Typescript 如何使用泛型类型访问对象

我需要一种通过课堂访问obj的方法. 为此，我想使用泛型类型.

问题是，在类中，我得到了一个错误—它是不可能判断这个. locators是否包含输入字段.

 type '{ form1: { item2: { checkbox: string; input: string; }; }; form2: { item1: { checkbox: string; }; }; }[T][F]' is not assignable to type 'object'.
  Type '{ form1: { item2: { checkbox: string; input: string; }; }; form2: { item1: { checkbox: string; }; }; }[T][keyof { form1: { item2: { checkbox: string; input: string; }; }; form2: { item1: { checkbox: string; }; }; }[T]]' is not assignable to type 'object'.
    Type '{ form1: { item2: { checkbox: string; input: string; }; }; form2: { item1: { checkbox: string; }; }; }[T][string] | { form1: { item2: { checkbox: string; input: string; }; }; form2: { item1: { checkbox: string; }; }; }[T][number] | { ...; }[T][symbol]' is not assignable to type 'object'.
      Type '{ form1: { item2: { checkbox: string; input: string; }; }; form2: { item1: { checkbox: string; }; }; }[T][string]' is not assignable to type 'object'

如何解决？我想提一下，类型必须从const obj中推断，我不想定义任何接口/类型.

const obj = {
    form1: {
        item2: {
            checkbox: 'checkbox',
            input: 'input',
        }
    },
    form2: {
        item1: {
            checkbox: 'busin'
        }
    }
};

type ObjType = typeof obj;
type ProgramKeys = keyof ObjType;
type FormKeys<T extends ProgramKeys> = keyof ObjType[T];



class CheckboxFormOne<T extends ProgramKeys, F extends FormKeys<T>> {
    locators:  ObjType[T][F];

    constructor(public program: T, public form: F) {
        this.locators = obj[program][form];
    }


    fillInput(input: keyof typeof obj[T][F]) {  
        if ('input' in this.locators && typeof this.locators[input] === 'string') {
            this.locators.input
        }
    }
}
new CheckboxFormOne('form1', 'item2').fillInput('input')