在这个 TypeScript 示例中，如何解决“X 被简化为‘never’，因为属性‘Y’在某些成分中具有冲突类型”？

Question

I have a fairly large compiler project I'm working on, and have copied out the relevant TypeScript portions into this TypeScript playground (it's a lot of types, which I think might be part of the problem).

The function at the top is this, which is throwing an error:

export function createNest<T extends Nest>(
  like: T,
  scope: SiteStepScopeType,
): NestType<T> {
  return {
    children: [],
    like,
    scope,
  }
}

It is throwing:

Type '{ children: never[]; like: Nest; scope: SiteStepScopeType; }' is not assignable to type 'NestType<T>'.
  Type '{ children: never[]; like: Nest; scope: SiteStepScopeType; }' is not assignable to type 'never'.
    The intersection 'NestBaseType & { like: Nest.Assertion; } & { like: Nest.Bind; } & SiteModuleBaseType & { like: Nest.BookModule; } & { like: Nest.Boolean; } & { like: Nest.BorrowVariable; } & ... 31 more ... & { ...; }' was reduced to 'never' because property 'like' has conflicting types in some constituents.(2322)

Basically:

...was reduced to 'never' because property 'like' has conflicting types in some constituents

I checked like on the Nest* types, and they all have a relevant value, I can't see anything that could be wrong. Is it because I have so many types? Or what is it exactly that's wrong?

Answer 1

The error message is rather confusing, but the problem the compiler doesn't understand the case where one of the values in Nest is not a key of the NestMappingType type. To my knowledge, there is no way to specify this relationship, but you can inform the compiler what's happening by using an additional constraint on T in createNest and a type assertion, like so;

export function createNest<T extends keyof NestMappingType & Nest>(
  like: T,
  scope: SiteStepScopeType,
): NestType<T> {
  return {
    children: [],
    like,
    scope,
  } as NestType<T>
}

If you prefer, instead of the additional type constraint you could also redefine Nest like this:

enum NestMappingKey {
  Assertion = "nest-assertion",
  Bind = "nest-bind",
}

export type Nest = NestMappingKey & keyof NestMappingType;