TypeScript is unsound

unsound.ts

class Base {

}
class Derived extends Base {
    y:string
}

//pretend we have variance
class Collection1<T>{
    accept(value: T) { }
}

var derivedCollection: Collection1<Derived> = new Collection1<Base>();
derivedCollection.accept(new Derived())

class Collection2<T>{
    getResult(): T { return null }
}
var baseCollection2: Collection2<Base> = new Collection2<Derived>();
var result: Base = baseCollection2.getResult()

//but adding an indexer breaks everything
class Base {

}
class Derived extends Base {
    y: string

}

class Collection3<T>{
    ["string"]: T;
}

var derived: Collection3<Derived>
derived["foo"] = new Base();

var base: Collection3<Base>
derived["foo"] = new Derived();
  
//unsound
var original = new Map<string, Derived>();
var viaBase: Map<string, Base> = original;
var viaDerived: Map<string, Derived> = original;
viaBase["fff"] = new Base();
console.log(viaDerived["fff"])