Narrowed types from user defined type guards are lost in subsequent method calls

TypeScript Version: 2.6.1.

Code

{
    // no error
    const arr1: Array<number> = ['foo', 1].filter(
        (x): x is number => typeof x === 'number',
    );

    /* unexpected error:
[ts]
Type '(string | number)[]' is not assignable to type 'number[]'.
Type 'string | number' is not assignable to type 'number'.
Type 'string' is not assignable to type 'number'.
*/
    const arr2: Array<number> = ['foo', 1]
        .filter((x): x is number => typeof x === 'number')
        .map(x => x);

    // workaround. no error
    const arr3: Array<number> = ['foo', 1]
        .filter((x: any): x is number => typeof x === 'number')
        .map(x => x);
}

Related ReactiveX/IxJS#162

possibly related to #19334. defining the type-guard predicate before the filter call works fine:

function isNumber(x: string | number): x is number {
  return typeof x === 'number';
}
// no error
const arr: Array<number> = ['foo', 1].filter(isNumber).map((x) => x);

Oliver Joseph Ash (@OliverJAsh)

Oliver Joseph Ash (@OliverJAsh) based on #19640 (comment) it looks like this may be fixed by #17600, as this form also works in 2.6.1:

// no error
const arr: Array<number> = ['foo', 1]
  .filter<number>((x): x is number => typeof x === 'number')
  .map((x) => x);

This is indeed fixed by #17600.

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Narrowed types from user defined type guards are lost in subsequent method calls #20186

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Narrowed types from user defined type guards are lost in subsequent method calls #20186

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trxcllnt commented on Nov 21, 2017

trxcllnt commented on Nov 21, 2017

ahejlsberg commented on Dec 3, 2017

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