Discriminating items of filtered array of generic values in generic class-CodePudding

I know that the question may be not too clear, but it's very difficult to phrase questions and find answers to such abstract cases, and I honestly don't know how to put it better.

I have the following code:

enum TagType {
    SIMPLE = 'simple',
    COLLECTOR = 'collector',
}

interface Tag {
    type: TagType,
}

interface SimpleTag extends Tag {
    type: TagType.SIMPLE,
}

interface CollectorTag extends Tag {
    type: TagType.COLLECTOR,
}

interface ServiceDefinition<T extends Tag> {
    tags: T[],
}

// This works.
const definition: ServiceDefinition<Tag> = {tags: []};
const collectorTags: CollectorTag[] = definition.tags.filter((t: Tag): t is CollectorTag => t.type === TagType.COLLECTOR);

// This doesn't work.
class CollectorDefinitionProcessor <T extends Tag> {
    process(definition: ServiceDefinition<T>): void {
        const collectorTags: CollectorTag[] = definition.tags.filter((t: T): t is CollectorTag => t.type === TagType.COLLECTOR);
    }
}

Playground

Why code in the class is wrong? How can we discriminate items of this array in the generic class like the one above?

CodePudding user response：

This happens because the definition.tags can contain all types of tags defined in the enum. You could explicitly cast the CollectorTags to your desired type like this:

class CollectorDefinitionProcessor<T extends Tag> {
  process(definition: ServiceDefinition<T>): void {
    const collectorTags: CollectorTag[] = definition.tags.filter(
      (t: T) => t.type === TagType.COLLECTOR
    ) as CollectorTag[]
  }
}