打字稿泛型。 无法将B的实例传递给参数a：T，其中T扩展了B。为什么？

Question

I have class:

class X<T extends B> [...]
  // this.p.a :: B | null
  methodA(a: T):void {[...]}

  methodB(): void {
    if(this.p.a){ // :: B
      this.methodA(this.p.a) // Error

As you can see, I'm relaying on T being any type as long as it extends B , and then I'm passing some property to it of said type B . However Typescript can't infer that B is a type that satisfy T type parameter in the last line of above code snippet.

Error message:

error TS2345: Argument of type 'B' is not assignable to parameter of type 'T'.

Why is this not working? Should I change any offending type declaration to B instead of T and relay on inheritance?

Answer 1

You're getting an error because someone might use X<T> where T is a subclass of B , and then this.pa might be a B that is not a T . To know the best way to resolve the error, we'd need to know more about what you are trying to achieve.