TypeScript Version:

nightly (1.9.0-dev.20160323)

Code

letnumbers:number[]=[1,'2',3,'4'].filter(x=>typeofx!=='string');declarefunctionisNumber(arg:any):arg isnumber;letnumbers2:number[]=[1,'2',3,'4'].filter(isNumber);

Playground

Expected behavior:
.filter with a type guard returns an array of the specified type. This would be especially useful with--strictNullChecks, so we could do something likefoo.map(maybeReturnsNull).filter(x => x != null)....

Actual behavior:
.filter returns an array of the same type, and the twolets above are errors.