TypeScript泛型参数推导报错：string|number不可赋值给never类型问题排查

问题原因 #

• 基类State的抽象doAction方法定义的签名和子类实现不兼容，基类固定data为any类型，子类泛型实现的签名无法被TS正确识别
• 子类doAction方法中多余声明的GData extends ActionData<GAction>泛型约束，会导致TS将联合类型的推导结果收窄为never，触发类型不匹配报错

修复代码 #

完整可运行的修复后代码如下：

abstract class Action<GData> {
  abstract isValid(data : GData): boolean

  static dostuff<GData>(action: Action<GData>, data : GData): boolean {
    return action.isValid(data);
  }
}

// 修改基类State的抽象方法签名，支持泛型推导
abstract class State<GAction extends Action<any>> {
  abstract doAction<T extends GAction>(action: T, data: ActionData<T>): State<GAction> 
}

class NextAction extends Action<string> {
  isValid(data: string): boolean {
    return (data === "222")
  }
}

class LastAction extends Action<number> {
  isValid(data : number): boolean {
    return (data === 254);
  }
}

type AllStateActions = LastAction | NextAction 

type ActionData<S> = S extends Action<infer H> ? H : never;

// 子类实现匹配基类签名即可
class OneState extends State<AllStateActions> {
  doAction<T extends AllStateActions>(action: T , data: ActionData<T>): OneState {
    if(!action.isValid(data)) {
      return this;
    }
    return this; 
  }
}

验证效果 #

调用时会自动推导正确的data类型：

const state = new OneState();
state.doAction(new NextAction(), "222"); // 类型校验通过
state.doAction(new LastAction(), 254); // 类型校验通过
state.doAction(new NextAction(), 123); // 触发类型错误，符合预期

内容的提问来源于stack exchange，提问作者Jacob Heath