weird-contraction-of-union-type.ts

interface superType {
    a: string,
    b: number,
}

interface subType extends superType {
    c: string | number
}

declare const xs: Array<superType | null> | Array<subType>

const isNotNull = <T>(a: T): a is NonNullable<T> => a !== null

/** Should be enough to guarantee no-nulls but somehow isn't */
const whyAreThereNulls = xs.filter(isNotNull) // => (superType | null)[] 

// need to add another filter before TS gives all-clear
const definitelyNoNulls = xs.filter(isNotNull).filter(isNotNull) // superType[]

/*** simpler type works */ 
declare const ys: Array<superType | null>

const oneFilter = ys.filter(isNotNull) // superType[]