What does the "=" do in ```export class NgForOfContext<T, U extends NgIterable<T> = NgIterable<T>> {```

I was looking at the source for ngForOf in Angular and I came across this line:

export class NgForOfContext<T, U extends NgIterable<T> = NgIterable<T>> {

see: https://github.com/angular/angular/blob/master/packages/common/src/directives/ng_for_of.ts#L14

I played around with the = operator and changed it to & and |. However, even with the typescript docs I couldn't figure out what the = operator actually does.

Kind Regards