Can't call forEach on number[] | ReadonlyArray<never> #16716

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1. jcalz commented on Jun 23, 2017

   

   Joe Calzaretta (jcalz)

   on Jun 23, 2017

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   I think the first error message is from the same issue reported in #16644:

   var a = [1, 2, 3];
   (a as number[]).filter(x => typeof x === 'string'); // fine
   (a as (number | string)[]).filter(x => typeof x === 'string'); // fine
   (a as number[] | (number | string)[]).filter(x => typeof x === 'string'); // error!
   
   var b = [];
   (b as number[]).forEach(x => console.log(x)); // fine
   (b as never[]).forEach(x => console.log(x)); // fine
   (b as number[] | never[]).forEach(x => console.log(x)); // error!

   In that issue, Jakub Korzeniowski (@kujon) was asking that A[] | B[] should reduce to (A|B)[] which is consistent with the way TypeScript currently treats arrays as covariant in their content type. But possibly-mutable arrays should be invariant in their type, so maybe that fix would be going farther in the wrong direction.

   The other way of looking at this issue is:

   type Overloaded<A, B> = {
     (a: A): void;
     (b: B): void;
   }
   function exploded<A, B>(f: Overloaded<A,B>, ab: A | B): void {
     f(ab); // error!
     var isA: (ab) => ab is A;
     isA(ab) ? f(ab) : f(ab); // workaround, no error 
   }

   TypeScript can't take an overloaded or generic function and realize that if it can resolve a parameter of type A and separately resolve a parameter of type B, then it "should" be able to handle a parameter of type A|B. But here be dragons.
3. ghost closed this as completedon Jun 23, 2017
4. ghost added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Jun 23, 2017
5. microsoft

   locked and limited conversation to collaborators on Jun 14, 2018