Type definition of Array.prototype.filter is not fully correct

[yenshih]

TypeScript Version: 2.8.3

Code

interface Obj {
}
const obj: Obj = {};
const xs: boolean[] = [false, true, true];
const ys: Array<Obj | null> = [null, obj, obj];


function identity(): undefined;
function identity<T>(x: T): T
function identity(...args: any[]): any {
    if (args.length) {
        return args[0];
    }
}

/**
 * Or import {identity} from 'lodash/fp';
 * 
 * Type definition of LodashIdentity is
 * 
 * interface LodashIdentity {
 *     (): undefined;
 *     <T>(value: T): T;
 * }
 */

xs.filter(identity).map(x => x /* x is still boolean */);
ys.filter(identity).map(y => y /* y is still Obj | null */);

Expected behavior:

x is inferred as true.
y is inferred as Obj.

Actual behavior:

x is still boolean.
y is still Obj | null.

Playground Link: Reproduce

[s-ve]

Possibly related : #7657, #2388, #5101, #10734

[yenshih]

@s-ve You're right. Duplicated.

[RyanCavanaugh]

@yenshih please fill out the entire template (including search terms) for any future issues. Thanks!