在类成员和方法中扩展泛型类型

如何解决在类成员和方法中扩展泛型类型

我不太确定这个实现，我可以清楚地理解问题，但我不确定正确的语法。 private db: Map<string,any> 有效，但这不是正确的方法。

class Collection {
  private db: Map<string,GenericItem> = new Map(); // I would like to use 'GenericItem' here,but it is not assignable to 'T'.

  setItemByName<T extends GenericItem>(name: string,item: T): void {
  this.db.set(name,item);
  }

  getCollectionItemByName<T extends GenericItem>(name: string): T {
    // Type 'GenericItem' is not assignable to type 'T'.
    // 'GenericItem' is assignable to the constraint of type 'T',// but 'T' Could be instantiated with a different subtype of constraint 'GenericItem'.
    return this.db.get(name);
  }
}

class GenericItem { name = null; }
class Geometry extends GenericItem { faces = 'squares';}
class Human extends GenericItem { age = 12; }

const collection = new Collection();
const cube = new Geometry();

collection.setItemByName<Geometry>('cube',cube);

const cubeItem = collection.getCollectionItemByName<Geometry>('cube');

// narrowing the type with generics let inference instance properties and have an error message
console.log(cubeItem.age); // Property 'age' does not exist on type 'Geometry'.