Array filter/map predicate not type checking properly

[OliverJAsh]

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
}

[aluanhaddad]

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

[OliverJAsh]

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

[RyanCavanaugh]

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