TypeScript 3.9.5- 交叉点“...”被简化为“从不”，因为属性“使用”在某些成分中有冲突的类型

Question

I have a function which take in a class definition and uses that to return a new abstract class. Up to TS v3.8.3 this worked fine. Now while upgrading to TS v3.9.5 Typescript appears to be getting a different type from the class passed into the function. It's best if I just reproduce the code here.

enum AttributeUsageModel {
  ContractHash = 0x00,
  ECDH02 = 0x02,
  ECDH03 = 0x03,
  Script = 0x20,
  Vote = 0x30,
  DescriptionUrl = 0x81,
  Description = 0x90,
  Hash1 = 0xa1,
  Hash2 = 0xa2,
  Hash3 = 0xa3,
  Hash4 = 0xa4,
  Hash5 = 0xa5,
}

type BufferAttributeUsageModel =
  | 0x81
  | 0x90
  | 0xf0
  | 0xfa
  | 0xfb
  | 0xfc
  | 0xfd
  | 0xfe
  | 0xff;

type Constructor<T> = new (...args: any[]) => T;

abstract class AttributeBaseModel<T extends AttributeUsageModel> {
  public abstract readonly usage: T;
}

class AttributeModel extends AttributeBaseModel<BufferAttributeUsageModel> {
  public readonly usage: BufferAttributeUsageModel;

  public constructor(usage: BufferAttributeUsageModel) {
    super();
    this.usage = usage;
  }
}

function AttributeBase<
  Usage extends AttributeUsageModel,
  TBase extends Constructor<AttributeBaseModel<Usage>>
>(Base: TBase) {
  // Replacing `Base` with `AttributeModel` apparently fixes it
  abstract class AttributeBaseClass extends Base {} // Base constructor return type is apparently 'never'

  return AttributeBaseClass;
}

// Base constructor return type 'never' is not an object type or intersection of object types with
// statically known members.
//   The intersection 'AttributeBase<AttributeUsageModel, typeof
//   AttributeModel>.AttributeBaseClass & AttributeModel' was
//   reduced to 'never' because property 'usage' has conflicting types in some constituents.
class BufferAttribute extends AttributeBase(AttributeModel) {
  public constructor(usage: BufferAttributeUsageModel) {
    super(usage);
  }
}

The error from Typescript is highlighted on AttributeBase(AttributeModel) . In this example AttributeModel is being passed into the function as the Base argument. If we use AttributeModel directly inside the function instead of the Base argument the error goes away. But this is the same thing, right?

I'm not an expert in Typescript yet, but we do need some way to use this AttributeBase function, or some other method, to dynamically extend multiple base classes. Perhaps the answer is obvious, but this does seem like rather odd behavior. I've tried a bunch of other things but nothing that maintains the same type strictness we get from this setup.

So can someone please explain to me what's going on here and how I might fix this so we get the same type safety without the never return type from AttributeBase(AttributeModel) ?

I've also posted this code to a repo here if that helps.

Answer 1

Problem is that your BufferAttributeUsageModel does not extend AttributeUsageModel .

Your AttributeBaseModel<T extends AttributeUsageModel> requires T extends AttributeUsageModel , but then you define class AttributeModel extends AttributeBaseModel<BufferAttributeUsageModel> with BufferAttributeUsageModel that does not extend AttributeUsageModel . That's where your usage property "has conflicting types in some constituents." - as the error states.

To fix that, you can define BufferAttributeUsageModel as:

type BufferAttributeUsageModel =
    AttributeUsageModel
    | 0x81
    | 0x90
    | 0xf0
    | 0xfa
    | 0xfb
    | 0xfc
    | 0xfd
    | 0xfe
    | 0xff;

Perhaps, you should try to simplify those type definitions.