Indexing constraints on mapped type should use the constraint type

[mhegazy]

when indexing into a mapped type using a key that is constraint to the same constraint type, indexing should be allowed. consider:

type Optional<T> = {[K in keyof T]?: T[K]}; 

function setFromOther<T, K extends keyof T>(obj: T, k: K, other: Optional<T>) {
 obj[k] = other[k]; // K is not constraint to keyof Optional<T>
}

[ahejlsberg]

This is fixed by #12351. However, in --strictNullChecks mode the example above still errors because T[K] | undefined is not assignable to T[K], but that's to be expected.