为什么该TypeScript代码未抛出类型错误且无智能提示？

问题复现代码 #

interface payloadCollection {
    [key: string]: payloadObject<object, object>
}

 type payloadObject<TExpectedData extends object = {}, TExpectedResponse extends object = {}> = {
    request: TExpectedData,
    response: TExpectedResponse
}

class Communicator<
    TListeners extends payloadCollection, 
    TBroadcasters extends payloadCollection
> {

    broadcasters!: TBroadcasters;

    send<
        TMessage extends keyof TBroadcasters, 
        TData extends TBroadcasters[TMessage]['request'], 
        TResponse extends TBroadcasters[TMessage]['response']>(message: TMessage, data: TData): TResponse {
            return '' as any;
    }

    addListener<
        TMessage extends keyof TListeners,
        TData extends TListeners[TMessage]['request'],
        TResponse extends TListeners[TMessage]['response']
    >(message: TMessage, callback: (data: TData) => TResponse) {}
}

type extendedCommunicatorListeners = {
    'extendedListener': payloadObject<{
        type: string
    }, {
        responseType: string
    }>;
}

interface extendedCommunicatorBroadcasters extends payloadCollection {
    'extendedBroadcaster': payloadObject<{
        type: string
    }, {
        responseType: string
    }>;
}

class ExtendedCommunicator<
    TListeners extends extendedCommunicatorListeners = extendedCommunicatorListeners,
    TBroadcasters extends extendedCommunicatorBroadcasters = extendedCommunicatorBroadcasters
> 
    extends Communicator<TListeners, TBroadcasters> {


        private innerMethod() {
            const response = this.send("extendedBroadcaster", {
                type: 'test'
            });
        }   
    }

const testExtendedCommunicator: ExtendedCommunicator = new ExtendedCommunicator();

testExtendedCommunicator.send("invalidMethod", {
    invalidData: 15
});

异常表现 #

• 在ExtendedCommunicator类的innerMethod方法内调用this.send时，首个消息名参数无智能补全提示；只有手动输入合法的首个参数后，第二个data参数才会出现正确的类型提示，返回值类型推导符合预期。
• 实例化ExtendedCommunicator后调用send方法，传入不存在的invalidMethod作为消息名、传入不符合类型约束的data参数时，TypeScript 不会抛出任何类型错误。

问题根源 #

核心问题出在payloadCollection的定义上：该接口声明了[key: string]: payloadObject<object, object>字符串索引签名，意味着任意字符串键都符合类型约束，此时keyof TBroadcasters会被推导为string类型，而非定义的具体消息名字面量联合类型。

• 对IDE来说，string类型包含无限可能值，无法给出有限的补全列表，因此第一个参数没有提示。
• 对类型校验来说，任意字符串作为消息名传入都满足TMessage extends string的约束，因此非法的invalidMethod不会触发类型错误。
  此外send、addListener方法声明了多余的TData、TResponse泛型参数，会干扰TypeScript的泛型推导优先级，进一步加剧第一个参数的补全失效问题。

修复方案 #

1. 替换带开放索引签名的payloadCollection约束，改用同态映射类型做泛型校验，仅约束传入类型的所有属性符合payloadObject结构，不引入多余的字符串索引，保证keyof能拿到精确的字面量联合类型。
2. 移除send、addListener方法中多余的泛型参数，直接通过消息名类型索引对应请求、响应类型，提升泛型推导效率。
   修复后可正常运行的代码如下：

type payloadObject<TExpectedData extends object = {}, TExpectedResponse extends object = {}> = {
    request: TExpectedData,
    response: TExpectedResponse
}

// 同态映射类型约束：仅校验属性类型，不引入开放字符串索引
type ValidPayloadCollection<T> = {
    [K in keyof T]: payloadObject<object, object>
}

class Communicator<
    TListeners extends ValidPayloadCollection<TListeners>, 
    TBroadcasters extends ValidPayloadCollection<TBroadcasters>
> {
    broadcasters!: TBroadcasters;

    send<TMessage extends keyof TBroadcasters>(
        message: TMessage, 
        data: TBroadcasters[TMessage]['request']
    ): TBroadcasters[TMessage]['response'] {
        return '' as any;
    }

    addListener<TMessage extends keyof TListeners>(
        message: TMessage, 
        callback: (data: TListeners[TMessage]['request']) => TListeners[TMessage]['response']
    ) {}
}

type extendedCommunicatorListeners = {
    'extendedListener': payloadObject<{
        type: string
    }, {
        responseType: string
    }>;
}

// 直接定义结构，不继承带索引签名的接口
interface extendedCommunicatorBroadcasters {
    'extendedBroadcaster': payloadObject<{
        type: string
    }, {
        responseType: string
    }>;
}

class ExtendedCommunicator<
    TListeners extends ValidPayloadCollection<TListeners> = extendedCommunicatorListeners,
    TBroadcasters extends ValidPayloadCollection<TBroadcasters> = extendedCommunicatorBroadcasters
> 
    extends Communicator<TListeners, TBroadcasters> {
        private innerMethod() {
            // 此处输入this.send(即可看到extendedBroadcaster的补全提示
            const response = this.send("extendedBroadcaster", {
                type: 'test'
            });
            // response自动推导为{ responseType: string }类型
        }   
}

const testExtendedCommunicator = new ExtendedCommunicator();

// 此处传入invalidMethod会直接触发类型错误，data参数也会严格校验类型
testExtendedCommunicator.send("invalidMethod", {
    invalidData: 15
});

内容的提问来源于stack exchange，提问作者Adam Baranyai