Type guards wrapped in lambda lose the guard type

[jovdb]

TypeScript Version: 2.7.0-dev.20180119

Search Terms:
array guard lambda

Code

declare function isString(arg: any): arg is string;

var items: (number | string)[] = [];
var a = items.filter(isString); // a: string[]
var b = items.filter(i => isString(i)); // b: (string | number)[]

When guard is wrapped in a lambda, guard type is gone
Other example:

// Get non-empty strings
var c = items.filter(i => isString(i) && i.length > 0); // c: (string | number)[]

Expected behavior:
b: string[]

Actual behavior:
b: (string | number)[]

Playground
link

Related Issues:
#18562

[mhegazy]

Duplicate of #5101 and #20812

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.