Unable to assert(implicit down cast) narrow types from intersection types with partial object types #7531

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1. mhegazy commented on Mar 16, 2016

   

   Mohamed Hegazy (mhegazy)

   on Mar 16, 2016

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   Do not really know how better to explain it than the error message does :)

   Neither type '{ a: number & string; }' nor type '{ a: number; b: number; }' is assignable to the other.
2. Mohamed Hegazy (mhegazy)

   added

   QuestionAn issue which isn't directly actionable in codeAn issue which isn't directly actionable in code

   on Mar 16, 2016
3. RyanCavanaugh commented on Mar 16, 2016

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Mar 16, 2016

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   We have the rule in type assertions that your assertion has to be plausibly correct.

   In other words, you can't write this code

   var x = 'hello';
   var y = <number>x; // Nope

   because there's really no way that's true unless you've been telling some big lies about the type of something.

   So in this case, if you want this to be legal (as a suggested change, not a bug), we'd need to see some rule which a) makes this legal, b) has some compelling justification as for why it should be legal, and c) doesn't allow any random type to be asserted to any other random type.
4. falsandtru commented on Mar 16, 2016

   

   falsandtru (falsandtru)

   on Mar 16, 2016

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   TypeScript implicitly destructs intersection types in declarations.

   var x: number & string;
   var y: number = x;

   Why TypeScript does not have the same rule in partial object?
5. RyanCavanaugh commented on Mar 16, 2016

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Mar 16, 2016

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   There's no implicit destructuring there, just a simple rule of assignability.

   A simpler case to think about would be something like

   interface Farm1 {
     x: Horse;
     y: Animal;
   }
   interface Farm2 {
     x: Animal;
     y: Dog;
   }
   
   var f1: Farm1;
   var f2: Farm2;
   
   f1 = <Farm1>f2;

   This assertion isn't allowed because neither Farm1 nor Farm2 is assignable to the other. You can't have one property go "up" while the other goes "down" -- either all the properties are being asserted to a supertype, or all the properties are asserted to a subtype. There's no mixing and matching allowed.

   #5300 is closely related.
6. falsandtru commented on Mar 16, 2016

   

   falsandtru (falsandtru)

   on Mar 16, 2016

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   I understood that. Thanks Ryan Cavanaugh (@RyanCavanaugh) .
7. Ryan Cavanaugh (RyanCavanaugh)

   marked this as a duplicate of Downcast leads to compiler error TS2352 #17338 on Jul 21, 2017
8. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018