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Array filter/map predicate not type checking properly #12498

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Array filter/map predicate not type checking properly#12498
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@OliverJAsh
Oliver Joseph Ash (OliverJAsh)
opened on Nov 25, 2016

TS 2.0.3

See the comments and last line in the code below for expected/actual.

{
    type Foo = { foo: true };
    const isFoo = (foo: Foo) => foo.foo;

    const xs = [{ foo: true }, undefined];

    isFoo(xs[1]) // good, errors
    const x = undefined as Foo | undefined;
    isFoo(x) // good, errors
    xs.filter(isFoo) // bad, didn't fail, expected filter arg to fail, should be Foo | undefined
}

Activity

  1. aluanhaddad commented on Nov 25, 2016

    @aluanhaddad
    Aluan Haddad (aluanhaddad)
    on Nov 25, 2016
    Contributor

    This is expected. Consider

    declare function f(x: number): any;
declare function g(x?: number): any;
declare function h(x: number | undefined): any;
let i = f;
i = g;
i = f;
i = h;
let k = f;
k = h;
k = g;

    Also, see this section of the handbook. type-compatibility

  2. OliverJAsh commented on Nov 25, 2016

    @OliverJAsh
    Oliver Joseph Ash (OliverJAsh)
    on Nov 25, 2016
    ContributorAuthor

    In the example above, I would receive a type error at runtime. Is there any way to enforce this at compile time?

  3. Ryan Cavanaugh (RyanCavanaugh)
    added
    DuplicateAn existing issue was already created
    on Nov 28, 2016

  4. RyanCavanaugh commented on Nov 28, 2016

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Nov 28, 2016
    Member

    See https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant

  5. Ryan Cavanaugh (RyanCavanaugh)
    closed this as completedon Nov 28, 2016
  6. Anders Hejlsberg (ahejlsberg)
    added
    FixedA PR has been merged for this issue
    on Sep 21, 2017
  7. microsoft
    locked and limited conversation to collaborators on Jun 19, 2018
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