[] = array.map(i => ({
           prop1: 'asdf',
           prop2: 1234,
           extra: 'extra'
   }) as DataModel);
   
   // NO ERROR
   const datamodels7: DataModel[] = array.map<DataModel>(i => ({
           prop1: 'asdf',
           prop2: 1234,
           extra: 'extra'
   }));
   

   `
   Is this the expected behavior or a bug?
5. RyanCavanaugh commented on Jan 16, 2019

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Jan 16, 2019

   Member

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   @whiteblackkeys Unrelated. Root cause is #241 - function return expression widening causes the object literal freshness to be lost, causing excess properties to not be checked.
6. ahejlsberg commented on Mar 22, 2019

   

   Anders Hejlsberg (ahejlsberg)

   on Mar 22, 2019

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   Fixed by #29478.

   🎉React with 🎉1Reacted by Jan Žák
7. Anders Hejlsberg (ahejlsberg)

   closed this as completedon Mar 22, 2019