main.ts

class F {
  f = "I am F"
}

class A extends F { a = "I am a" }
class B extends F { b = "I am b" }
class C extends F { c = "I am c" }

interface Something<T extends { [i: string]: F } = {}> {
  register: <K extends string, C extends F>(name: K, classInstance: C) => Something<T & { [key in K]: C }>
  getClass: <K extends keyof T>(className: K) => T[K]
}

function something<T extends { [i: string]: F } = {}>() {
  const classes = {} as { [i: string]: F }

  const somethingInstance: Something = {
    register: <K extends string, C extends F>(name: K, classInstance: C) => {
      classes[name] = classInstance
      return somethingInstance as Something<T & { [key in K]: C }>
    },

    getClass: <K extends keyof T>(className: K): T[K] => {
      return classes[className as string] as T[K]
    }
  }

  return somethingInstance
}

const somethingInstance = something().register("a", new A).register("b", new B)
const aInstance = somethingInstance.getClass("a")